molecules

Review
Flavour Volatiles of Fermented Vegetable and Fruit Substrates:
A Review
Sarathadevi Rajendran 1,2 , Patrick Silcock 1, * and Phil Bremer 1

1 Department of Food Science, University of Otago, Dunedin 9054, New Zealand

2 Department of Agricultural Chemistry, Faculty of Agriculture, University of Jaffna,
Kilinochchi 42400, Sri Lanka
* Correspondence: pat.silcock@otago.ac.nz

Abstract: Health, environmental and ethical concerns have resulted in a dramatic increase in demand
for plant-based dairy analogues. While the volatile organic compounds (VOCs) responsible for the
characteristic flavours of dairy-based products have been extensively studied, little is known about
how to reproduce such flavours using only plant-based substrates. As a first step in their develop-
ment, this review provides an overview of the VOCs associated with fermented (bacteria and/or
fungi/yeast) vegetable and fruit substrates. Following PRISMA guidelines and using two English
databases (Web of Science and Scopus), thirty-five suitable research papers were identified. The
number of fermentation-derived VOCs detected ranged from 32 to 118 (across 30 papers), while
5 papers detected fewer (10 to 25). Bacteria, including lactic acid bacteria (LAB), fungi, and yeast
were the micro-organisms used, with LAB being the most commonly reported. Ten studies used a
single species, 21 studies used a single type (bacteria, fungi or yeast) of micro-organisms and four
studies used mixed fermentation. The nature of the fermentation-derived VOCs detected (alcohols,
aldehydes, esters, ketones, acids, terpenes and norisoprenoids, phenols, furans, sulphur compounds,
alkenes, alkanes, and benzene derivatives) was dependent on the composition of the vegetable/fruit
matrix, the micro-organisms involved, and the fermentation conditions.

Keywords: volatile organic compounds (VOCs); plant-based substrates; lactic acid bacteria (LAB)

Citation: Rajendran, S.; Silcock, P.;

Bremer, P. Flavour Volatiles of
Fermented Vegetable and Fruit 1. Introduction
Substrates: A Review. Molecules 2023, The demand for plant-based foods is rapidly increasing. Consumers’ reasons for
28, 3236. https://doi.org/10.3390/ eating more plant-based foods are varied but mainly revolve around a desire to enhance
molecules28073236 their health by reducing their risk of diseases such as heart disease, cancer, and type
Academic Editors: Mirela Kopjar and 2 diabetes [1], concerns around the sustainability of meat and dairy-based food production
Anita Pichler systems [2], and/or a desire to move away from the exploitation of animals [3]. Accordingly,
the global market for plant-based foods is predicted to reach USD 480.43 billion by 2024
Received: 27 January 2023 with a predicted CAGR (compound annual growth rate) of 13.82% between 2019 and
Revised: 27 March 2023
2024 [4]. Despite plant-based diets becoming increasingly popular, many consumers still
Accepted: 28 March 2023
prefer to prepare their meals from foods that are familiar to them in terms of their flavour,
Published: 4 April 2023
appearance, preparation, and cooking method [3]. In response to these needs, the number
and range of dairy analogues available on the market are rapidly increasing [5]. While
currently available dairy analogues generally have a realistic texture and appearance, their
Copyright: © 2023 by the authors.
flavour is often uncharacteristic of the dairy products they are attempting to mimic.
Licensee MDPI, Basel, Switzerland. Flavour is an important sensory attribute of food, and it is a multimodal sensory
This article is an open access article experience with the major modality contributors being taste and smell (retronasal). A
distributed under the terms and wide range of non-volatile organic compounds (NVOCs) along with multiple odour-active
conditions of the Creative Commons volatile organic compounds (VOCs) contribute to the taste, aroma, and overall perceived
Attribution (CC BY) license (https:// flavour of a food [6,7]. VOCs are typically small compounds (up to C20 ), which have a
creativecommons.org/licenses/by/ low molecular weight (<300–400 Da), and a relatively high vapour pressure at ambient
4.0/). temperature. Such compounds can be easily transferred into the vapour phase, which

Molecules 2023, 28, 3236. https://doi.org/10.3390/molecules28073236 https://www.mdpi.com/journal/molecules

Molecules 2023, 28, 3236 2 of 36

means depending on their concentration, they have the potential to induce an odour
sensation [8,9].
Fat, protein (casein, whey/amino acids), sugars (lactose), and citrate are the key com-
ponents of raw/processed milk, and they are responsible for a wide range of flavour VOCs.
The sensory perception of fresh milk is largely determined by a pleasant mouthfeel owing
to the physical composition of milk, a slight sweet/salty taste derived from lactose and
milk salts, and a delicate aroma due to the VOCs present. In addition, lactic acid bacteria
(LAB), when present in fermented dairy products, produce flavour VOCs from sugars
or citrate (acetoin, diacetyl, propionic acid, and acetaldehyde), amino acids (benzalde-
hyde, 3-methylbutanal/3-methyl butanol, methional, methanethiol, phenyl acetic acid,
dimethyl sulphide, dimethyl disulphide, and dimethyl trisulphide), and lipids (butanoic
acid, butanone, octene-3-ol, hexanal, and δ-decalactone) [10,11].
As many consumers wish to avoid dairy-derived products completely, dairy-derived
flavours are generally not acceptable components of dairy analogues. For this reason,
the flavours or the substrates needed to generate appropriate flavours need to originate
from other sources, with plants owing to diversity, availability and affordability being
the most logical choice. However, as dairy flavours contain specific VOCs, as outlined
above, producing these flavours and simulating the desired flavour from plant substrates
is challenging. In addition, when using plant-based substrates, the flavours of interest may
also be associated with other less desirable flavours contained in the plant material matrix,
as plants typically contain complex mixtures of substrates [10,12].
The use of micro-organisms to produce desirable flavours from plant-based substrates
has gained interest owing to the diversity of the micro-organisms, and the wide range of
metabolic reactions they can carry out. Micro-organisms growing on plant-based substrates
utilise the available sugars, lipids, and proteins for their energy, synthesis, and growth
needs and, in this process, produce a wide range of volatile secondary metabolites through
various metabolic pathways [13].
Microbial flavours can therefore be produced in a relatively simple and environ-
mentally friendly/food-grade manner, and such approaches are generally scalable and
affordable [14]. The VOCs (composition and concentration) produced by microbial fermen-
tation are dependent on the substrate, type of fermenting micro-organisms (bacteria, LAB,
fungi, and yeast), substrate treatments prior to fermentation, and fermentation conditions
(aerobic/anaerobic, time, temperature, moisture content, and pH) [9,15].
A better understanding of the substrates available in plants and the metabolic pathways
present in micro-organisms will facilitate the use of microbial biosynthesis/fermentation
as a means of synthesising VOCs from plant-based substrates that are able to be used
as dairy flavours or flavour precursor compounds. As the first step in developing this
understanding, the current review presents the range of VOCs reported to be produced
during the fermentation of fruit or vegetable substrates, with a focus on flavour VOCs that
are known to contribute to dairy-like flavours.

2. Research Methods
A review based on PRISMA (Preferred Reporting Items for Systematic Reviews and
Meta-Analyses) guidelines [16] was carried out to search for research articles describing the
VOCs obtained from the microbial fermentation of plant-based substrates. The goal of the
review was to determine the main VOCs reported as being generated from fermented fruit
or vegetable substrates, the range of fruit and vegetable substrates used, the different micro-
organisms used in previous studies, the fermentation conditions used for VOC production,
and the methods used for VOC detection. A search of the electronic databases, Web of
Science and Scopus for original research articles written in English over the last six years
(from 2017 to April 2022) using the following keywords; “Fermentation flavor volatiles”,
“Fermentation flavour volatiles”, “Fermentative aromatic compounds”, “Fermentation
flavours vegetable and fruit juices”, “Fermentation flavours vegetable and fruit substrates”,
“Fermentation flavours plant substrates” (Figure 1) generated 855, and 1700 research
Molecules 2022, 27, x FOR PEER REVIEW 3 of 41

Molecules 2023, 28, 3236 “Fermentation flavours vegetable and fruit juices”, “Fermentation flavours vegetable 3 of 36and
fruit substrates”, “Fermentation flavours plant substrates” (Figure 1) generated 855, and
1700 research papers, respectively. The titles and then abstracts of these papers were
screened to identify potentially relevant publications. This gave 277 and 408 initially
papers, respectively. The titles and then abstracts of these papers were screened to identify
screened
potentiallyarticles from
relevant Web of Science
publications. and277
This gave Scopus,
and 408respectively, for which
initially screened articlesthe
fromfull
Web texts
were
of Science and Scopus, respectively, for which the full texts were assessed against inclusion as
assessed against inclusion and exclusion criteria (Table 1). A technique known
“snowballing” identified
and exclusion criteria a further
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papers dealing with were
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into consideration for thisand fruit substrates
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Records found from Web of Science Records found from Scopus

855 1700
Criteria: Research articles/6 years Criteria: Research articles/6 years

No of articles excluded Title screening: Fermentation No of articles excluded

flavour volatiles from plant
542 1150
substrates

No of articles screened No of articles screened

313 550

No of articles excluded
No of articles excluded
Abstracts screening
142
36

No of articles further screened No of articles further screened

277 408

No of articles excluded Full paper screening: Fermented No of articles excluded

flavours from fruit and vegetable
247 substrates 381

Included articles Included articles

30 27

Included articles after removal of

duplicates Snowballing

36 02

Screening criteria: Plant juices No of articles excluded

added in milk
3

Final articles
35

Figure
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PRISMAflowchart of the
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on flavour of fermented
volatiles vegetable
of fermented and fruit
vegetable and
substrates.
fruit substrates.
Molecules 2023, 28, 3236 4 of 36

Table 1. Inclusion and exclusion criteria for screening papers.

Inclusion Criteria Exclusion Criteria

Original research papers published in recognized journals Papers published in predatory journals
Fermented flavours of wine, beer, cocoa, meat, and milk-based substrates
Fermented flavours of fruit and vegetable substrates
Plant juices added in milk substrates

3. Results
3.1. Research Profiles
A total of 35 scientific papers met the criteria used for the review. An overview
of the study conditions reported in the papers is presented in Table 2. The number of
papers published by year was: 2017 (4 articles, 11.44%); 2018 (10 articles, 28.57%); 2019
(7 articles, 20%); 2020 (2 articles, 5.71%); 2021 (10 articles, 28.57%); and 2022 (2 articles,
5.71%). The papers were published in 20 different journals, which had an impact factor
ranging from 1.713 to 7.514 (Figure 2). The greatest number of papers (6) were published in
Food Chemistry.

Table 2. Overview of the study conditions used in the 35 papers that met the inclusion criteria on the
VOCs associated with fermented vegetable and fruit substrates.

Fermentation Extraction and Analysis Major Flavour

No Substrate Micro-organisms Reference
Conditions of Flavour Volatiles Volatile Classes

Apple, carrot, tomato, L. rhamnosus Acids (1)

L. plantarum Inoculum: 2% (v/v) Alcohols (2)
1 cucumber, and haw L. casei Extracted and analysed Aldehydes (3) [17]
mixed juice using HS-SPME-GC-MS
L. acidophilus at 30 ◦ C for 24 h Esters (2)
(40: 25: 15: 15: 5) L. fermentum Ketones (2)
Acids (4)
Alcohols (20)
Inoculum: Strain Aldehydes (17)
powder Esters (3)
Momordica charantia L. 0.01% (w/v) (11.0 log Extracted and analysed Heterocyclic (2)
2 L. plantarum Hydrocarbons (4) [18]
juice CFU/mL) using HS-SPME-GC-MS
Ketones (8)
at 37 ◦ C for 48 h Terpenes and
norisoprenoids (13)
Others (2)
Inoculum:
L. plantarum 1% (v/v)
Apple juice L. helveticus (8.0 log CFU/mL) Alcohols (13)
L. casei Extracted and analysed Aldehydes (5)
3 (Fuji apple (Malus at 37 ◦ C for 48 h [19]
L. paracasei using HS-SPME-GC-MS Esters (24)
pumila Mill.)) L. acidophilus Ketones (10)
B. lactis pH adjusted to 5.0 using
food-grade Na2 CO3
Inoculum: 0.5% (v/v)
Jujube juice L. acidophilus Acids (14)
at 37 ◦ C for 48 h
L. casei Alcohols (10)
4 (Two varieties L. helveticus Extracted and analysed Aldehydes (19)
pH adjusted to 5.0 using [20]
Ziziphus Jujuba cv. L. plantarum using HS-SPME-GC-MS
Muzao and food-grade Na2 CO3 and Esters (8)
Hetian) Ketones (12)
brix adjusted to 10.0
◦ Brix with potable water

Inoculum: 7.0 log Acids (2)

L. acidophilus CFU/mL Aroma profile analysed Alcohols (16)
L. rhamnosus Aldehydes (5)
by electronic nose
5 Apple juice L. casei pH adjusted to 6.0 using Esters (12) [21]
system and
L. plantarum food grade Na2 CO3 Ketones (6)
characterized by Phenols (4)
at 37 ◦ C for 80 h HS-SPME/GC-MS. Terpenes and
norisoprenoids (6)
Acids (6)
Alcohols (19)
Inoculum: 7.0 log Aldehydes (3)
L. plantarum CFU/mL Alkenes (3)
Elderberry juice L. rhamnosus Extracted and analysed
6 Esters (8) [22]
(Sambucus nigra L.) L. casei at 30 ◦ C (L. plantarum), using HS-SPME-GC-MS Ketones (9)
and 37 ◦ C (L. rhamnosus Terpenes and
and L. casei) for 48 h
norisoprenoids (26)
Others (7)
Molecules 2023, 28, 3236 5 of 36

Table 2. Cont.

Fermentation Extraction and Analysis Major Flavour

No Substrate Micro-organisms Reference
Conditions of Flavour Volatiles Volatile Classes
Inoculum: 5% mass ratio Acids (8)
Alcohols (20)
at 37 ◦C for 48 h
Papaya juice Aldehydes (10)
7 L. acidophilus Extracted and analysed
Esters (23) [23]
(Carica papaya L.) L. plantarum 5% of edible glucose and using HS-SPME-GC-MS
Ketones and
5% of skim milk powder lactones (13)
added into the juice Phenols (3)
Alcohols (23)
Aldehydes (15)
Alkanes (8)
Alkenes (2)
Extracted and analysed Benzene
Inoculum: 7.0 log by Purge and trap derivatives (5)
Pomegranate juice CFU/mL coupled with gas Esters (14)
8 L. plantarum Furans (9) [24]
(Punica granatum L.) chromatography-mass
at 30 ◦ C for 120 h spectrometry Ketones (24)
Sulphur
(PT-GC-MS)
compounds (6)
Terpenes and
norisoprenoids (8)
Others (2)
Inoculum: 0.5% (v/v)
L. plantarum
L. rhamnosus at 37 ◦ C for 20h under Acids (5)
L. reuteri anaerobic conditions Alcohols (14)
Goji berry/wolfberry B. velezensis Aldehydes (12)
9 juice B. licheniformis pH adjusted to 6.5 using Extracted and analysed
using HS-SPME-GC-MS Esters (6) [25]
(Lycium barbarum L.) baking soda Ketones (7)
Different Phenols (5)
combinations of 5% sucrose added into Others (9)
mixed culture the juice before pH
adjustment
Acids (3)
L. acidophilus Inoculum: 8.18 log Alcohols (8)
Cashew apple juice
L. casei CFU/mL Extracted and analysed Aldehydes (1)
10 (Anacardium [26]
L. plantarum using HS-SPME-GC-MS. Esters (6)
occidentale) at 37 ◦ C for 48 h Ketones (1)
Others (1)
Alcohols (7)
Aldehydes (5)
Inoculum: 4.0 log Aromatics (9)
Carrot juice CFU/mL Extracted and analysed Esters (3)
11 L. plantarum Ketones (10) [27]
(Daucus carota L.) using HS-SPME-GC-MS
at 37 ◦ C for 120 h Terpenes and
norisoprenoids (17)
Others (3)
Acids (3)
Inoculum: 8.0 log Alcohols (8)
CFU/mL Aldehydes (4)
Broccoli juice
12 P. pentosaceus Extracted and analysed Esters (2)
(Brassica oleracea L. var. [28]
at 37 ◦ C for 36 h under using HS-SPME-GC-MS Furans (3)
italica)
anaerobic Ketones (2)
incubator Sulfides (3)
Others (8)
Acids (3)
Alcohols (15)
Inoculum: 7.0 log Aldehydes (9)
L. plantarum CFU/mL Benzene
Cherry juice L. rhamnosus Extracted and analysed derivatives (5)
13 Esters (2) [29]
(Prunus avium L.) L. paracasei using HS-SPME-GC-MS
at 30 ◦ C (L. plantarum), Ketones (7)
and 37 ◦ C (L. rhamnosus, Terpenes and
and L. paracasei) for 48 h norisoprenoids (11)
Others (5)
Inoculum: 1% (v/v)

L. casei monoculture:
at 37 ◦ C for 3 days and Acids (3)
Alcohols (2)
30 ◦ C for 32 days Aldehydes (1)
Durian pulp L. casei
14 W. saturnus var. Extracted and analysed
(Durio zibethinus Sequential inoculation: Esters (14) [30]
saturnus using HS-SPME-GC-MS
Murr.) Ketones (1)
after 3 days of L. casei Sulphur containing
inoculation, 1% (v/v) W. compounds (4)
saturnus var. saturnus
inoculated at 30 ◦ C for
another 32 days
Molecules 2023, 28, 3236 6 of 36

Table 2. Cont.

Fermentation Extraction and Analysis Major Flavour

No Substrate Micro-organisms Reference
Conditions of Flavour Volatiles Volatile Classes
Inoculum: 9.0 log
CFU/mL

at 30 ◦ C for 72 h Acids (2)

Alcohols (5)
L. plantarum Fresh cashew apple juice Extracted and analysed Aldehydes (1)
15 Cashew apple juice [31]
with 11.4 ◦ brix using HS-SPME-GC-MS Esters (5)
and concentrated Ketones (1)
cashew apple juice with Others (1)
78.6 ◦ brix was adjusted
to 11.4 ◦ brix with
distilled water
Inoculum: not reported Acids (7)
Alcohols (46)
at 37 ◦ C for 48 h under Aldehydes (10)
L. plantarum anaerobic condition
16 Tomato juice L. casei Extracted and analysed
Esters (6) [32]
using HS-SPME-GC-MS
Total brix was adjusted Hydrocarbons (19)
from 4.6 to 12.4 ◦ Brix Ketones (17)
with glucose and sucrose Others (13)

Inoculum: 8.0 log

CFU/mL Acids (13)
L. plantarum or L. Alcohols (33)
Yam/Soil potato juice Extracted and analysed
17 plantarum and at 37 ◦ C for 24 h Aldehydes (8) [33]
(Rhizoma dioscoreae) S. thermophilus using HS-SPME-GC-MS
Esters (19)
The sugar content was Ketones (8)
adjusted to 20 ◦ Brix
Inoculum: 9.0 log
CFU/mL Acids (3)
Alcohols (11)
Bog bilberry juice at 23 ◦ C for 14 days Aldehydes (2)
18 L. plantarum Extracted and analysed Esters (9) [34]
(Vaccinium uliginosum using HS-SPME-GC-MS
L.) Juice at nature (2.65) and Terpenes and
pH adjusted to 3.50 norisoprenoids (4)
using NaOH solid Others (5)
powder
Inoculum: 7.0 log
CFU/mL Alcohols (11)
Apple juice, orange L. plantarum Esters (4)
19
juice, carrot juice, and B. breve at 37 ◦ C for 48 h under Extracted and analysed Hydrocarbons (14)
S. thermophilus [35]
Chinese jujube juice anaerobic conditions using HS-SPME-GC-MS
mixed starters Ketones and
blend Aldehydes (5)
Blended juice enriched
with Se
Inoculum: 0.5% (w/v) Acids (4)
Alcohols (11)
at 25 ◦ C for 7 days Aldehydes (6)
Cabbage L. plantarum
20 Lentilactobacillus Extracted and analysed Alkenes (12)
Celery Vegetables (cabbages—8 [36]
diolivorans using HS-SPME-GC-MS Benzenes (6)
Carrot kg, celeries—1 kg, and Esters (13)
Carrots—1 kg) mixed Ketones (8)
with 500 mL, 4% brine Phenols (3)
Acids (5)
Inoculum: 8.0 log
Horse gram sprouts Alcohols (8)
CFU/mL Aldehydes (3)
21 (Macrotyloma uniflorum L. plantarum (3mL inoculum added to Extracted and analysed
Esters (6) [37]
var. uniflorum 2 g of crushed sprouts) using HS-SPME-GC-MS
Paiyur 1) Ketones (5)
◦C Phenols (4)
at 37 for 5 days
Others (3)
Initial fungal inoculum:
5.2 log CFU R. Acids (6)
oligosporus/g okara (wet Alcohols (12)
basis) Aldehydes (13)
Co culture of Extracted and analysed Esters (20)
22 Okara (Soybean pulp) R. oligosporus After one day: inoculum Furans (5) [38]
using HS-SPME-GC-MS
and Y. lipolytica 6 log CFU Y. lipolytica/g Ketones (10)
okara (wet basis) Phenol and phenol
derivatives (5)
at 30 ◦ C for 3 days under Others (2)
aerobic condition
Inoculum: 9.0 log
CFU/mL Alcohols (1)
Esters (9)
Extracted and analysed Terpenes and
23 Cashew apple juice L. casei at 30 ◦ C for 16 h using HS-SPME-GC-MS
[39]
norisoprenoids (1)
pH adjusted to 6.4 with Others (1)
3.0 M NaOH
Inoculum: 9.0 log
CFU/mL Alcohols (5)
Extracted and analysed Aldehydes (8)
Melon juice L. casei at 31 ◦ C for 8 h [39]
using HS-SPME-GC-MS Esters (18)
pH adjusted to 6.1 with Ketones (1)
0.1 M citric acid
Molecules 2023, 28, 3236 7 of 36

Table 2. Cont.

Fermentation Extraction and Analysis Major Flavour

No Substrate Micro-organisms Reference
Conditions of Flavour Volatiles Volatile Classes
Acids (3)
Inoculum: 7.0–8.0 log Alcohols (6)
spores /mL Volatiles extracted by Aldehydes (7)
T. atroviride HS-SPME and analysed Esters (1)
24 Tomato pomace ◦C Ketones (2) [40]
A. sojae at 30 for 120 h in a using GC-MS and
shaking incubator at GC-Olfactometry (GCO) Pyrazines (1)
120 rpm Sulphur
compounds (1)
Inoculum: 7.0–8.0 log Acids (2)
spores/mL Volatiles extracted by Alcohols (6)
T. atroviride HS-SPME and analysed Aldehydes (4)
Red pepper pomace A. sojae [40]
at 30 ◦ C for 120 h in a using GC-MS and Esters (2)
shaking incubator at GC-Olfactometry (GCO) Ketones (6)
120 rpm Pyrazines (1)
Inoculum: mass ratio
of 5% Acids (5)
Alcohols (11)
S. cerevisiae at 25 ◦ C for Aldehydes (2)
L. plantarum 48 h
Alkanes (5)
25 S. thermophilus Extracted and analysed Esters (15)
Mango slurry L. casei LAB [41]
using HS-SPME-GC-MS Ketones (7)
S. cerevisiae at 37 ◦ C for 48 h
Terpenes and
Total brix adjusted to norisoprenoids (10)
22.3 ◦ brix with glucose Others (2)
and sucrose
Acids (7)
Inoculum: 8.30 Log Alcohols (6)
CFU/mL juice Aldehydes (7)
L. plantarum subsp. Alkanes (3)
Sea buckthorn juice plantarum at 30 ◦ C for 36 and 72 h Extracted and analysed Esters (53)
26 (Hippophae rhamnoides Ketones (9) [42]
using HS-SPME-GC-MS
L.) L. plantarum subsp. Juice at native pH (2.7) Terpenes and
argentoratensis and pH adjusted to 3.5 norisoprenoids (4)
with 1 M NaOH Sulphur
compounds (3)
Acids (8)
Alcohols (26)
L. plantarum Aldehydes (6)
Grape juice L. brevis Inoculum: 5% (v/v)
27 Extracted and analysed
mixed culture 1:2 Alkenes (1) [43]
(Jumeigui variety) using HS-SPME-GC-MS
ratio, respectively at 36 ◦ C for 60 h Esters (5)
Ketones (3)
Phenols (2)
Alcohols (18)
Aldehydes (12)
Esters (7)
Extracted by simple and Furans (3)
Inoculum: 7.0 Log
Orange pomace vacuum distillation Ketones (10)
28 L. rhamnosus CFU/g [44]
(Citrus sinensis) Sulphur
Distillates analysed
at 37 ◦ C for 72 h compounds (1)
using HS-SPME-GC-MS
Terpenes and
norisoprenoids (38)
Others (2)
Alcohols (22)
Aldehydes (19)
Alkenes (1)
Inoculum: 7.0 Log Extracted by simple and Esters (25)
Melon by-product vacuum distillation Furans (1)
L. rhamnosus CFU/g [44]
(Cucumis melon) Ketones (6)
Distillates analysed Sulphur
at 37 ◦ C for 72 h using HS-SPME-GC-MS compounds (2)
Terpenes and
norisoprenoids (11)
Extracted and analysed
Inoculum: 8.0 log using a static headspace Acids (1)
CFU/mL gas chromatography ion Alcohols (5)
mobility spectrometry Aldehydes (9)
29 L. plantarum at 37 ± 1 ◦ C for 48 h (GC-IMS) Alkenes (1)
Mung beans [45]
Esters (11)
Mass ratio of mung bean Aroma characteristics of Furans (1)
to water of 1:3 (g:g) samples were analysed Ketones (6)
by electronic nose Others (3)
(E-nose)
Molecules 2023, 28, 3236 8 of 36

Table 2. Cont.

Fermentation Extraction and Analysis Major Flavour

No Substrate Micro-organisms Reference
Conditions of Flavour Volatiles Volatile Classes
Inoculum: 6.0–7.0 log
L. acidophilus CFU/g okara Alcohols (13)
L. rhamnosus Aldehydes (13)
30 Okara
P. acidilactici at 37 ◦ C for 72 h Extracted and analysed
[46]
using HS-SPME-GC-MS Furans (4)
monoculture and Ketones (9)
Solid state
co-culture Others (2)
fermentation process

Inoculum: 8% (v/v)
Jujube pulp mixed lactic acid Acids (15)
L. plantarum bacteria culture (L. Volatiles extracted by Alcohols (7)
(Ziziphus jujuba Mill.) L. rhamnosus HS-SPME and analysed Aldehydes (9)
31 plantarum/L. rhamnosus/
S. thermophilus [47]
S. thermophilus = 1:1:2, using GC-MS and Esters (5)
(Charkhlik Hui-jujube, Mixed culture v/v/v) GC-Olfactometry (GCO) Ketones (6)
Xinjiang variety) Others (7)
at 37 ◦ C for 24 h
Acids (1)
Alcohols (15)
Aldehydes (14)
Inoculum: 1% (v/v) Alkanes (2)
L. plantarum Esters (1)
Watermelon juice L. rhamnosus Furans (4)
32 L. casei at 30 ◦ C (L. plantarum, P. Extracted and analysed
(Citrullus lanatus Ketones (6) [48]
L. brevis pentosaceus and L. brevis) using HS-SPME-GC-MS
[Thunb]) and 37 ◦ C (L. casei and L. Terpenes and
P. pentosaceus norisoprenoids (7)
rhamnosus) for 24 h
Sulphur
compounds (1)
Others (3)
Inoculum: 1% (v/v)
Alcohols (20)
Kiwifruit juice at 37 ◦ C for 48 h Aldehydes (13)
(Actinidia deliciosa cv. L. acidophilus
Extracted and analysed
Esters (6)
33 Xuxiang and Actinidia L. helveticus pH adjusted to 4.0 using Ketones (11) [49]
L. plantarum food-grade Na2 CO3 using HS-SPME-GC-MS
chinensis cv. Terpenes and
Hongyang) (50.0 mg/mL) and brix norisoprenoids (6)
adjusted to 12.0 ◦ Brix
with potable water
Acids (3)
L. plantarum Alcohols (20)
L. casei Aldehydes (8)
Chinese wolfberry L. paracasei Inoculum: 0.5% (v/v)
Extracted and analysed Esters (18)
34 juice L. acidophilus [50]
using HS-SPME-GC-MS Ketones (11)
(Lycium barbarum) L. helveticus at 37 ◦ C for 48 h Terpenes and
B. lactis
norisoprenoids (3)
Others (1)
Inoculum: 7%
Acids (7)
Sequential fermentation: Alcohols (11)
Aldehydes (5)
S. cerevisiae at 25 ◦ C for
35 L. plantarum 5–6 days, after Extracted and analysed Esters (13)
Apple juice S. cerevisiae [51]
using HS-SPME-GC-MS Ethers (1)
dealcoholization of Ketones (10)
fermented juice, then
Olefins (3)
inoculate with Others (1)
L. plantarum for 16 h at
◦
39 C in static condition

3.2. Fermented Flavours of Vegetables and Fruits

Twenty-three of the thirty-five papers described VOCs originating from the fermenta-
tion of fruit juices (with 2 of the 23 on goji/wolfberry juice, 2 on jujube juice/pulp, 3 on
cashew apple juice, and 3 on apple juice). Nine of the papers described VOCs originating
from the fermentation of vegetable substrates (with 2 of the 9 on okara (soybean pulp)),
with one investigating tomato and red pepper pomace. Three articles investigated the
VOCs generated from a mixture of vegetables and mixtures of vegetables and fruits, with
carrot juice being a component in all three studies and apple juice in two.
A wide range of micro-organisms were reported as being used, encompassing bacteria,
LAB, fungi, and yeast. Thirty-three papers described the use of LAB, with twenty-nine
studies only referring to LAB strains, one paper referring to LAB combined with other
bacteria, and three papers referring to LAB and yeasts. In the remaining two studies,
one study referred to the use of fungi, while the other referred to fungi in combination
with yeast.
 Ketones (10)
of fermented MS
Olefins (3)
juice, then inocu-
Others (1)
late with
L. plantarum for
Molecules 2023, 28, 3236
16 h at 39 °C in 9 of 36
static condition

Figure 2. The journals in which the papers were published and the number of papers in each journal.
Figure 2. The journals in which the papers were published and the number of papers in each journal.

The most commonly mentioned LAB was Lactiplantibacillus plantarum (L. plantarum)
(28 papers), followed by Lacticaseibacillus casei (L. casei) (12 papers), Lactobacillus acidophilus
(L. acidophilus) (9 papers), Lacticaseibacillus rhamnosus (L. rhamnosus) (9 papers), Lactobacillus
helveticus (L. helveticus) (4 papers), and Streptococcus thermophilus (S. thermophilus) (4 papers).
Only four of the papers investigated the use of yeast in combination with LAB and fungi.
The most common fermentation temperature and time combination was 37 ◦ C for 48 h
(10 papers), as shown in Table 2. As additional examples, 9 papers described studies carried
out at 37 ◦ C with fermentation times (excluding 48 h) ranging from 20 to 120 h. Six papers
described studies carried out at 30 ◦ C with fermentation times ranging from 24 to 120 h.
In 4 papers where multiple LAB strains were investigated, fermentation temperatures of
37 ◦ C and 30 ◦ C were used at different times.
Thirty-three of the papers isolated VOCs using headspace solid-phase microextraction
(HS-SPME), and one study using a purge and trap method, while other one using a
static headspace technique. HS-SPME is a simple, rapid, solvent-free method that can
extract a diverse mixture of VOCs using a fibre coated with an adsorbent resin. The fibre
most frequently stated as being used was 50/30 µm DVB/CAR/PDMS (Divinylbenzene-
Carboxen/polydimethylsiloxane) (17 papers), followed by 75 µm CAR/PDMS (6 papers),
75 µm DVB/CAR/PDMS (1 paper), and 50/30 µm DVB/CAR (2 papers). For SPME, the
most frequently reported adsorption time was 30 min (19 papers) for temperatures ranging
from 35 to 85 ◦ C, with 9 papers using 30 min at 40 ◦ C. The remaining papers reported the
use of a wide range of time and temperature combinations, ranging from 7 to 60 min at 40
to 80 ◦ C. Gas chromatography-mass spectrometry (GC-MS) was used in 34 papers to detect
the extracted VOCs, and the remaining paper reporting using gas chromatography-ion
mobility spectrometry (GC-IMS).
In the papers reviewed, the main VOCs detected due to fermentation were alcohols,
esters, aldehydes, ketones, acids, terpenes and norisoprenoids, sulphur compounds, phe-
nols, furans, alkanes, alkenes, and benzene derivatives (Table 2 and Table S1). The flavour
notes of the major VOCs from each of the above classes are listed in Table 3. Additional
Molecules 2023, 28, 3236 10 of 36

details about these different classes of compounds are presented in the following sections.
Note that the type of information available from the publications reviewed ranged from
the reporting of concentration values to peak area comparisons to simply reporting the
presence or absence of a compound; in all cases, the maximum amount of information
available in the reported studies has been presented.

Table 3. Flavour notes of important VOCs.

Flavour Groups Volatiles Flavour Notes Reference

Sharp, pungent, vinegar, cheesy,

Acetic acid [20,22,25]
fatty, sour
Butanoic acid (Butyric acid) Cheesy, sweet [20,40]
2-Methyl-1-butanoic acid Cheese, sweaty [31]
1 Acids Sweet, acid, rancid, stinky feet,
3-Methyl-1-butanoic acid (Isovaleric acid) cheese, pungent, sour, [22,26,31,40]
fruity
Hexanoic acid Sour, fatty, cheesy [22]
Fruity, sweet, warm, herbal
2-Hexenoic acid (E) [20,22]
must, fat
Faint, fruity-acid, irritating,
Octanoic acid [18,20,34,41]
brandy, cheese, sweet
Hexadecanoic acid Waxy, fatty [18]

Ethanol Apple, sweet, strong, alcoholic [20,22,24]

1-Butanol Sweetness [24]
2-Methyl-1-butanol Malt, wine, onion, fruity [17,20,21]
Whiskey, alcoholic, malt, burnt,
3-Methyl-1-butanol (Isoamyl alcohol) [22,26,31,40]
overripe cashew apple, banana
1-Pentanol Fermented yeasty [28]
Phenylmethanol (Benzyl alcohol) Floral [22]
2-Phenylethanol/phenyl ethyl alcohol Floral, soft, rose, jasmine [22,31,34,40]
Benzenepropanol Balsamic [31]
2 Alcohols Light-apple, sweetness, resin,
1-Hexanol flower, green, herbal [18,20–22,24,28,31]
ethereal, fruity
2-Ethyl hexanol Rose, sweety, floral, green [20,21,24]
(Z)-3-Hexen-1-ol Grassy-green, leafy [22,28,34]
Heptanol Fruity, green [20–22]
4-Methyl-4-heptanol Piney, citrus [31]
2,6-Dimethyl-4-heptanol Fruity, sweet [31]
1-Octanol Pungent, waxy, green, orange [22,41]
(E)-2-Octenol Dust, cement, mushroom [40]
1-Octen-3-ol Earthy, mushroom, vegetable-like [22,28,34,40]
Dodecanol Earthy, soapy, waxy, fatty [22]
Molecules 2023, 28, 3236 11 of 36

Table 3. Cont.

Flavour Groups Volatiles Flavour Notes Reference

Acetaldehyde Pungent, ether, fruity and juicy [20,21]

3-Methylbutanal (Isovaleraldehyde) Ethereal, aldehydic [22]
Almonds, cherry, sweetness,
Benzaldehyde [18,20,23–25,31]
burnt sugar
3 Aldehydes Hexanal Fatty, grassy, green [25,28,40]
(E)-2-Hexenal Green leaf, aldehydic, fatty [19,28]
Octanal Fat, lemon, green, soap [20]
(E)-2-Octenal Oxide nutty, mushroom [40]
Nonanal Citrus, green, fruity, fat [20,21,34]
(2-Nonenal Hay, cucumber [40]
Decanal Soap, orange peel, tallow [20]

Butyl acetate Pear, solvent-like [21,35]

Ethyl acetate Pineapple, fruity, ethereal [21,22,26,41]
Methyl 3-methyl butanoate
Strong, apple, pineapple [22]
(Methyl isovalerate)
3-Methylbutyl acetate (Isoamyl
Banana, pear, fruity, sweet [20,22,34,38,40]
acetate/Isopentyl acetate)
2-Phenylethyl acetate Floral, rose, sweet, honey [22]
Apple, strawberry, fruity,
Ethyl butanoate (Ethyl butyrate) [20,21,34,38]
pineapple
Ethyl isobutyrate Sweet [21]
4 Esters
Ethyl 3-methyl butanoate
Fruit, sweet, apple [22,31]
(Ethyl isovalerate)
Ethyl 2-methyl-2-butenoate Fruity, cashew-like [31]
Propyl butanoate (Propyl butyrate) Pineapple [21]
3-Methylbutyl 3-methyl-butanoate
Fruity, sweet, green [22,31]
(Isoamyl isovalerate)
(E)-2-Hexenyl acetate Green, fruity [22]
Methyl salicylate Wintergreen, mint [22]
Ethyl hexanoate Apple peel, fruit, green apple [20,31,34]
Ethyl 2-hydroxyhexanoate Fruity [31]
Ethyl octanoate Fruit, fat [31]

Acetone Pungent, ethereal, apple, pear [20,22,23]

2-Butanone Pungent [23]
2,3-Butanedione (Diacetyl) Creamy, butter [17,20,22,24,40]
5 Ketones Milk/dairy, butter, cream, sweet,
3-Hydroxy-2-butanone (Acetoin) [20,22,23,25,31]
vanilla
Methylheptenone Lemon grass [19]
6-Methyl-5-hepten-2-one Green [25]
3-Octanone Butter, herb, resin [20]
2,3-Octanedione Dill, cooked, broccoli [22]

Furfural Bread, almond, sweet [20]

6 Furans 2-Ethyl furan Musty, earthy [28]
Floral, grassy, fruity, green,
2-Pentyl furan [25,28,38]
Earthy, beany

Methanethiol Pickle, sulphur [28,40]

7 Sulphurs Sulfurous, vegetable, cabbage,
Dimethyl disulfide onion [28]

Dimethyl trisulfide Sulfurous, cooked onion, savory [28]

 Earthy, beany

Methanethiol Pickle, sulphur [28,40]

Sulfurous, vegetable, cab-
7 Sulphurs Dimethyl disulfide [28]
bage, onion
Molecules 2023, 28, 3236 12 of 36
Sulfurous, cooked onion,
Dimethyl trisulfide [28]
savory

Table 3. Cont.
(β)-Citronellol Floral, rose, citrus [19,22]
Flavour Groups Myrcene
Volatiles Peppery,Flavourspicy Notes [22]
Reference
Flower, lavender, citrus
Linalool [21,22,35]
(β)-Citronellol leaf, fruity
Floral, rose, citrus [19,22]
Myrcene Citrus, lemon, confection-
Peppery, spicy [22]
Linalool
D-Limonene Flower, lavender,
ery pineapple, fruity,citrus
an- leaf, fruity [21,22,35]
[22,35]
Citrus,
ise lemon, confectionery
D-Limonene [22,35]
pineapple, fruity, anise
Terpenes and Oily, woody, terpenic,
8 (ɤ)-Terpinene
( )-Terpinene Oily, woody, terpenic, tropical [22]
[22]
Terpenes and nor-
norisoprenoids tropical
8 Geraniol
Rose, geranium, sweet, floral,
[21,22]
isoprenoids Rose, geranium, fruity sweet,
Geraniol [21,22]
floral, fruitylilac, minty, floral,
Pine, terpene,
(α)-Terpineol citrusy, orange [22,35]
Pine, terpene, lilac, minty,
(α)-Terpineol
(β)-Ionone Violet [22,35]
[25]
floral, citrusy, orange
Woody, sweet, fruity, earthy,
(β)-Ionone
(β)-Damascenone Violet
stewed apple, iced tea, rose, honey [25]
[20,22,35]
Woody,
Woody,sweet,
pine, fruity,
balsam, sweet, mint,
Myrtenol [18]
(β)-Damascenone earthy, stewed apple, medicaliced [20,22,35]
tea, rose, honey
3.3. Alcohols Woody, pine, balsam,
Myrtenol [18]
Alcohols, with their characteristic sweet, mint, comprised
aromas, medical the largest volatile group de-
tected in 33 of the 35 studies reviewed. Alcohols are produced from carbohydrate degrada-
tion or amino 3.3.
acidAlcohols
catabolism [52]. Across the 33 studies, the alcohols most commonly de-
tected after fermentation werewith
Alcohols, ethanol,
their3-methyl-1-butanol
characteristic aromas, (isoamyl alcohol/isopentyl
comprised alco- group
the largest volatile
hol), 2-methyl-1-butanol (amyl alcohol), 3-methyl-3-buten-1-ol (isoprenol),
tected in 33 of the 35 studies reviewed. Alcohols are produced from carbohydrate de 2, 3-butanediol,
2-ethylhexanol, 1-hexanol,
dation or amino2-hexen-1-ol, 3 hexen-1-ol,
acid catabolism 2,6-dimethyl-4-heptanol,
[52]. Across benzyl alco-
the 33 studies, the alcohols most comm
hol (phenyl methanol/benzene methanol), were
detected after fermentation 2-phenylethyl alcohol (2-phenyl ethanol/benzene
ethanol, 3-methyl-1-butanol (isoamyl alcohol/isope
ethanol), 4-ethylphenol, 2-(4-methylphenyl)-2-propanol,
alcohol), 2-methyl-1-butanol (amyl alcohol),1-octanol, 1-octen-3-ol, (isoprenol),
3-methyl-3-buten-1-ol (Z)-1,5- 2, 3
octadien-3-ol, tanediol,
octenol, 2-octenol, 1-nonanol,
2-ethylhexanol, (Z)-3-nonen-1-ol,
1-hexanol, 2-hexen-1-ol, 2-undecanol, 3,7,11-trimethyl-
3 hexen-1-ol, 2,6-dimethyl-4-hepta
1-dodecanol, andbenzyl2-tridecanol.
alcohol (phenyl methanol/benzene methanol), 2-phenylethyl alcohol (2-ph
Ethanol is synthesised
ethanol/benzene from sugars4-ethylphenol,
ethanol), naturally present in plants; LAB utilise sugars
2-(4-methylphenyl)-2-propanol, 1-octanol, 1
via the phosphoketolase (PK) pathway, and yeast utilise
ten-3-ol, (Z)-1,5-octadien-3-ol, octenol, 2-octenol, 1-nonanol, sugars through the(Z)-3-nonen-1-ol,
Embden– 2
Meyerhof–Parnas (EMP)
decanol, pathway [13]. In mango slurry,
3,7,11-trimethyl-1-dodecanol, and fermentation
2-tridecanol. involving the yeast
Saccharomyces cerevisiae
Ethanol cerevisiae)
(S. is synthesised generated 30–100
from sugars times more
naturally presentethanol than LAB
in plants; LAButilise
fer- sugar
mentation [41]. When Williopsis saturnus var. saturnus (W. saturnus)
the phosphoketolase (PK) pathway, and yeast utilise sugars through the Embd yeast was combined
with LAB for fermentation, there was also a marked increase in the ethanol concentration
(six-fold increase) compared to LAB alone in durian pulp [30]. However, ethanol can be
generated by heterofermentative LAB, which possess the alcohol dehydrogenase enzyme
that converts acetaldehyde into ethanol [26]. In kiwifruit juice (cultivars of Actinidia deliciosa
cv. Xuxiang and Actinidia chinensis cv. Hongyang), the ethanol concentration was 10,316.5,
17,249.2, and 8652.7 ng/mL in Xuxiang cultivar juice fermented by either L. acidophilus,
L. helveticus, or L. plantarum, respectively, compared to 6242.9 ng/mL in the unfermented
juice. However, the ethanol concentration was 13,042.5, 7004.2, and 9551.9 ng/mL in
Hongyang cultivar juice fermented by either L. acidophilus, L. helveticus, or L. plantarum,
respectively, compared to 19,642.6 ng/mL in the unfermented juice [49]. The different
ethanol concentrations produced from the two kiwifruit cultivars after LAB fermentation
could be a result of the different substrate compositions of the cultivars, which are sub-
jected to various metabolic pathways by LAB. After fermentation of orange pomace by
L. rhamnosus, 0.3 µg/mL of ethanol was detected in a distillate prepared using vacuum
distillation to extract VOCs. However, ethanol was not detected in a fermented orange
pomace distillate prepared using a simple distillation method. In the same study, the
ethanol concentration detected in distillates prepared from L. rhamnosus fermented melon
by-product using vacuum distillation was 6.5 µg/mL, compared to 1.3 µg/mL in the distil-
late from the unfermented melon by-products, while ethanol was not detected in distillates
Molecules 2023, 28, 3236 13 of 36

prepared from the same samples using simple distillation [44]. In papaya juice, the ethanol
concentration was significantly (p < 0.05) increased by 7 and 11 times after fermentation
by either L. plantarum or L. acidophilus, respectively, compared to the concentration in the
unfermented juice [23]. On the other hand, only small changes in ethanol were observed
in two studies: (1) Ricci et al. [29] found that LAB fermented cherry juice had an ethanol
concentration of 4.1–8.5 ng/mL, compared to 3.1–3.7 ng/mL in the unfermented juice, and
(2) in watermelon juice fermented by either L. rhamnosus, L. plantarum, L. casei, or Pediococcus
pentosaceus (P. pentosaceus), the ethanol concentration was 16.8, 15.2, 15.1, and 15 ng/mL,
respectively, compared to 14.6 ng/mL in the unfermented juice; however, after fermen-
tation by Levilactobacillus brevis (L. brevis), the ethanol concentration was 13.9 ng/mL [48].
Six studies reported that LAB fermentation reduced the ethanol concentration in fermented
fruit and vegetable juices, compared to the unfermented juices, with the synthesis of vari-
ous esters speculated to have caused this decrease: (1) In unfermented Chinese wolfberry
juice, the ethanol concentration was 5501.3 µg/mL, compared to 1364.7 µg/mL in the L.
acidophilus fermented juice, where it was not detected in the juice fermented by other LAB
strains [50]; (2) in two varieties of unfermented jujube (Muzao, and Hetain) juices, the
ethanol concentration was 6850, and 6130 ng/mL, respectively, compared to 5740, 5100,
and 1530 ng/mL in Muzao fermented with either L. helveticus, L. casei, or L. plantarum,
respectively, and 5380, 4400, 2660, and 2410 ng/mL in Hetain fermented with either L. casei,
L. acidophilus, L. plantarum, or L. helveticus, respectively [20]; (3) in okara, the initial ethanol
concentration was 44 µg/g which reduced to 20.4 and 13.8 µg/g after fermentation with
LAB monoculture (L. rhamnosus or Pediococcus acidilactic (P. acidilactic), respectively) and
to 19.6 µg/g after co-culture fermentation (L. acidophilus, L. rhamnosus, and P. acidilactic).
However, in okara fermented with an L. acidophilus monoculture, the ethanol concentration
increased from 44 to 57.4 µg/g [46]; (4) in unfermented apple juice, the ethanol concentra-
tion was 188.4 ng/mL, compared to 83.4–123.5 ng/mL after fermentation with various LAB
strains [19]; (5) in non-pH-adjusted (2.7) sea buckthorn juice, the ethanol concentration was
170.7 ng/mL, which reduced after fermentation for 36 and 72 h by L. plantarum to 165.4
and 152 ng/mL, respectively. However, if the pH of the juice was adjusted to pH 3.5, the
initial ethanol concentration of 166.3 ng/mL increased after L. plantarum fermentation for
36 and 72 h to 194.6 and 206.5 ng/mL, respectively [42]; and (6) in tomato juice, the ethanol
concentration after fermentation with either L. plantarum or L. casei was 2.7 and 1.2 times
lower, respectively, compared to its concentration in the unfermented juice [32].
1-Octanol is a fatty alcohol produced by micro-organisms utilizing glucose as a sub-
strate through a fatty acid synthesis pathway using various enzymes [53]. The 1-octanol
concentration was generally reported to increase after LAB fermentation in the 5 studies
reviewed: (1) In Chinese wolfberry juice fermented by either L. plantarum, L. casei, Lacticas-
eibacillus paracasei (L. paracasei), L. acidophilus, L. helveticus, or Bifidobacterium Lactis (B. lactis),
the 1-octanol concentration was 172.9, 119.1, 137.2, 209.3, 131.4, and 142.4 µg/mL, where it
was not detected in the unfermented juice [50]; (2) the 1-octanol concentration in a distil-
late prepared using vacuum distillation from orange pomace fermented by L. rhamnosus
was 1.5 µg/mL, compared to 0.1 µg/mL in the distillate from unfermented pomace; in
distillates prepared using a simple distillation method, the 1-octanol concentration in the
fermented orange pomace distillate was 2.1 µg/mL, compared to 1.8 µg/mL in the unfer-
mented pomace distillate. Interestingly in the same study, using the simple distillation
method, the 1-octanol concentration in the fermented melon by-product distillate was
21.5 µg/mL, compared to 7.1 µg/mL in the unfermented by-product distillate, whereas in
extracts prepared by vacuum distillation, the 1-octanol concentration in the distillate from
melon by-product fermented by L. rhamnosus was 1.1 µg/mL, compared to 0.2 µg/mL in
the unfermented by-product distillate [44]; (3) in kiwifruit juice (Xuxiang and Hongyang
cultivars), the 1-octanol concentration was 285.5 and 325.5 ng/mL in Xuxiang cultivar juice
fermented by either L. helveticus or L. plantarum, respectively, compared to 146.4 ng/mL in
the unfermented juice, where it was not detected in Xuxiang cultivar juice fermented by
L. acidophilus. Interestingly, with the Hongyang cultivar juice, 1-octanol was not detected in
Molecules 2023, 28, 3236 14 of 36

the unfermented juice or in any of the LAB fermented juices. [49]; (4) in cherry juice fer-
mented by either L. plantarum, L. rhamnosus, or L. paracasei, the 1-octanol concentration was
4.5–7.8, 8.4, and 5.2 ng/mL, respectively, compared to 3.4–3.6 ng/mL in the unfermented
juice [29]; and (5) in apple juice fermented by either L. acidophilus, L. rhamnosus, L. casei
or L. plantarum, the 1-octanol concentration was 4.2, 3.5, 3.8, and 4.0 ng/g, respectively,
compared to 1.0 ng/g in the unfermented juice [21]. However, in okara fermented by LAB
monoculture of either L. rhamnosus, P. acidilactic or co-culture (L. acidophilus, L. rhamnosus,
and P. acidilactic), the 1-octanol concentration was 11.7, 17.8, and 1.7 µg/g, respectively, com-
pared to 30.0 µg/g in the unfermented okara, whereas okara fermented with L. acidophilus
had an 1-octanol concentration of 34.7 µg/g [46].
1-Hexanol is produced from the enzymatic oxidation of the fatty acid linoleic acid [54].
In 11 experiments, the 1-hexanol concentration increased after fermentation: (1) In Chinese
wolfberry juice fermented by either L. paracasei, L. acidophilus, or B. lactis, the 1-hexanol
concentration was 566.3, 728.2, and 682.2 µg/mL, respectively, where it was not detected in
the unfermented juice and the juice fermented by other LAB strains [50]; (2) for kiwifruit
juice (Xuxiang, and Hongyang cultivars), the 1-hexanol concentration in the Xuxiang cul-
tivar juice fermented by either L. acidophilus, L. helveticus, or L. plantarum was 11,239.2,
13,280.9, and 11,713.8 ng/mL, respectively, compared to 7054.8 ng/mL in the unfermented
juice, where for the Hongyang cultivar juice fermented by either L. acidophilus, L. helveticus,
or L. plantarum, the 1-hexanol concentration was 12,313.5, 12,357.1, and 11,461.1 ng/mL,
respectively, compared to 2462.9 ng/mL in the unfermented juice [49]; (3) the 1-hexanol
concentration in a distillate prepared using simple distillation of orange pomace fermented
by L. rhamnosus was 1.1 µg/mL, compared to 0.1 µg/mL in the unfermented orange po-
mace distillate. In the same study, using simple distillation, the 1-hexanol concentration in
fermented melon by-product distillate was 21.2 µg/mL, compared to 10.2 µg/mL in the
unfermented melon by-product distillate, whereas, when using vacuum distillation, the
1-hexanol concentration in fermented melon by-product distillate was 4.3 µg/mL, com-
pared to 0.4 µg/mL in the unfermented melon by-product distillate [44]; (4) in watermelon
juice fermented by either L. rhamnosus, L. casei, L. plantarum, L. brevis, or P. pentosaceus,
the 1-hexanol concentration was 79.9, 121.0, 141.0, 170.0, and 171.0 ng/mL, respectively,
compared to 74.7 ng/mL in the unfermented juice [48]; (5) in cherry juice fermented by
either L. plantarum, L. rhamnosus, or L. paracasei, the 1-hexanol concentration was 1.9–3.5,
2.1, and 1.1 ng/mL, respectively, compared to 0.7–0.8 ng/mL in the unfermented juice [29];
(6) in okara fermented by either L. acidophilus, L. rhamnosus, or P. acidilactic, the 1-hexanol
concentration was 206.7, 217, and 217.5 µg/g, respectively, compared to 64.2 µg/g in the
unfermented okara. However, for okara fermented with LAB co-cultures (L. acidophilus,
L. rhamnosus, and P. acidilactic), the 1-hexanol concentration reduced to 16.0 µg/g [46];
(7) in jujube juice fermented by a mixture of L. plantarum, L. rhamnosus, and S. thermophilus,
the 1-hexanol concentration was 824 ng/g, and it was not detected in the unfermented
juice [47]; (8) in apple juice fermented by either L. plantarum, L. rhamnosus, L. acidophilus,
or L. casei, the 1-hexanol concentration was 21.1, 22.0, 47.4, and 52.4 ng/g, respectively,
compared to 1.2 ng/g in the unfermented juice [21]; (9) in grape juice fermented by LAB,
the 1-hexanol concentration was 3.3-fold higher compared to the concentration in the
unfermented juice [43]; (10) in papaya juice fermented by either L. acidophilus or L. plan-
tarum, the 1-hexanol concentration was 3 and 4 times higher, respectively, compared to its
concentration in the unfermented juice [23]; and (11) in Momordica charantia juice fermented
by L. plantarum, the 1-hexanol concentration was 2.5 times higher compared to the concen-
tration in the unfermented juice [18]. However, the 1-hexanol concentration has also been
reported to reduce after fermentation in 3 studies: (1) In goji juice fermented by different
combinations of bacterial strains (either L. plantarum, L. rhamnosus, Limosilactobacillus reuteri
(L. reuteri), Bacillus velezensis (B. velezensis), or Bacillus licheniformis (B. licheniformis)), the
1-hexanol concentration was 43.0–61.4 ng/g, compared to 80.2 ng/g in the unfermented
juice [25]; (2) in tomato juice fermented by L. plantarum, the 1-hexanol concentration was
reduced by half, compared to its concentration in the unfermented juice [32]; and (3) in
Molecules 2023, 28, 3236 15 of 36

mung bean fermented by L. plantarum, 1-hexanol was not detected, where it was detected
in the unfermented bean [45].
Benzyl alcohol is an aromatic alcohol produced by microbial fermentation either
from glucose [55] or the amino acid phenylalanine [56]. Benzyl alcohol was detected after
bacterial fermentation in 7 studies [19,25,29,34,35,45,47] with concentrations ranging from
2.3 ng/mL to 270 ng/mL.
3-Methyl-1-butanol and 2-phenylethyl alcohol were the most common amino acid-
derived alcohols detected after fermentation. 3-Methyl-1-butanol and 2-phenylethyl alcohol
are synthesised from the catabolism of the amino acids leucine [57] and phenylalanine [58],
respectively. 3-Methyl-1-butanol has a malt/alcoholic/whiskey odour that is considered
unpleasant when present in concentrations greater than 400 µg/mL [59]. In fermented fruit
and vegetable juices, 3-methyl-1-butanol was detected in 10 studies [21–24,29,31,38,44,45,49].
However, in fermented fruit and vegetable juices, the 3-methyl-1-butanol concentration
was reduced in 5 studies: (1) In Chinese wolfberry juice fermented by either L. casei,
L. paracasei, L. acidophilus, L. helveticus, or B. lactis, the 3-methyl-1-butanol concentration
was 825.9, 674.0, 833.6, 799.6, and 820.6 µg/mL, respectively, compared to 2065.3 µg/mL in
the unfermented juice, and it was not detected in the juice fermented by L. plantarum [50];
(2) in non-pH-adjusted (2.7) sea buckthorn juice, the 3-methyl-1-butanol concentration was
122.9 ng/mL, which was reduced after fermentation for 36 and 72 h by L. plantarum to
98.9 and 91.4 ng/mL, respectively. However, if the pH of the juice was adjusted to pH 3.5,
the initial 3-methyl-1-butanol concentration of 121.4 ng/mL increased after L. plantarum
fermentation for 36 and 72 h to 217.9 and 233 ng/mL, respectively [42]; (3) in apple
juice fermented by different LAB strains, the 3-methyl-1-butanol concentration ranged
from 4.5–16.9 ng/mL, compared to 73.2 ng/mL in the unfermented juice, among the
strains studied, L. acidophilus fermented juice had a 93% reduction in 3-methyl-1-butanol
concentration [19]; (4) in apple juice fermented by the yeast S. cerevisiae, the 3-methyl-1-
butanol concentration was 644 ng/mL; however, the concentration of 3-methyl-1-butanol
reduced to 42.1 ng/mL after L. plantarum sequential fermentation [51]; and (5) in cashew
apple juice fermented by L. acidophilus, the 3-methyl-1-butanol concentration reduced by
2 times, compared to other LAB strains (either L. plantarum or L. casei) studied [26].
In 10 studies, the concentration of 2-phenylethyl alcohol, which has a flowery smell,
was increased or it was detected after fermentation: (1) In Chinese wolfberry juice fermented
by L. plantarum, the 2-phenylethyl alcohol concentration was 246.4 µg/mL, where it was
not detected in the unfermented juice or the fermented juice by other LAB strains [50]; (2)
in non-pH-adjusted (2.65) bog bilberry juice fermented by two strains of L. plantarum, the 2-
phenylethyl alcohol concentration was 1731.2 and 1775.8 ng/mL, compared to 663.5 ng/mL
in the unfermented juice. However, if the pH of the juice was adjusted to pH 3.5, the initial
2-phenylethyl alcohol concentration of 617.5 ng/mL was decreased after fermentation by
two strains of L. plantarum to 459.7 and 463.4 ng/mL [34]; (3) in grape juice fermented by
LAB, the 2-phenylethyl alcohol concentration was 40.6 ng/mL, where it was not detected in
the unfermented juice [43]; (4) in horse gram sprouts fermented by two L. plantarum strains,
the 2-phenylethyl alcohol concentration was 1290 and 780 ng/g, compared to 40 ng/g in
raw seed [37]; (5) in goji juice fermented by different combinations of bacterial strains (ei-
ther L. plantarum, L. rhamnosus, L. reuteri, B. velezensis, or B. licheniformis), the 2-phenylethyl
alcohol concentration ranged from 362.6 to 494.0 ng/g, compared to 103.1 ng/g in the un-
fermented juice [25]; (6) in jujube pulp fermented by a mixture of L. plantarum, L. rhamnosus,
and S. thermophilus, the 2-phenylethyl alcohol concentration was 283 ng/g, where it was
not detected in the unfermented juice [47]; (7) in okara fermented with a monoculture of
Rhizopus oligosporus (R. oligosporus) fungi, the 2-phenylethyl alcohol concentration increased
by 20 times, compared to its concentration in the unfermented okara, whereas with a mixed
culture of R. oligosporus fungi and Yarrowia lipolytica (Y. lipolytica) yeast, the concentra-
tion increased by 8.5 times [38]; (8) in mango slurry fermented by yeast S. cerevisiae, the
2-phenylethyl alcohol concentration was 4 to 23 times higher, compared the concentration
after LAB fermentation, where it was not detected in the unfermented mango slurry [41];
Molecules 2023, 28, 3236 16 of 36

(9) in papaya juice fermented by L. plantarum, the 2-phenylethyl alcohol concentration was
doubled, compared to the concentration in L. acidophilus fermented juice, where it was not
detected in the unfermented juice [23]; and (10) in durian pulp fermented by L. casei mixed
with the yeast W. saturnus, 2-phenylethyl alcohol was detected, where it was not detected
in the unfermented pulp, or in the pulp fermented by a L. casei monoculture [30].
Furthermore, compared to unfermented juice, one study reported that almost half of
the alcohols detected decreased in L. casei fermented tomato juice, likewise in L. plantarum
fermented tomato juice, most of the alcohols detected decreased. However, due to the
generation of new alcohols, the relative peak area (RPA) for total alcohols increased to 59.9%
and 49.7% in juice fermented by either L. casei or L. plantarum, respectively, compared to a
49.3% RPA in the unfermented juice [32]. On the other hand, LAB fermentation increased
the overall combined alcohol concentration of fruit and vegetable juices in 4 studies: (1)
LAB fermentation of apple juice increased the overall combined alcohol concentration by
10 times compared to its concentration in the unfermented juice, demonstrating that most
of the alcohols were produced during fermentation [21]; (2) LAB fermentation of grape juice
increased the total combined alcohol concentration by 102.4% [43]; (3) LAB fermentation
of kiwifruit juice (Xuxiang and Hongyang cultivars) increased the total combined alcohol
concentration by 39, 107, and 56% in Xuxiang cultivar juice fermented by either L. acidophilus,
L. helveticus, or L. plantarum, respectively, and by 25, 30, and 26% in Hongyang cultivar juice
fermented by either L. acidophilus, L. helveticus or L. plantarum, respectively [49]; and (4) LAB
fermentation increased the total combined alcohol concentration of jujube juice (Varieties of
Muzao and Hetian) by 66.5% in L. acidophilus fermented Muzao juice and 33.7% in L. casei
fermented Hetian juice [20]. In another study, the total combined alcohol concentration of
mango slurry fermented with yeast strains was nearly 10 times higher compared to mango
slurry fermented with LAB strains [41].
Overall, alcohols such as 1-hexanol (11 papers), 3-methyl-1-butanol (10 papers), 2-
phenylethyl alcohol (10 papers), benzyl alcohol (7 papers), ethanol (7 papers), and 1-octanol
(5 papers) have been reported to have increased or were only detected after the fermentation
of fruit and vegetable juices, mainly by LAB. However, it is important to note that for some
substrates, the concentration of ethanol (6 papers), 3-methyl-1-butanol (5 papers), 1-hexanol
(3 papers), and 1-octanol (1 paper) has been reported to have decreased after fermentation.

3.4. Esters
Esters, which have sweet and fruity notes, are formed when carboxylic acids linked
with coenzyme-A (CoA) are esterified with alcohols [60]. Volatile esters were found in
fermented fruit and vegetable juices in 27 of the 35 papers reviewed in this report. The
sensory detection threshold for esters is lower than that of the corresponding alcohol
or acid [61]. The majority of esters reported were either ethyl esters or acetate esters.
Ethanol and acyl-CoA derivatives of fatty acids combine to form ethyl esters. Acetyl-CoA
and alcohols, such as ethanol or higher alcohols, produced from amino acid metabolism,
combine to form acetate esters [62]. The key ester compounds reported in the reviewed
studies were the acetate esters: ethyl acetate (ethyl ethanoate); 3-methylbutyl acetate
(isoamyl acetate/isopentyl acetate); 2-phenylethyl acetate and hexyl acetate, and ethyl
esters: ethyl butanoate (ethyl butyrate); ethyl-3-methyl butanoate (ethyl isovalerate/ethyl
isopentanoate); ethyl hexanoate (ethyl caproate); ethyl octanoate (ethyl caprylate); ethyl
dodecanoate (ethyl laurate); ethyl propanoate (ethyl propionate); ethyl-2-methyl-butanoate;
ethyl hexadecanoate (ethyl palmitate) and others: hexyl formate; methyl 3-methylbutanoate
(methyl isovalerate/methyl isopentanoate); 3-methylbutyl 3-methylbutanoate (isoamyl
isovalerate/isopentyl isopentanoate) and methyl 2-hydroxybenzoate (methyl salicylate).
Eleven of the papers stated that ethyl acetate, which is formed by alcohol acetyltrans-
ferases from the reaction between acetyl Co-A and ethanol [20], was primarily respon-
sible for the fruity flavour of fermented fruits and vegetable juices. A variety of LAB
strains [19–23,33,44,45,50] were used in all studies except two; one used LAB combined
with a yeast [30], and the other used fungi combined with a yeast [38]. In these studies, it
Molecules 2023, 28, 3236 17 of 36

was reported that for Chinese wolfberry juice fermented by either L. plantarum, L. paracasei,
L. acidophilus, or L. helveticus, the ethyl acetate concentration was 6931, 4827.1, 4925.4, and
7323.3 µg/mL, respectively, compared to 774.5 µg/mL in the unfermented juice, where it
was not detected in the juice fermented by L. casei or B. lactis [50]. In Muzao jujube juice
fermented by either L. plantarum or L. acidophilus, the ethyl acetate concentration was 111.7
and 64.2 µg/mL, respectively, where it was not detected in the unfermented juice or juice
fermented by other LAB [20]. Further, in durian pulp fermented by L. casei combined with
a yeast W. saturnus, ethyl acetate was detected, whereas it was not detected in sole L. casei
fermentation or unfermented pulp [30]. However, Liu et al. [32] found that in tomato
juice, prior to fermentation, ethyl acetate was detected, where it was not detected after
fermentation by LAB, and in mango slurry fermented by either yeast or LAB, the ethyl
acetate concentration was reduced by 1.2–1.8 times, compared to its concentration in the
unfermented slurry [41].
In 3 experiments, ethyl butanoate, which is formed by a reaction between ethanol and
butyryl-CoA during LAB fermentation [21], was the next most common ester compound:
(1) The ethyl butanoate concentration in a distillate prepared using simple distillation
of L. rhamnosus fermented melon by-product was 8760 ng/mL, compared to 700 ng/mL
in the unfermented melon by-product distillate, and the ethyl butanoate concentrations
of both the fermented and unfermented by-product distillates produced using vacuum
distillation were at least 10 times lower than in the simple distillation distillates [44]; (2) in
apple juice fermented by different LAB, the ethyl butanoate concentration ranged from
16.3 to 23.1 ng/g, compared to 2.1 ng/g in the unfermented juice [21]; and (3) in papaya
juice fermented by LAB, ethyl butanoate was detected, where it was not detected in the
unfermented juice [23].
2-Phenylethyl acetate, which is formed by a reaction between 2-phenylethyl alcohol
and acetyl CoA, was the third most commonly reported (3 studies) ester in juices after
fermentation: (1) In horse gram sprouts fermented by L. plantarum, the 2-phenylethyl
acetate concentration was 220 ng/g, where it was not detected in raw seeds [37]; (2) in
grape juice fermented by a mixed culture of L. plantarum and L. brevis, the 2-phenylethyl
acetate concentration was 3.5 ng/mL, compared to its concentration in the unfermented
juice (1.6 ng/mL) [43]; and (3) in durian pulp fermented by a combination of L. casei and
W. saturnus yeast, 2-phenylethyl acetate was detected, whereas it was not detected during
L. casei only fermentation or in the unfermented pulp [30].
Propyl acetate, which is formed by a reaction between propanol and acetyl-CoA, was
detected in cherry juice fermented by either L. plantarum, L. rhamnosus, or L. paracasei, where
the propyl acetate concentration was 18.5–83.4, 1186.7, and 201.6 ng/mL, respectively,
compared to about 0.01 ng/mL in the unfermented juice. In this study, the formation of
propyl acetate during fermentation appeared to correlate with acetic acid production. As
there was a low concentration of acetic acid after fermentation by L. rhamnosus or L. paracasei,
this was taken as evidence of the conversion of acetic acid to the corresponding ester. In
contrast, in the same study, fermentation by L. plantarum resulted in a high concentration of
acetic acid and a lower concentration of propyl acetate [29].
Overall the total combined ester concentration increased in 6 studies after fermentation
due to the availability of alcohol precursors [52]: (1) In Muzao jujube juice fermented by
either L. acidophilus or L. plantarum, the total combined ester concentration was 65.4 and
156.7 µg/mL, respectively, compared to 4.7 µg/mL in the unfermented juice [20]; (2) in
mixed juices (apple juice, orange juice, carrot juice, and Chinese jujube juice) fermented by
LAB mixed culture (L. plantarum, Bifidobacterium breve (B. breve), and S. thermophilus), the
total combined ester concentration was 415 ng/mL, compared to 239 ng/mL in the unfer-
mented juice [35]; (3) in apple juice fermented by different LAB, the total combined ester
concentration ranged from 81.8 to 92.9 ng/g, compared to 33.7 ng/g in the unfermented
juice [21]; (4) in grape juice fermented by LAB, the total combined ester concentration
increased by 83.76%, compared to its concentration in the unfermented juice [43]; (5) in
mango slurry fermented by LAB and yeast, the total combined ester concentration increased,
Molecules 2023, 28, 3236 18 of 36

compared to the unfermented slurry, with the yeast S. cerevisiae generating a significantly
(p < 0.05) higher number of esters present at a high concentration than LAB [41]; and (6) in
pomegranate juice fermented by LAB, the total combined ester concentration increased,
compared to the concentration in the unfermented juice [24].
On the other hand, 3 studies reported a reduction in the total combined ester concen-
tration after fermentation, possibly due to hydrolysis into their corresponding acids and
alcohols [47]: (1) In apple juice fermented by either L. plantarum, L. helveticus, L. casei, L.
paracasei, L. acidophilus or B. lactis, the total combined ester concentration was 1090, 1279.6,
787.5, 695.9, 702.3, and 643.1 ng/mL, respectively, compared to 1410.7 ng/mL in the unfer-
mented juice [19]; (2) in jujube juice fermented by a mixture of L. plantarum, L. rhamnosus,
and S. thermophilus, the total combined ester concentration was 1541 ng/g, compared to
5814 ng/g in the unfermented juice [47]; and (3) in tomato juice fermented by either L. casei
or L. plantarum, the total combined ester concentration was 1.6 times and 7 times lower,
respectively, compared to the concentration in the unfermented juice [32].
Overall, esters such as ethyl acetate (11 papers), ethyl butanoate (3 papers), 2-phenylethyl
acetate (3 papers), and propyl acetate (1 paper) have been reported to increase or were only
detected after fermentation of fruit and vegetable juices, mainly by LAB. However, the
concentration of ethyl acetate did decrease after fermentation in 2 studies.

3.5. Ketones
A number of ketones were identified in 26 studies investigating the fermentation of
vegetable and fruit juices. 3-Hydroxy-2-butanone (acetoin), 2,3-butanedione (diacetyl),
2-propanone (acetone), 1-hydroxy-2-propanone (hydroxy acetone), 2-butanone (methyl
ethyl ketone), 2-pentanone, 2-hydroxy-3-pentanone, 3-methyl-4-methylene-2-hexanone,
2-heptanone, 4-heptanone, 4-methyl-2-heptanone, 6-methyl-5-hepten-2-one (sulcatone),
2-octanone, 2-nonanone, 2-dodecanone, 2-undecanone, 2-tridecanone, 2-tetradecanone,
(E)-6,10-dimethylundeca-5,9-dien-2-one (geranyl acetone), 4-cyclopentene-1,3-dione, and
1-phenylethanone (acetophenone) were identified in the reviewed papers.
3-Hydroxy-2-butanone (acetoin), which imparts a creamy/buttery note, was the most
frequently detected ketone produced during the fermentation of vegetables and fruits.
Citrate in vegetable and fruit juices can be directly converted to acetoin (Figure 3) by some
LAB strains exhibiting citrate permease and citrate lyase activities. Citrate can be converted
by LAB to pyruvate via oxaloacetate, then to acetaldehyde-thiamine pyrophosphate (TPP)
through a decarboxylation process, and finally to acetaldehyde-TPP through an enzymatic
reaction involving α-acetolactate synthase, resulting in the synthesis of α-acetolactate.
α-Acetolactate synthase has a low affinity for pyruvate; therefore, an excess of pyruvate is
required to favour this reaction. In the presence of citrate and sugars, homofermentative
LAB will convert pyruvate directly to α-acetolactate when less NADH is generated than
pyruvate. Heterofermentative LAB will, however, accumulate pyruvate at low pH when
citrate is the sole carbon source. Further, due to the instability of α-acetolactate, it is
readily decarboxylated enzymatically or chemically to yield acetoin. Acetoin can also be
synthesised from diacetyl via the enzyme diacetyl acetoin reductase [63–66]. In addition,
when the pH of the medium is between 5 and 8, Lactococcus lactis can also produce acetoin
from the catabolism of aspartic amino acid [67] (Figure 4). The acetoin concentration
increased after LAB fermentation of fruit and vegetable juices in 8 studies: (1) In Chinese
wolfberry juice fermented by either L. plantarum, L. paracasei, L. helveticus, or B. lactis, the
acetoin concentration was 346.3, 267.4, 528.1, and 422.1 µg/mL, respectively, compared to
29.1 µg/mL in the unfermented juice, where it was not detected in the juice fermented by
L. casei or L. acidophilus [50]; (2) in Muzao jujube juice fermented either L. plantarum or L.
helveticus, the acetoin concentration was 29.9 and 30.8 µg/mL, respectively, compared to
17.5 µg/mL in the unfermented juice. However, the acetoin concentration was reduced
to 10.5 µg/mL in L. acidophilus fermented juice and acetoin was not detected in L. casei
fermented juice [20]; (3) in kiwifruit juice (Xuxiang, and Hongyang cultivars), the acetoin
concentration was 2621.6 and 1348.9 ng/mL in the Xuxiang cultivar juice fermented by
Molecules 2023, 28, 3236 19 of 36

either L. helveticus or L. plantarum, respectively, where it was not detected in the L. acidophilus
fermented juice or in the unfermented juice. The acetoin concentration was 8431.7 and
4390.6 ng/mL in Hongyang cultivar juice fermented by either L. helveticus or L. plantarum,
respectively, where it was not detected in the L. acidophilus fermented juice or in the
unfermented juice [49]; (4) in elderberry juice fermented by either L. plantarum, L. casei, or
L. rhamnosus, the acetoin concentration was 83.1–496.7, 90.7–314.5, and 41.4–456.2 ng/mL,
respectively, compared to 1.4–22.1 ng/mL in the unfermented juice [22]; (5) in cherry
juice fermented by either L. rhamnosus or L. paracasei, the acetoin concentration was 260.7
and 5.9 ng/mL, respectively, compared to 0.001 ng/mL in the unfermented juice, and in
cherry juice fermented by different L. plantarum strains, the acetoin concentration ranged
from 44 to 287.9 ng/mL, compared to 0.002 ng/mL in the unfermented juice [29]; (6) the
acetoin concentration in a distillate prepared using vacuum distillation from orange pomace
fermented by L. rhamnosus was 450 ng/mL, compared to 110 ng/mL in the unfermented
pomace distillate, where acetoin was not detected in distillates from either fermented
or unfermented pomace using the simple distillation method [44]; (7) in mung beans
fermented by two L. plantarum strains, the acetoin concentration was 2.8 and 7.5 times
higher, compared to the concentration in the unfermented mung beans [45]; and (8) in
papaya juice fermented by either L. plantarum or L. acidophilus, the acetoin concentration
was 2.2 and 3.7 times higher, respectively, compared to the unfermented juice [23]. In
5 studies, acetoin was only detected after fermentation of juices: (1) In okara fermented by
a LAB co-culture (L. acidophilus, L. rhamnosus, and P. acidilactici), the acetoin concentration
was 166.3 µg/g, where it was not detected in LAB monocultures [46]; (2) in horse gram
sprouts fermented by L. plantarum, the acetoin concentration was 440 ng/g [37]; (3) in goji
juice fermented by a bacterial mixture of L. rhamnosus, L. reuteri, and B. velezensis, the acetoin
concentration was 87.2 ng/g juice [25]; (4) in mango slurry fermented by S. thermophilus,
acetoin was detected, where it was not detected after fermentation by yeast S. cerevisiae or
other LAB [41]; and (5) in durian pulp fermented by L. casei in sequential co-culture with W.
saturnus yeast, acetoin was detected, where it was not detected in L. casei monoculture [30].
Note, that in apple juice fermented by either L. acidophilus, L. helveticus, or L. paracasei,
the acetoin concentration was reported to have decreased to 0.8 ng/mL, 1.2 ng/mL, and
4.3 ng/mL, respectively, compared to 5.4 ng/mL in the unfermented juice, where it was
not detected in L. plantarum, L. casei, or B. lactis fermented apple juice [19].
The second most commonly reported ketone was 2,3-butanedione (diacetyl), which
imparts creamy/buttery notes. It is produced by LAB from citrate present in juice (Figure 3).
As discussed for the acetoin production pathway, α-acetolactate can be directly converted
into diacetyl through nonenzymatic oxidative carboxylation in the presence of molecular
oxygen [63,65]. Diacetyl was reported to be increased by the presence of some LAB during
fermentation of juices in 6 studies: (1) In Chinese wolfberry juice fermented by either L.
plantarum, L. casei, L. paracasei, or L. acidophilus, the diacetyl concentration was 45.1, 51,
71.9, and 28.1 µg/mL, respectively, where it was not detected in the unfermented juice
nor the juice fermented by L. helveticus or B. lactis [50]; (2) in elderberry juice fermented by
either L. rhamnosus, L. plantarum, or L. casei strains, the diacetyl concentration ranged from
37–586.8, 16.2–400.7, and 221.9–276.6 ng/mL, respectively, compared to 3.3–12.2 ng/mL
in the unfermented juice [22]; (3) in kiwifruit juice (Xuxiang, and Hongyang cultivars),
the diacetyl concentration was 261.1 ng/mL in Xuxiang cultivar juice fermented by L.
helveticus, where it was not detected in Xuxiang cultivar juice fermented by other LAB or
in the unfermented juice. Interestingly, with the Hongyang cultivar juice, diacetyl was
not detected in the unfermented juice or in any of the LAB fermented juices [49]; (4) in
watermelon juice fermented by either L. plantarum, L. brevis, L. casei, or L. rhamnosus, the
diacetyl concentration was 1.46, 1.47, 62.5, and 85.7 ng/mL, respectively, where it was
not detected in the P. pentosaceus fermented juice and the unfermented juice [48]; (5) in
mango slurry fermented by L. casei, diacetyl was detected, whereas it was not detected
in other LAB or yeast fermentations [41]; and (6) in pomegranate juice fermented by L.
plantarum strains, the diacetyl concentration increased, compared to the concentration in
 unfermented juice, whereas in mango slurry fermented by the yeast S. cerevisiae, the total
combined ketone concentration was 1.4–2.7 times lower compared to the concentration in
the unfermented juice [41].
Overall, ketones, which are key contributors to dairy notes, such as acetoin (13 pa-
pers) and diacetyl (6 papers) have been reported to increase or were only detected after
the fermentation of fruit and vegetable juices. However, in two studies, the concentration
Molecules 2023, 28, 3236 20 of 36
of acetoin (1 paper), and diacetyl (1 paper) decreased after fermentation by LAB and fungi,
respectively.
In 8 studies, L. plantarum was the main LAB producing acetoin in fermented juices,
followed by L. helveticus (3 studies) and L. rhamnosus (3 studies). The two main LAB that
the unfermented juicehigh
produced [24].diacetyl
However, in tomato
concentrations and pepper
in fermented juicespomace fermented
were L. plantarum by either
(4 papers)
and L. casei
Trichoderma atroviride (T.(4atroviride)
papers). Overall, for the papers
or Aspergillus reviewed,
sojae L. plantarum
(A. sojae), produced
the diacetyl more of
concentration
the creamy flavours of acetoin and diacetyl compared to other LAB strains studied.
was reduced compared to the concentration present in the unfermented pomace [40].

Figure 3. Citrate metabolic

Figure 3. Citratepathway
metabolic by LAB.byCL:
pathway LAB.citrate lyase,
CL: citrate ODC:
lyase, ODC:oxaloacetate decarboxylase,
oxaloacetate decarboxylase,
Molecules 2022, 27, x FOR PEER REVIEW LDH: lactate dehydrogenase, PDC: pyruvate decarboxylase, TPP: thiamine 26 of 41
thiamine pyrophosphate, ALS: α-ALS:
LDH: lactate dehydrogenase, PDC: pyruvate decarboxylase, TPP: pyrophosphate,
acetolactate synthase, ALDC: α-acetolactate decarboxylase, DAR: diacetyl acetoin reductase, and
α-acetolactate synthase, ALDC: α-acetolactate
BDH: 2,3-butanediol decarboxylase, DAR: diacetyl acetoin reductase, and
dehydrogenase [63–66].
BDH: 2,3-butanediol dehydrogenase [63–66].

Figure Catabolism
Figure 4.4.Catabolism of aspartic
of aspartic aminoamino
acid byacid by Lactococcus
Lactococcus lactis.
lactis. ODC: ODC: oxaloacetate
oxaloacetate decarboxylase,decarboxylase,
LDH: lactatedehydrogenase,
LDH: lactate dehydrogenase, PDH:PDH: pyruvate
pyruvate dehydrogenase,
dehydrogenase, PDC:decarboxylase,
PDC: pyruvate pyruvate decarboxylase,
TPP: TPP:
thiamine pyrophosphate,
thiamine pyrophosphate, ALS:ALS:
α-acetolactate synthase,
α-acetolactate ALDC: α-acetolactate
synthase, decarboxylase,
ALDC: α-acetolactate DAR:
decarboxylase, DAR:
diacetyl acetoin reductase, and BDH: 2,3-butanediol dehydrogenase [67].
diacetyl acetoin reductase, and BDH: 2,3-butanediol dehydrogenase [67].
3.6. Aldehydes
Aldehydes were present at lower concentrations after the fermentation of fruit and
vegetables in 26 out of 35 studies. During the fermentation process, aldehydes are gener-
ated via alcohol oxidation or acid decarboxylation. The main aldehyde compounds de-
tected after fermentation included ethanal (acetaldehyde), phenyl methanal (benzalde-
hyde), 2-methyl butanal, 3-methyl butanal (isovaleraldehyde), pentanal (valeraldehyde),
Molecules 2023, 28, 3236 21 of 36

Overall, after LAB fermentation, the total combined ketone concentration increased
in 5 studies: (1) In okara fermented by LAB co-culture with L. acidophilus, L. rhamnosus,
and P. acidilactici, the total combined ketone concentration was 2355.6 µg/g, compared to
116.1 µg/g in the unfermented okara; however, in okara fermented by monocultures of
either L. acidophilus, L. rhamnosus, or P. acidilactici, the total combined ketone concentration
was 98.8, 64.3, and 57.8 µg/g, respectively. During a monoculture fermentation, unstable
aldehydes and ketones may be reduced to primary and secondary alcohols, whereas in a
co-culture fermentation, synergic interactions between strains may instead result in the pro-
duction of higher levels of ketones, which could be linked to the oxidation of alcohols [46];
(2) in cherry juice fermented by either L. rhamnosus or L. paracasei, the total combined ketone
concentration was 285.1 and 11.3 ng/mL, respectively, compared to 7.2 ng/mL in the
unfermented juice. In the cherry juice fermented by different L. plantarum strains, the total
combined ketone concentration ranged from 48.3 to 292.4 ng/mL, compared to 6.9 ng/mL
in the unfermented juice [29]; (3) in apple juice fermented by either L. plantarum, L. helveticus,
L. casei, L. paracasei, L. acidophilus, or B. lactis, the total combined ketone concentration was
17.0, 27.6, 29.6, 56.6, 22.5, and 26.4 ng/mL, respectively, compared to 16.6 ng/mL in the
unfermented juice [19]; (4) in another study, using apple juice fermented by different LAB
strains, the total combined ketone concentration ranged from 10.1 to 11.7 ng/g, compared
to 2.6 ng/g in the unfermented juice [21]; and (5) LAB fermentation of kiwifruit juice
(Xuxiang, and Hongyang cultivars) increased the total combined ketone concentration
by 2.6, 5.2, and 2.6 times in Xuxiang cultivar juice fermented by either L. acidophilus, L.
helveticus, or L. plantarum, respectively, and by 6.3, 75, and 37 times in Hongyang cultivar
juice fermented by either L. acidophilus, L. helveticus, or L. plantarum, respectively [49]. More-
over, in mango slurry fermented by LAB, the total combined ketone concentration was
1.2–1.8 times higher compared to the concentration in the unfermented juice, whereas in
mango slurry fermented by the yeast S. cerevisiae, the total combined ketone concentration
was 1.4–2.7 times lower compared to the concentration in the unfermented juice [41].
Overall, ketones, which are key contributors to dairy notes, such as acetoin (13 papers)
and diacetyl (6 papers) have been reported to increase or were only detected after the
fermentation of fruit and vegetable juices. However, in two studies, the concentration of
acetoin (1 paper), and diacetyl (1 paper) decreased after fermentation by LAB and fungi,
respectively.
In 8 studies, L. plantarum was the main LAB producing acetoin in fermented juices,
followed by L. helveticus (3 studies) and L. rhamnosus (3 studies). The two main LAB that
produced high diacetyl concentrations in fermented juices were L. plantarum (4 papers) and
L. casei (4 papers). Overall, for the papers reviewed, L. plantarum produced more of the
creamy flavours of acetoin and diacetyl compared to other LAB strains studied.

3.6. Aldehydes
Aldehydes were present at lower concentrations after the fermentation of fruit and
vegetables in 26 out of 35 studies. During the fermentation process, aldehydes are generated
via alcohol oxidation or acid decarboxylation. The main aldehyde compounds detected
after fermentation included ethanal (acetaldehyde), phenyl methanal (benzaldehyde), 2-
methyl butanal, 3-methyl butanal (isovaleraldehyde), pentanal (valeraldehyde), hexanal
(caproaldehyde), (E)-2-hexenal, heptanal (enanthaldehyde), octanal (caprylaldehyde), (E)-
2-octenal, nonanal (pelargonaldehyde), (E)-2-nonenal, decanal (capraldehyde), dodecanal
(lauraldehyde), 2,4-dimethyl-benzaldehyde, octadecanal (stearaldehyde), 2-undecenal,
tridecanal, 3,5-dimethyl-benzaldehyde, and benzeneacetaldehyde/phenylacetaldehyde.
Acetaldehyde provides fermented juices their distinct flavour, and it is produced
by LAB from the amino acid threonine [68] or from sugars via the PK (phosphoketolase)
pathway, and by yeast from sugars via the EMP (Embden–Meyerhof–Parnas) pathway [13].
At lower concentrations, acetaldehyde improves the flavour of fermented juice; however,
at higher concentrations (200 µg/g or 200 µg/mL or above) [20,21], it may negatively
influence the flavour of fermented juices. Acetaldehyde was detected in 6 studies after
Molecules 2023, 28, 3236 22 of 36

LAB fermentation: (1) In Muzao jujube juice fermented by L. acidophilus, the acetaldehyde
concentration was 19.9 µg/mL, compared to 1.5 µg/mL in the unfermented juice, with
other LAB strains generating slightly higher or lower acetaldehyde concentrations com-
pared to the unfermented juice [20]; (2) in kiwifruit juice (Xuxiang and Hongyang cultivars),
Xuxiang cultivar juice fermented by either L. plantarum, L. acidophilus, or L. helveticus the
acetaldehyde concentration was 1013.1, 136.1, and 124.3 ng/mL, respectively, compared to
109.5 ng/mL in the unfermented juice, whereas in the Hongyang cultivar juice fermented
by either L. plantarum or L. acidophilus, the acetaldehyde concentration was 1075.6 and
159.7 ng/mL, respectively, compared to 95.7 and 25.5 ng/mL in the unfermented juice and
fermented juice by L. helveticus, respectively [49]; (3) in apple juice fermented by either
L. casei, L. rhamnosus, L. plantarum, or L. acidophilus, the acetaldehyde concentration was
40.4, 15.0, 27.5, and 21.9 ng/g, respectively, whereas it was not detected in the unfermented
juice [21]; (4) in apple juice fermented by either L. plantarum, L. helveticus, L. casei, L. paraca-
sei, L. acidophilus, or B. lactis, the acetaldehyde concentration was 5.4, 2.1, 4.5, 2.4, 1.9, and
3.0 ng/mL, respectively, whereas it was not detected in the unfermented juice [19]; (5) in
non-pH-adjusted (2.7) sea buckthorn juice, the acetaldehyde concentration was 10.6 ng/mL,
which increased after fermentation for 36 and 72 h by L. plantarum to 13.9 and 15.8 ng/mL,
respectively. However, if the pH of the juice was adjusted to pH 3.5, the initial acetaldehyde
concentration of 10.8 ng/mL decreased after L. plantarum fermentation for 36 and 72 h to 1.1
and 1.2 ng/mL, respectively [42]; and (6) in watermelon juice fermented by either L. plan-
tarum or P. pentosaceus, the acetaldehyde concentration was 4.6 and 3.2 ng/mL, respectively,
compared to 2.3, 2.0, 0.5, and 0.5 ng/mL in the unfermented juice, and L. brevis, L. casei,
or L. rhamnosus fermented juices, respectively [48]. The concentration of acetaldehyde
in 6 studies reported here was still well below the concentration that has been reported
to adversely affect flavour, indicating that acetaldehyde may have a positive impact on
the overall flavour profile of fermented juices if it is above the minimum concentration
required for perception. Note, in Chinese wolfberry juice fermented by either L. plantarum,
L. casei, L. paracasei, L. helveticus, or B. lactis, the acetaldehyde concentration was reduced
to 52.8, 124.2, 123.3, 23.4, and 13.3 µg/mL, respectively, compared to its concentration
in the unfermented juice (155.9 µg/mL), where in the juice fermented by L. acidophilus,
the acetaldehyde concentration was 188.2 µg/mL [50]. Further, the initial acetaldehyde
concentration in a distillate prepared using simple distillation from unfermented melon by-
product was 1320 ng/mL, which was reduced to 470 ng/mL in the L. rhamnosus fermented
melon by-product distillate. Moreover, when using the vacuum distillation method, in the
unfermented melon by-product distillate, the acetaldehyde concentration was 160 ng/mL,
where it was only 20 ng/mL in the distillate from L. rhamnosus fermented melon by-product.
Some LAB can convert acetaldehyde to ethanol and acetic acid, which could explain the
decrease in acetaldehyde concentration in some fermentations [44].
Another important aldehyde from a flavour perspective as it imparts a pleasant aroma
to fermented juices is benzaldehyde, which is generated by LAB from the amino acid
phenylalanine. The conversion of phenylalanine to benzaldehyde by LAB is initiated by
the aminotransferase enzyme. The resulting phenyl pyruvic acid is chemically converted
to benzaldehyde in the presence of oxygen and manganese [58,69]. The benzaldehyde
concentration increased after fermentation of vegetable and fruit juices in 7 studies: (1) In
Chinese wolfberry juice fermented by either L. plantarum, L. paracasei, or L. acidophilus, the
benzaldehyde concentration was 117.2, 68.1, and 40.7 µg/mL, respectively, where it was not
detected in the unfermented juice nor in juice fermented by other LAB strains [50]; (2) in
kiwifruit juice (Xuxiang and Hongyang cultivars), Xuxiang cultivar juice fermented by L.
acidophilus, the benzaldehyde concentration was 490.3 ng/mL, compared to 369.7 ng/mL
in the unfermented juice. In contrast, it was not detected in the unfermented Hongyang
cultivar juice nor in the Xuxiang cultivar juice fermented by L. helveticus or L. plantarum and
all LAB fermented Hongyang cultivar juices [49]; (3) in bog bilberry juice fermented by two
L. plantarum strains, the benzaldehyde concentration was 55.5 and 62.3 ng/mL, compared to
41.8 ng/mL in the unfermented juice [34]; (4) in non-pH-adjusted (2.7) sea buckthorn juice,
Molecules 2023, 28, 3236 23 of 36

the benzaldehyde concentration was 2.7 ng/mL, which increased after fermentation for 36
and 72 h by L. plantarum to 5.4 and 7.9 ng/mL, respectively. However, if the pH of the juice
was adjusted to pH 3.5, the initial benzaldehyde concentration of 2.3 ng/mL decreased after
L. plantarum fermentation for 36 and 72 h to 1.1 and 1.7 ng/mL, respectively [42]; (5) in goji
juice fermented by different combinations of bacterial strains (either L. plantarum, L. rhamno-
sus, L. reuteri, B. velezensis, or B. licheniformis), the benzaldehyde concentration ranged from
55.5 to 101.4 ng/g, compared to 46.3 ng/g in the unfermented juice [25]; (6) in durian pulp
fermented by L. casei monoculture, the benzaldehyde concentration was 2.9 times higher,
compared to its concentration in the sequential co-culture with yeast W. saturnus, and it
was not detected in the unfermented pulp. This difference is because LAB can convert
phenylalanine amino acid to benzaldehyde; however, yeast preferentially convert pheny-
lalanine amino acid to phenylethyl alcohol via the Ehrlich pathway, resulting in a higher
quantity of benzaldehyde in LAB fermentations [30]; and (7) in papaya juice fermented by
L. plantarum, the benzaldehyde concentration was 2 times higher, compared to the concen-
tration after the L. acidophilus fermentation or in the unfermented juice [23]. Though the
benzaldehyde concentration increased after LAB fermentation, it also reduced in 4 studies:
(1) in Hetain jujube juice fermented by LAB, the benzaldehyde concentration ranged from
22.1 to 29.7 µg/mL, compared to 33 µg/mL in the unfermented juice [20]; (2) in jujube pulp
fermented by a mixture of L. plantarum, L. rhamnosus, and S. thermophilus, the benzaldehyde
concentration was 3516 ng/g, compared to 4672 ng/g in the unfermented pulp [47]; (3) in
cherry juice fermented by various L. plantarum strains, the benzaldehyde concentration
ranged from 15.3 to 33.4 ng/mL, compared to 100.5 ng/mL in the unfermented juice, and
in cherry juice fermented by L. rhamnosus, the benzaldehyde concentration was 76 ng/mL,
compared to 90.5 ng/mL in the unfermented juice [29]; and (4) in okara fermented by LAB
monocultures of L. rhamnosus or P. acidilactici and a co-culture (L. acidophilus, L. rhamnosus,
and P. acidilactici), the benzaldehyde concentration was 22.4, 45, and 46.2 µg/g, respectively,
compared to 114.6 µg/g in the unfermented okara, and in okara fermented by L. acidophilus,
the benzaldehyde concentration was only slightly reduced to 109.9 µg/g [46].
Hexanal and nonanal are two other important aldehydes that may influence the flavour
profile of fermented juices. Hexanal, which is produced from linoleic fatty acid, imparts a
fresh, green, grassy, waxy, fatty, and unpleasant aroma to fermented fruit and vegetable
juices [54]. After fermentation, the hexanal concentration was reduced in goji juice [25],
mung bean [45], watermelon juice [48], sea buckthorn juice [42], kiwifruit juice [49], and
okara pulp [38,46]. In addition, it was not detected after the fermentation of Chinese
wolfberry juice [50]. Jin et al. [41] reported that in mango slurry fermented by LAB, the
nonanal was detected, where it was not detected after yeast fermentation. Further, the
nonanal concentration was reduced in fermented yam juice [33], okara pulp [46], mung
bean [45], watermelon juice [48], kiwifruit juice [49], and sea buckthorn juice [42]. It was
also not detected during the fermentation of goji juice and durian pulp [25,30].
Overall, the total combined aldehyde concentration was reduced after the fermenta-
tion of vegetable and fruit juices in 8 studies. This was probably due to the conversion
of aldehydes either by reduction to alcohols or oxidation to acids [23]: (1) In Chinese
wolfberry juice fermented by either L. plantarum, L. casei, L. paracasei, L. acidophilus, L.
helveticus, or B. lactis, the total combined aldehyde concentration was 332.8, 680.1, 500.5,
983.3, 422.1, and 492.6 µg/mL, respectively, compared to 1841.2 µg/mL in the unfermented
juice [50]; (2) in unfermented jujube (Muzao and Hetain varieties) juice, the total combined
aldehyde concentration was 102.8 and 121.3 µg/mL, respectively, compared to 65.4–94.7
and 61.9–83.9 µg/mL in the fermented Muzao, and Hetain varietal juices by different LAB
strains, respectively. However, the total combined aldehyde concentration increased by
38.3% in Hetain varietal juice fermented by L. plantarum and 158% in Muzao varietal juice
fermented by L. acidophilus [20]; (3) in kiwifruit juice (Xuxiang and Hongyang cultivars),
Xuxiang cultivar juice fermented by either L. acidophilus, L. helveticus, or L. plantarum, the
total combined aldehyde concentration was 2411.9, 4248.7, and 4915.8 ng/mL, respec-
tively, compared to 15,316.3 ng/mL in the unfermented juice, where in the Hongyang
Molecules 2023, 28, 3236 24 of 36

cultivar juice fermented by either L. acidophilus, L. helveticus, or L. plantarum, the total

combined aldehyde concentration was 2466.1, 2528.9, and 3866.4 ng/mL, respectively,
compared to 23,477.9 ng/mL in the unfermented juice [49]; (4) in cherry juice fermented
by various L. plantarum strains, the total combined aldehyde concentrations ranged from
43.5 to 78.2 ng/mL, compared to 208.5 ng/mL in the unfermented juice [29]; (5) in okara
fermented by L. acidophilus, P. acidilactici, or L. rhamnosus, and a co-culture (L. acidophilus,
P. acidilactici, and L. rhamnosus), the total combined aldehyde concentration was 866.1,
282.9, 25.3, and 68.3 µg/g, respectively, compared to 1450.6 µg/g in the unfermented
okara [46]; (6) in watermelon juice fermented by either L. plantarum, L. rhamnosus, L. casei,
L. brevis, or P. pentosaceus, the total combined aldehyde concentration reduced by > 50%
compared to the unfermented juice [48]; (7) in grape juice fermented by LAB, the total
combined aldehyde concentration reduced by 45.4% [43]; and (8) in tomato juice fermented
by either L. plantarum or L. casei, the total combined aldehyde concentration was 15 and
164 times lower, respectively, compared to the concentration in the unfermented juice [32].
Note, that in apple juice fermented by either L. plantarum, L. helveticus, L. casei, L. paracasei,
L. acidophilus, or B. lactis, the total combined aldehyde concentration was reported to have
increased to 22.4, 13.3, 7.6, 4.4, 5.4, and 6.0 ng/mL, respectively, compared to 2.5 ng/mL in
the unfermented juice [19].
Higher overall combined aldehyde concentrations, particularly if these are dominated
by lipid-derived aldehydes, are likely to have a negative impact on the flavour of fermented
juices. Reducing the concentrations of lipid-derived aldehydes is likely to increase the fruity
aroma of fermented fruits and vegetables while lowering their green odour character. In all
of the studies reviewed, most lipid-derived aldehyde compounds were reduced after LAB
fermentation (hexanal (8 papers) and nonanal (8 papers)), whereas acetaldehyde (6 papers)
and benzaldehyde (7 papers) were increased or were only detected after fermentation and
were responsible for the pleasant aroma of fermented fruit and vegetable juices.

3.7. Acids
Twenty-five studies out of the thirty-five reported that acids contributed to the flavour
of fermented vegetables and fruits. An extensive range of short- to long-chain fatty acids,
including acetic acid, propanoic acid (propionic acid), 2-methyl-propanoic acid (isobutyric
acid), butanoic acid (butyric acid), 3-methyl-1-butanoic acid (isovaleric acid), 2-methyl-
1-butanoic acid, caproic acid (hexanoic acid), enanthic acid (heptanoic acid), caprylic
acid (octanoic acid), pelargonic acid (nonanoic acid), capric acid (decanoic acid), lauric
acid (dodecanoic acid), palmitic acid (hexadecanoic acid), and oleic acid, were detected
in the fermented fruits and vegetables, which were formed from sugars or amino acid
catabolism [52].
Acetic acid, which is a key flavour compound of fermented juices, is produced mainly
by heterofermentative LAB (L. brevis, Limosilactobacillus fermentum (L. fermentum), L. reuteri,
L. plantarum, L. rhamnosus, and L. casei), which first utilise sugars via the PK pathway
and produce acetyl phosphate, which is subsequently converted into acetic acid by the
acetokinase enzyme [13]. However, homofermentative LAB (L. acidophilus, S. thermophilus,
and L. helveticus), with high glycolytic flux rates, ferment sugars into solely lactic acid, while
under slow growth conditions and low glycolytic flux rates, the homofermentative LAB
change to mixed acid fermentation (formic acid, lactic acid, ethanol, and acetic acid) [70].
Acetic acid can also be produced from citrate, which is found in fruit and vegetable
juices [65]. The acetic acid concentration increased after fermentation of fruit and vegetable
juices in 15 studies: (1) In Chinese wolfberry juice fermented by either L. casei, L. paracasei,
L. helveticus, or B. lactis, the acetic acid concentration was 19,773.7, 16,093.9, 12,698.4, and
14,011.1 µg/mL, respectively, and it was not detected in the unfermented juice or juice
fermented by L. plantarum or L. acidophilus [50]; (2) In two varieties of jujube (Muzao and
Hetain) juices fermented by different LAB strains, the acetic acid concentration ranged from
161 to 234.3 µg/mL, and 211.2 to 278.1 µg/mL, respectively, compared to 90.8 µg/mL in
Muzao and 104.2 µg/mL in Hetain unfermented varietal juices. Interestingly, L. helveticus
Molecules 2023, 28, 3236 25 of 36

increased the acetic acid concentration by 158.2% in Muzao varietal juice and L. casei
increased it by 166.9% in Hetian varietal juice [20]; (3) in elderberry juice fermented by
either L. plantarum, L. casei, or L. rhamnosus strains, the acetic acid concentration ranged from
205.9–1012.3, 62.3–122.1, and 47.2–132.1 ng/mL, respectively, compared to 0.3–12.2 ng/mL
in the unfermented juice [22]; (4) in cherry juice fermented by various L. plantarum strains,
the acetic acid concentration ranged from 54.8 to 184.8 ng/mL, compared to 0.01 ng/mL
in the unfermented juice [29]; (5) in non-pH-adjusted (2.7) sea buckthorn juice, the acetic
acid concentration was 1.0 ng/mL which increased after fermentation for 36 and 72 h by
L. plantarum to 2.1 and 3.5 ng/mL, respectively, where the pH of the juice was adjusted
to pH 3.5, the initial acetic acid concentration of 0.8 ng/mL increased after fermentation
with L. plantarum for 36 and 72 h to 50.5 and 85.9 ng/mL, respectively [42], 6. In grape juice
fermented by LAB, the acetic acid concentration was 25.5 ng/mL, and it was not detected
in the unfermented juice [43]; (7) in jujube juice fermented by a mixture of L. plantarum, L.
rhamnosus, and S. thermophilus, the acetic acid concentration was 12.2 µg/g, compared to
2.8 µg/g in the unfermented juice [47]; (8) in horse gram sprouts fermented by L. plantarum
strains, the acetic acid concentration ranged from 4.8 to 5 µg/g, and it was not detected
in the raw seed [37]; (9) in goji juice fermented by different combinations of bacterial
strains (either L. plantarum, L. rhamnosus, L. reuteri, B. velezensis, or B. licheniformis), the
acetic acid concentration ranged from 25.4 to 88.9 ng/g, where it was not detected in the
unfermented juice [25]; (10) in mung bean fermented by L. plantarum strains, the acetic acid
concentration ranged from 0.17 to 0.29 ng/g, where it was not detected in the unfermented
mung bean [45]; (11) in papaya juice fermented by either L. acidophilus or L. plantarum,
the acetic acid concentration was 5.7 and 2.4 times higher, respectively, compared to the
concentration in the unfermented juice [23]; (12) in mango slurry fermented by either
L. plantarum or S. thermophilus, the acetic acid concentration was 2.4 and 2.7 times higher,
respectively, compared to the concentration in the unfermented mango slurry, and in
addition, it was not detected in L. casei or yeast fermentations [41]; (13) in tomato juice
fermented by L. plantarum, the acetic acid concentration was 8.6 times higher compared to
the concentration in the unfermented juice, where it was not detected in L. casei fermented
juice [32]; (14) in yam juice fermented by LAB, the acetic acid concentration was 1.5 times
higher compared to the concentration in the unfermented yam juice [33], and 15. in durian
pulp fermented by L. casei, the acetic acid concentration was 1.4 times higher compared to
the concentration in a co-culture of L. casei and yeast W. saturnus, where it was not detected
in the unfermented pulp [30].
3-Methyl-1-butanoic acid is an important acid flavour compound in dairy foods, which
is produced from the amino acid leucine by an aminotransferase enzyme [57]. However,
when present at high concentrations, it may negatively impact on the flavour of fermented
juices [26]. The 3-methyl-1-butanoic acid concentration increased in 3 studies after LAB
fermentation: (1) In non-pH-adjusted (2.7) sea buckthorn juice, the 3-methyl-1-butanoic
acid concentration was 19.2 ng/mL, which increased after fermentation for 36 and 72 h
by L. plantarum to 25 and 39 ng/mL, respectively, where the pH of the juice was adjusted
to pH 3.5, the initial 3-methyl-1-butanoic acid concentration of 13.1 ng/mL increased
after L. plantarum fermentation for 36 and 72 h to 129.3 and 185 ng/mL, respectively [42];
(2) in tomato juice fermented by L. plantarum, the 3-methyl-1-butanoic acid concentration
was 3 times higher compared to the concentration in the unfermented juice, where, it
was not detected in L. casei fermented juice [32]; and (3) in papaya juice fermented by
L. plantarum, the 3-methyl-1-butanoic acid concentration was 4.1 times higher compared
to the concentration in L. acidophilus fermented juice, where it was not detected in the
unfermented juice [23].
Butanoic acid, which is produced from the fermentation of sugars through the fatty
acid biosynthesis pathway [71] and can confer a dairy/cheesy aroma to fermented juices,
was detected in 3 studies after fermentation: (1) In jujube juice fermented by a mixture
of L. plantarum, L. rhamnosus, and S. thermophilus, the butanoic acid concentration was
1487 ng/g, compared to 345 ng/g in the unfermented juice [47]; (2) in apple juice fer-
Molecules 2023, 28, 3236 26 of 36

mented by different LAB, the butanoic acid concentration ranged from 2.3 to 4.7 ng/g,
where it was not detected in the unfermented juice [21]; and (3) in okara fermented by a
combination of R. oligosporus and Y. lipolytica, the butanoic acid concentration was 34 times
higher compared to the concentration in the juice fermented by R. oligosporus monocul-
ture [38]. However, in mango slurry fermented by the yeast S. cerevisiae, the butanoic
acid concentration was 4.4–5.6 times lower compared to the concentration in the unfer-
mented mango slurry, where the butanoic acid concentration was slightly higher after LAB
fermentation compared to the unfermented mango slurry [41].
Hexanoic acid, which can be produced from the fermentation of sugars through
the fatty acid biosynthesis pathway or from the cleavage of linoleic acid via hexanal,
may impart a fatty/cheesy/sour flavour to fermented juices [22,54,72,73]. Hexanoic acid
was detected after fermentation in 4 studies: (1) In jujube juice fermented by a mixture
of L. plantarum, L. rhamnosus, and S. thermophilus, the hexanoic acid concentration was
15.1 µg/g, compared to 7.1 µg/g in the unfermented juice [47]; (2) in non-pH-adjusted (2.65)
bog bilberry juice fermented by two strains of L. plantarum, the hexanoic acid concentration
was 266 and 272.1 ng/mL, compared to 147 ng/mL in the unfermented juice. However,
if the pH of the juice was adjusted to pH 3.5, the initial hexanoic acid concentration of
132.6 ng/mL was slightly changed after fermented by two strains of L. plantarum to 136.9
and 123.2 ng/mL [34]; (3) in grape juice fermented by LAB, the hexanoic acid concentration
was 14.2 ng/mL, where it was not detected in the unfermented juice [43]; and (4) in mango
slurry fermented by different LAB, the hexanoic acid concentration was 1.4–1.7 times
higher than the concentration in the unfermented slurry. However, in yeast fermentation,
the hexanoic acid concentration was 1.2–1.6 times lower compared to the concentration
in the unfermented juice [41]. According to Li et al. [20], in Muzao varietal jujube juice,
the initial hexanoic acid concentration of 69 µg/mL was reduced after fermentation by
different LAB to between 9.2 and 54.5 µg/mL, with the highest reduction being reported
for L. acidophilus fermented juice. In the same study, in Hetain varietal jujube juice, the
initial hexanoic acid concentration was 19.0 µg/mL, which increased after fermentation
by different LAB to 31.2–75.9 µg/mL, with the highest concentration being obtained in
L. acidophilus fermented juice.
Octanoic acid, which is synthesised from sugars (glucose) through the fatty acid
biosynthesis pathway [74], was detected in 6 studies after the fermentation of juices: (1)
In non-pH-adjusted (2.65) bog bilberry juice fermented by two strains of L. plantarum, the
octanoic acid concentration was 3921.6 and 4013.2 ng/mL, compared to 1538.2 ng/mL
in the unfermented juice. However, if the pH of the juice was adjusted to pH 3.5, the
initial octanoic acid concentration of 1156.5 ng/mL was reduced after fermentation by
two strains of L. plantarum to 655.3 and 722.9 ng/mL [34]. (2) in goji juice fermented by a
bacterial mixture, the octanoic acid concentration ranged from 251.8 to 321.5 ng/g, and it
was not detected in the unfermented juice [25]; (3) in grape juice fermented by LAB, the
octanoic acid concentration was 5.7 ng/mL, and it was not detected in the unfermented
juice [43]; (4) in yam juice fermented by LAB, the octanoic acid concentration was 2 times
higher compared to the concentration in the unfermented juice [33]; (5) in durian pulp
fermented by L. casei, the octanoic acid concentration was 2.3 times higher compared
to the concentration in the unfermented pulp, and when the pulp was fermented by
L. casei combined with yeast W. saturnus, the octanoic acid concentration was 2 times lower
compared to the concentration in the unfermented pulp [30]; and (6) in mango slurry,
the octanoic acid was detected after fermentation by the yeast S. cerevisiae, but it was not
detected after LAB fermentation [41].
Chen et al. [33] reported that L. plantarum alone or in combination with S. thermophilus
increased acetic, nonanoic, and decanoic acids in fermented yam juice, reducing the astrin-
gent odour of the fermented juice.
Overall, volatile acids after LAB fermentation made contributions to the flavour of
fruit and vegetable juices, where acetic acid (15 papers) was the most commonly detected
 2,3-Octanedione Dill, cooked, br

Furfural Bread, almond,

2-Ethyl furan Musty, eart
Molecules 2023, 28, 3236 6 Furans 27 of 36 grassy, f
Floral,
2-Pentyl furan green,
Earthy, bea
volatile acid present in fermented juices, followed by octanoic acid (6 papers), hexanoic
acid (4 papers), 3-methyl-1-butanoic acid (3 papers), and butanoic acid (3 papers).
Methanethiol Pickle, sulph
3.8. Terpenes and Norisoprenoids Sulfurous, vegeta
7 Sulphurs Dimethyl disulfide
bage, onio
Terpenes, which are comprised of isoprene (C5) units, impart floral, rose, and fruity
Sulfurous, cooked
flavours. In 13 studies reported here, terpenes were classified as being
Dimethyl terpenes or noriso-
trisulfide
prenoids; interestingly, in 14 studies, some terpene compounds (β-linalool, α-terpineol, savory
β-citronellol, geraniol, β-damascenone, D-limonene, and trans-β-ionone) were classified
as either alcohol/hydrocarbon/ketone, or as alkenes in 2 studies. (β)-Citronellol
Terpenes are classified Floral, rose, c
based on the number of isoprene units they contain: hemiterpenoids Myrcene
(C5), monoterpenoids Peppery, sp
(C10), sesquiterpenoids (C15), diterpenoids (C20), sesterterpenoids (C25), triterpenoids Flower, lavender
Linalool
(C30), tetraterpenoids (C40) (β-carotene), and polyterpenoids (C > 40) [12,75]. β-carotene leaf, fruity
can be further oxidized into β-ionone, β-damascenone, and β-ionol. Bacteria biosynthesize Citrus, lemon, con
terpenoids via the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway [76], while
D-Limonene ery yeast
pineapple, fru
produce them via the mevalonate pathway [75]. (β)-Myrcene, D-limonene (1-Methyl-4- ise
(prop-1-en-2-yl) cyclohex-1-ene), ocimene, (β)-linalool (3,7-Dimethyl-1,6-octadien-3-ol),
Oily, woody, ter
camphene, p-cymene, α-terpinolene, (α)-terpinene, 1,8-cineole, ((ɤ)-Terpinene
)-terpinene, (α)-terpineol (2-
Terpenes and nor- tropical
(4-Methylcyclohex-3-en-1-yl)propan-2-ol),
8 Citronellyl formate, (β)-damascenone ((E)-1-(2,6,6-
isoprenoids Rose, geranium,
Trimethyl-1-cyclohexa-1,3-dienyl) but-2-en-1-one), (β)-citronellol (3,7-Dimethyloct-6-en-1-ol),
Geraniol
geraniol ((2E)-3,7-Dimethylocta-2,6-dien-1-ol), trans-(β)-ionone (4-(2,6,6-Trimethylcyclohex- floral, fruit
1-en-1-yl) but-3-en-2-one), m-cymene, prenol, (α)-ionene, D-germacrene, valencene, Pine, terpene, lila
cedrol,
(α)-Terpineol
phytol, myrtenol, eugenol (2-Methoxy-4-(prop-2-en-1-yl) phenol), and β- phellandrene werecitrusy, o
floral,
the major terpene compounds detected in the reviewed papers. (β)-Ionone Violet
Linalool, an important flavour terpene, was detected after fermentation of juices Woody,in sweet,
7 studies: (1) The linalool concentration of a distillate prepared using simple distillation
(β)-Damascenone earthy, stewed ap
from unfermented orange pomace was 1.6 µg/mL and after fermentation by L. rhamnosus tea, rose, hon
of the orange pomace, the concentration in the resulting distillate was 2.3 µg/mL. Similarly, Woody, pine, b
Myrtenol
in distillates prepared using vacuum distillation, the linalool concentration in the sweet,
unfer- mint, m
mented orange pomace distillate was lower than that in the distillate from L. rhamnosus
fermented orange pomace, at 0.1 µg/mL and 1.1 µg/mL, respectively. 3.3. AlcoholsIn the same study,
in the unfermented melon by-product distillate prepared using simple distillation, the
linalool concentration was 0.5 µg/mL and in the L. rhamnosus fermented Alcohols,melonwith their characterist
by-product
distillate, the concentration was 4.4 µg/mL. However, linalool was not detectedstudies
tected in 33 of the 35 in the reviewed
dation or amino
distillates of fermented and unfermented melon by-products prepared using vacuum dis- acid catabolism [52].
detected after fermentation
tillation [44]; (2) in elderberry juice fermented by either L. plantarum or L. rhamnosus, the were etha
linalool concentration was 328.5 and 339.7 ng/mL, respectively, alcohol), 2-methyl-1-butanol
compared to 79.3 and (amyl a
tanediol, 2-ethylhexanol,
189.3 ng/mL in the unfermented juices, respectively [22]; (3) in cherry juice fermented 1-hexanol, 2
benzyl alcohol (phenyl
by either L. rhamnosus, L. paracasei, or L. plantarum, the linalool concentration was 39.6, methanol/be
21.5, and 19.6–29 ng/mL, respectively, compared to 13.7–15.7ethanol/benzene ethanol), 4-ethylphen
ng/mL in the unfermented
juice [29]; (4) in goji juice fermented by a bacterial mixture (L. plantarum, L. rhamnosus, B. octen
ten-3-ol, (Z)-1,5-octadien-3-ol,
velezensis, and B. licheniformis), the linalool concentration wasdecanol,
22.3 ng/g,3,7,11-trimethyl-1-dodecanol
where it was not
detected in the unfermented juice [25]; (5) in apple juice fermented Ethanol is synthesised
by different LAB, the from suga
linalool concentration ranged from 3.0 to 4.5 ng/g, compared to 2.0 ng/g in the unfer-pathway
the phosphoketolase (PK)
mented juice [21]; (6) in yam juice fermented by L. plantarum, the linalool concentration was
21 times higher compared to the concentration in the juice fermented by a combination of
L. plantarum and S. thermophilus, and it was not detected in the unfermented yam juice [33];
and (7) in mango slurry fermented by LAB, linalool was detected, where it was not de-
tected in yeast fermented or unfermented slurry [41]. Though the linalool concentration
increased in these studies after fermentation, it was also reduced after fermentation in
2 studies: (1) In papaya juice fermented by either L. acidophilus or L. plantarum, the linalool
concentration was 1.6 and 1.2 times lower, respectively, compared to the concentration in
the unfermented juice [23]; and (2) in Momordica charantia juice fermented by L. plantarum,
the linalool concentration was reduced compared to the concentration in the unfermented
juice [18].
Molecules 2023, 28, 3236 28 of 36

D-limonene, a significant flavour terpene compound, increased in 8 experiments after

the fermentation of juices: (1) In Chinese wolfberry juice fermented by either L. casei, L.
paracasei, L. acidophilus, L. helveticus, or B. lactis, the D-limonene concentration was 281.4,
218, 372.6, 271.8, and 273.5 µg/mL, respectively, compared to 143.5 µg/mL in the unfer-
mented juice, where in fermented juice by L. plantarum, the D-limonene concentration
was 67.4 µg/mL [50]; (2) the D-limonene concentration in distillates prepared using vac-
uum distillation from L. rhamnosus fermented melon by-product or unfermented melon
by-product was 140 ng/mL and 10 ng/mL, respectively. In the same study, distillates
prepared using vacuum distillation had concentrations of 100 ng/mL and 20 ng/mL in the
L. rhamnosus fermented orange pomace distillate and the unfermented pomace distillate,
respectively [44]; (3) in cherry juice fermented by either L. rhamnosus or L. paracasei, the
D-limonene concentration was 7.9 and 3.0 ng/mL, respectively, compared to 2.8 ng/mL
in the unfermented juice, where in cherry juice fermented by four L. plantarum strains,
the D-limonene concentration was 1.8, 1.9, 3.1, and 4.0 ng/mL, compared to 2.3 ng/mL
in the unfermented juice [29]; (4) in jujube juice fermented by a mixture of L. plantarum,
L. rhamnosus, and S. thermophilus, the D-limonene concentration was 964 ng/g, where it
was not detected in the unfermented juice [47]; (5) in apple juice fermented by different
LAB, the D-limonene concentration ranged from 2.5 to 3.1 ng/g, compared to 1.1 ng/g in
the unfermented juice [21]; (6) in mango slurry fermented by different LAB, the D-limonene
concentration was 7.8–10 times higher compared to the concentration in the unfermented
slurry, where in yeast fermentation, the D-limonene concentration was 2.2 to 4.5 times
lower compared to the concentration in the unfermented juice [41]; (7) in pomegranate juice
fermented by L. plantarum strains, the D-limonene concentration was increased compared
to the concentration in the unfermented juice [24]; and (8) in tomato juice fermented by L.
casei, D-limonene was detected, where it was not detected after fermentation by L. plantarum
or in the unfermented juice [32]. However, the D-limonene concentration was reduced in
2 studies after fermentation: (1) The D-limonene concentration in a distillate prepared using
simple distillation from L. rhamnosus fermented orange pomace was 2.3 µg/mL, compared
to 4.8 µg/mL in the unfermented pomace distillate [44]; and (2) in blended apple, orange,
carrot, and jujube juices fermented by a mixed starter culture (L. plantarum, B. breve, and S.
thermophilus), the D-limonene concentration was 170 ng/mL, compared to 2093 ng/mL in
the unfermented blended juice [35].
In 5 studies, α-terpineol was detected after LAB fermentation: (1) The α-terpineol con-
centration in a distillate prepared using vacuum distillation from L. rhamnosus fermented
orange pomace was 1.5 µg/mL, compared to 0.3 µg/mL in the unfermented pomace
distillate, where it was not detected in distillates prepared using simple distillation. In
the same study, using simple distillation, in a distillate of melon by-product fermented
by L. rhamnosus, the α-terpineol concentration was 2.0 µg/mL, compared to 1.0 µg/mL
in the unfermented melon by-product distillate. Whereas using vacuum distillation, the
α-terpineol concentration was slightly increased to 0.25 µg/mL in the distillate from the
fermented melon by-product, compared to 0.18 µg/mL in the unfermented melon by-
product distillate [44]; (2) in kiwifruit juice (Hongyang and Xuxiang cultivars), Hongyang
cultivar juice fermented by either L. acidophilus, L. helveticus, or L. plantarum, the α-terpineol
concentration was 84.2, 154.9, and 151.1 ng/mL, respectively, where it was not detected in
the unfermented juice, and in addition, the α-terpineol was not detected in the unfermented
and the fermented Xuxiang cultivar juices [49]; (3) in goji juice fermented by a bacterial mix-
ture (L. rhamnosus, L. reuteri, and B. velezensis), the α-terpineol concentration was 38.7 ng/g,
compared to 33.9 ng/g in the unfermented juice [25]; (4) in non-pH-adjusted (2.65) bog
bilberry juice fermented by two strains of L. plantarum, the α-terpineol concentration was
5.2 and 5.9 ng/mL, compared to 4.3 ng/mL in the unfermented juice, where the pH of the
juice was adjusted to pH 3.5, the initial α-terpineol concentration was 3 ng/mL, which was
reduced after fermentation by two strains of L. plantarum to 2.8 and 2.5 ng/mL [34]; and (5)
in mango slurry fermented by different LAB, α-terpineol was detected, where it was not
detected after yeast fermentation or in the unfermented mango slurry [41].
Molecules 2023, 28, 3236 29 of 36

β-damascenone, another important flavour terpene compound, was detected after

fermentation in 4 studies: (1) In Hetain varietal jujube juice fermented by either L. acidophilus,
L. casei, L. helveticus, or L. plantarum, the β-damascenone concentration was 16.7, 22.4, 19.0,
and 21.4 µg/mL, respectively, where it was not detected in the unfermented juice [20]; (2)
the β-damascenone concentration in a distillate prepared using simple distillation from
a melon by-product fermented by L. rhamnosus was 8 µg/mL, compared to 1.2 µg/mL
in the unfermented melon by-product distillate, and when using vacuum distillation,
the β-damascenone concentration was 0.03 µg/mL in unfermented melon by-product
distillate and 0.12 µg/mL in the distillate of L. rhamnosus fermented melon by-product [44];
(3) in goji juice fermented by different combinations of mixed bacterial cultures, the β-
damascenone concentration ranged from 39.6 to 52 ng/g, where it was not detected in
the unfermented juice [25]; and (4) in apple juice fermented by different LAB, the β-
damascenone concentration ranged from 2.4 to 3.9 ng/mL, where it was not detected in
the unfermented juice [19]. However, Chen et al. [21] reported in apple juice that the initial
β-damascenone concentration was 12.8 ng/g and that after fermentation with LAB, it was
not detected.
Eugenol is a volatile phenol that imparts clove, honey, and spicy notes to fermented
fruit and vegetable juices. In the three papers, where it was mentioned, it was classified as
a terpene: (1) In horse gram sprouts fermented by two L. plantarum strains, the eugenol
concentration was 5.2 and 5.3 µg/g, and it was not detected in raw seeds [37]; (2) in cherry
juice fermented by either L. paracasei or L. rhamnosus, the eugenol concentration was 3.9,
and 26.8 ng/mL, respectively, compared to 2.4 ng/mL in the unfermented juice, and cherry
juice fermented by four L. plantarum strains, the eugenol concentration was 11.1, 13.0, 14.7,
and 22.4 ng/mL, compared to 3.2 ng/mL in the unfermented juice [29]; and (3) in apple
juice fermented by either L. plantarum, L. rhamnosus, L. casei, or L. acidophilus, the eugenol
concentration was 9.1, 6.2, 17.1, and 33.3 ng/g, respectively, compared to 5.4 ng/g in the
unfermented juice [21].
In addition, geraniol was detected in 3 studies after fermentation: (1) In watermelon
juice fermented by either L. plantarum, L. brevis, P. pentosaceus, L. casei, or L. rhamnosus,
the geraniol concentration was 5.6, 1.7, 1.7, 1.5, and 1.3 ng/mL, respectively, compared to
1.0 ng/mL in the unfermented juice [48]; (2) in goji juice fermented by bacterial mixture,
the geraniol concentration ranged from 119.3–246 ng/g, where it was not detected in the
unfermented juice [25]; and (3) in apple juice fermented by either L. plantarum, L. rhamnosus,
L. casei, or L. acidophilus, the geraniol concentration was 15.9, 13.2, 14.4, and 14.3 ng/g,
respectively, compared to 5.5 ng/g in the unfermented juice [21].
Overall, a wide range of terpenes were found to increase after LAB fermentation of
fruit and vegetable juices, with D-limonene being the most commonly reported (8 papers)
terpene, followed by linalool (7 papers), α-terpineol (5 papers), β-damascenone (4 papers),
eugenol (3 papers), and geraniol (3 papers). However, in some instances after fermenta-
tion, the concentration of linalool (2 papers), D-limonene (2 papers), and β-damascenone
(1 paper) decreased.

3.9. Phenols
Nine out of thirty-five studies detected phenolic compounds, which were categorised
into phenols or alcohols or others. Phenol, 4-vinylphenol-2-methoxy, 2,6-di-tert-butyl-4-
methylphenol, 4-ethyl-2-methoxyphenol, 4-ethyl-phenol, 2-methoxy-phenol, 2,5-dimethyl-
phenol, 2,4,5-trimethyl-phenol, and 2,6-dimethoxyphenol were the phenolic compounds
most frequently reported. The concentration of 2-methoxy-4-vinylphenol increased after
fermentation in 2 studies: (1) In goji juice fermented by a mixed bacterial culture, the
2-methoxy-4-vinylphenol concentration ranged from 532.5 to 678.6 ng/g, compared to
98.3 ng/g in the unfermented juice [25]; and (2) in apple juice fermented by either L.
plantarum, L. rhamnosus, L. casei, or L. acidophilus, the 2-methoxy-4-vinylphenol concentration
was 2.4, 1.7, 2.1, and 2.1 ng/g, respectively, compared to 0.2 ng/g in the unfermented
juice [21]. In addition, the concentration of phenol increased after fermentation in 3 studies:
Molecules 2023, 28, 3236 30 of 36

(1) In apple juice fermented by either L. plantarum or L. acidophilus, the phenol concentration
was 3.2 and 2.2 ng/g, respectively, compared to 0.1 ng/g in the unfermented juice [21]; (2)
in papaya juice fermented by L. acidophilus, the phenol concentration was 1.6 times higher
compared to the concentration in L. plantarum fermented juice, where it was not detected
in the unfermented juice [23]; and (3) in okara fermented by the fungi R. oligosporus in
combination with the yeast Y. lipolytica, the phenol concentration was 2.4 times higher
compared to the concentration when fermented by R. oligosporus in monoculture [38].

3.10. Furans
Furfural, 2-ethyl-furan, 2-propyl-furan, 2-pentyl-furan, 2,5-dimethyl-furan, 2,4-dimethyl-
furan, trans-2-(2-pentyl) furan, 2,3-Dihydrobenzofuran, and acetyl-furan were the major
furans identified in 13 studies under furans/aldehydes/others/heterocyclic compounds.
Furfural was detected in 8 studies, where it was mainly classified as an aldehyde. Pro-
duction of furfural is linked to Maillard reactions, and higher levels of furfural may have
a negative impact on the flavour of fermented substrates. However, LAB fermentation
reduces the amount of furfural, most likely as a result of the consumption of precursors
such as amino acids and reducing sugars [24,77,78]. The furfural concentration was re-
duced after LAB fermentation in 4 studies: (1) In jujube juice fermented by a mixture of
L. plantarum, L. rhamnosus, and S. thermophilus, the furfural concentration was 1886 ng/g,
compared to 3873 ng/g in the unfermented juice [47]; (2) in cherry juice fermented by four
different L. plantarum strains, the furfural concentration was 26.1, 36.8, 43.5, and 51.6 ng/mL,
compared to 101.7 ng/mL in the unfermented juice [29]; (3) in apple juice fermented by
either L. rhamnosus, L. casei, or L. acidophilus, the furfural concentration was 95.1, 106.9, and
91.8 ng/g, respectively, compared to 114.2 ng/g in the unfermented juice [21]; and (4) in
pomegranate juice fermented by different L. plantarum strains, the furfural concentration
was reduced compared to the concentration in the unfermented juice [24]. However, in
jujube (Muzao and Hetain varieties) juice, the initial furfural concentration in the Muzao
varietal juice was 2.6 µg/mL, which was increased after fermentation by different LAB
to a range of 2.9–5.2 µg/mL. In the same study, in Hetain varietal juice, the initial fur-
fural concentration of 4.2 µg/mL was increased after fermentation by LAB to a range
of 5.1–5.8 µg/mL [20]. Further, in goji juice fermented by a mixed bacterial culture, the
2-pentyl furan concentration ranged from 233.8 to 422.5 ng/g, compared to 36.3 ng/g in the
unfermented juice [25], and in watermelon juice fermented by either L. plantarum, L. brevis,
P. pentosaceus, L. casei, or L. rhamnosus, the 2-pentyl furan concentration was 96.7, 115, 117,
112, and 99 ng/mL, respectively, compared to 79.4 ng/mL in the unfermented juice [48].
However, the concentration of 2-pentyl furan was reduced in another 2 studies [18,38].

3.11. Sulphur Compounds

Seven studies found that sulphur compounds were present in fermented juices, and
the major compounds detected were methanethiol, dimethyl sulfide, dimethyl disulfide,
dimethyl trisulfide, methyl isopentyl disulfide, methyl ethyl disulphide, diethyl disulphide,
3,5-dimethyl, 1,2,4-Trithiolane (isomer 1), 3,5-dimethyl, 1,2,4-Trithiolane (isomer 2), methyl
propyl sulfide, and methyl (methylthio) methyl disulfide. Sulphur compounds such as
methanethiol, dimethyl sulfide, and dimethyl disulfide, depending upon concentration,
have been found to cause off-flavour in fruit and vegetable substrates [79]. Of the seven
studies, two reported that the concentration of sulphur compounds was reduced after
fermentation: (1) In durian pulp fermented by either L. casei monoculture or L casei co-
culture with the yeast W. saturnus, the diethyl disulfide concentration was 6.9 and 8.9 times
lower, respectively, compared to the concentration in the unfermented pulp [30]; and (2) in
pomegranate juice fermented by different L. plantarum strains, the methanethiol, dimethyl
sulfide, and dimethyl disulfide concentrations were reduced compared to the concentration
in the unfermented juice [24]. However, in non-pH-adjusted (2.7) sea buckthorn juice, the
dimethyl disulfide concentration was 2.1 ng/mL, which was increased after fermentation
for 36 and 72 h by L. plantarum to 12.2 and 10.9 ng/mL, respectively [42], and in watermelon
Molecules 2023, 28, 3236 31 of 36

juice fermented by either L. plantarum, L. brevis, P. pentosaceus, L. casei, or L. rhamnosus,

the dimethyl disulfide concentration was 6.4, 3.4, 4.1, 5.8, and 9.6 ng/mL, respectively,
compared to 1.1 ng/mL in the unfermented juice [48].

3.12. Alkanes, Alkenes, and Benzene Derivatives

Alkanes, alkenes, and benzene derivatives were reported less frequently, with only
eight of thirty-five studies mentioning them under alkanes/alkenes/benzene deriva-
tive/hydrocarbons/others.

3.12.1. Alkanes
In 5 studies, the alkanes, butane, pentane, heptane, octane, nonane, decane, un-
decane, dodecane, 6,10,14-tetramethyl-hexadecane, 2,6,10,14-tetramethyl-pentadecane,
4-methyloctane, and 2,4-dimethylheptane were detected after fermentation [24,41,42,46,48].

3.12.2. Alkenes
After fermentation, the alkenes 2-methyl-1-propene, 2-methyl-1,3-butadiene, and
3,4-dimethyl octene were reported in 3 studies [22,24,44].

3.12.3. Benzene Derivatives

Benzene, methyl benzene, 1,3,5-trimethyl benzene, naphthalene, p-methyl ethenyl
toluene, ethyl benzene, styrene, 1,3-bis(1,1-dimethylethyl) benzene, and 1-1-6-trimethyl-1,2-
dihydronaphthalene (TDN) were detected in two studies after fermentation [24,29].

4. Summary
Overall, a large array of VOCs was detected after the fermentation of fruit and veg-
etable juices. The major VOCs identified were ethanol, octanol, hexanol, benzyl alcohol,
3-methyl-1-butanol, and 2-phenylethyl alcohol (alcohols); ethyl acetate, 2-phenylethyl
acetate, and ethyl butanoate (esters); acetoin, and diacetyl (ketones); acetaldehyde, and
benzaldehyde (aldehydes); acetic acid, 3-methyl-1-butanoic acid, hexanoic acid, butanoic
acid, and octanoic acid (acids); linalool, D-limonene, β-damascenone, α-terpineol, eugenol,
and geraniol (terpenes and norisoprenoids).
The use of different fruit and vegetable substrates, micro-organisms, and fermentation
conditions are all likely to have had an impact on the production of fermentation VOCs.
With the exception of a few studies that used bacteria other than LAB, fungi, or yeast
either as a monoculture or in combination with LAB, most studies used LAB either as a
monoculture or in mixed cultures. The LAB most frequently used for producing desirable
VOCs were L. plantarum, L. casei, L. acidophilus, L. rhamnosus, and L. helveticus. In studies
that used two cultivars of a fruit, there were notable differences in the resulting fermented
VOCs present and their concentrations [20,49]. Additionally, there were variations in the
types and concentrations of VOCs between the pH-adjusted and non-adjusted juices of
sea buckthorn [42] and bog bilberry [34]. Moreover, there were observable differences
in the detected VOCs from distillates (simple or vacuum distillation) of LAB-fermented
orange pomace and melon by-product, assessed by SPME [44]. The results demonstrate
how VOCs detected after being produced during fermentation are greatly influenced by
the substrate (species and cultivar) being fermented, the LAB strain being used, and the
fermentation conditions.
Overall, in LAB-fermented fruit and vegetable juices, the concentrations of the main
dairy flavour VOCs, namely acetoin and diacetyl, ranged between 0.04 and 528.1 µg/mL,
and 0.01 and 71.9 µg/mL, respectively. It was apparent that LAB fermentation can yield
high concentrations of acetoin and diacetyl from plant-based substrates. However, a wide
variety of VOCs, including desirable and undesirable compounds, were detected in all of
the studies reviewed. Once desirable dairy flavour components have been produced, ex-
tracting and purifying them from the other components present will be the next challenge.
Current research is focusing on metabolic engineering techniques that involve overex-
Molecules 2023, 28, 3236 32 of 36

pressing rate-limiting enzymes that produce desirable VOCs or inactivating the enzymes
that produce undesirable VOCs in order to improve or create new metabolic pathways in
micro-organisms. For instance, pyruvate is a crucial intermediate in the synthesis of the
dairy flavours acetoin and diacetyl. Pyruvate in excess can be converted to α-acetolactate
by modifying the metabolic flux of pyruvate. If acetoin production is of particular in-
terest, α-acetolactate decarboxylase can be designed to be overexpressed, whereas when
diacetyl production is of interest, NADH-oxidase can be overexpressed and α-acetolactate
decarboxylase expression can be inactivated [66] (see acetoin/diacetyl production pathway
in the ketones section/Figure 3). However, due to the complexity of plant matrices, the
action of different metabolic processes, and the factors influencing the fermentation, such
as temperature, pH, and aeration, desirable VOCs might be metabolised, or their presence
masked by undesirable VOCs. To meet these challenges, research on the metabolic pathway
analysis of various micro-organism(s) on complex matrices of plants is required.

5. Conclusions
In conclusion, differences in substrates, micro-organisms, and fermentation conditions
influence the synthesis of microbial VOCs from vegetable and fruit substrates. In compari-
son to other bacteria, yeast, and fungi examined, LAB strains were most frequently used to
ferment fruit and vegetable substrates. Among LAB strains, Lactiplantibacillus plantarum was
the most frequently used species and it produced the highest concentration of VOCs. The
most frequently used fermentation temperature and time combination was 37 ◦ C for 48 h;
however, in the papers reviewed, most of the papers used temperatures of 30 and 37 ◦ C for
time combinations ranging from 20 to 120 h. Acids, alcohols, aldehydes, esters, ketones,
and terpenes/norisoprenoids were the most frequent VOCs reported after the fermentation
of vegetable and fruit substrates, whereas sulphur compounds, phenols, furans, alkanes,
alkenes, and benzene derivatives were reported less frequently. After LAB fermentation,
the concentration of alcohols, esters, ketones, acids, and terpenes/norisoprenoids generally
increased, whereas the concentration of aldehydes generally reduced. The fermentation of
vegetable and fruit substrates by different LAB strains generates a wide range of desired
VOCs, including the dairy flavours of acetoin and diacetyl. However, due to the complexity
of plant matrices, fermenting conditions, and different LAB and their metabolic charac-
teristics, producing dairy analogues with characteristic dairy flavours is still difficult. To
achieve the dairy flavours of interest for dairy analogues, in-depth research is still required
on the metabolic characteristics and pathways of LAB.

Supplementary Materials: The following supporting information can be downloaded at:

https://www.mdpi.com/article/10.3390/molecules28073236/s1, Table S1: The VOCs of fermented
fruit and vegetable substrates.
Author Contributions: Conceptualization, P.S. and P.B.; data curation, S.R.; writing—original draft,
S.R.; writing—review and editing, S.R., P.S. and P.B. All authors have read and agreed to the published
version of the manuscript.
Funding: This work was supported by the Accelerating Higher Education Expansion and Develop-
ment (AHEAD) operation (AHEAD/PhD/R3/Agri/394), a world bank funded project, Ministry of
Education, Sri Lanka, University of Otago doctoral scholarship, and Catalyst: Seeding funding was
provided by the New Zealand Ministry of Business, Innovation and Employment and administered
by the Royal Society Te Apārangi.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are not available from the authors.
Molecules 2023, 28, 3236 33 of 36

References
1. Lea, E.J.; Crawford, D.; Worsley, A. Consumers’ readiness to eat a plant-based diet. Eur. J. Clin. Nutr. 2006, 60, 342–351. [CrossRef]
[PubMed]
2. Austgulen, M.H.; Skuland, S.E.; Schjøll, A.; Alfnes, F. Consumer readiness to reduce meat consumption for the purpose of
environmental sustainability: Insights from Norway. Sustainability 2018, 10, 3058. [CrossRef]
3. Szejda, K.; Urbanovich, T.; Wilks, M. Accelerating Consumer Adoption of Plant-Based Meat: An Evidence-Based Guide for Effective
Practice; The Good Food Institute: Washington, DC, USA, 2020; Available online: https://gfi.org/images/uploads/2020/02/NO-
HYPERLINKED-REFERENCES-FINAL-COMBINED-accelerating-consumer-adoption-of-plant-based-meat.pdf (accessed on
10 September 2021).
4. BIS Research Report. 2019. Available online: https://bisresearch.com/industry-report/plant-based-food-beverages-alternatives-
market.html (accessed on 25 August 2021).
5. Euromonitor Internationals. 2016. Available online: https://www.euromonitor.com/article/tech-industry-driving-innovation-
meat-dairy-analogues (accessed on 12 August 2021).
6. Astray, G.; García-Río, L.; Mejuto, J.C.; Pastrana, L. Chemistry in food: Flavours. Electron. J. Environ. Agric. Food Chem. 2007, 6,
1742–1763.
7. Van Ruth, S.M.; Roozen, J.P. Delivery of Flavours from Food Matrices. In Food Flavour Technology, 2nd ed.; Taylor, A.J.,
Linforth, R.S.T., Eds.; Wiley-Blackwell Publishing Ltd.: Hoboken, NJ, USA, 2010; pp. 190–206. [CrossRef]
8. Paravisini, L.; Guichard, E. Interactions between aroma compounds and food matrix. In Flavour: From Food to Perception; John
Wiley & Sons: Hoboken, NJ, USA, 2016; pp. 208–234. [CrossRef]
9. Zareian, M.; Silcock, P.; Bremer, P. Effect of medium compositions on microbially mediated volatile organic compounds release
profile. J. Appl. Microbiol. 2018, 125, 813–827. [CrossRef]
10. Janssens, L.; De Pooter, H.L.; Schamp, N.M.; Vandamme, E.J. Production of flavours by microorganisms. Process Biochem. 1992, 27,
195–215. [CrossRef]
11. Marsili, R. Flavors and off-flavors in dairy foods. In Encyclopedia of Dairy Sciences, 2nd ed.; Academic Press: Cambridge, MA,
USA, 2011; pp. 533–551. [CrossRef]
12. Kallscheuer, N.; Classen, T.; Drepper, T.; Marienhagen, J. Production of plant metabolites with applications in the food industry
using engineered microorganisms. Curr. Opin. Biotechnol. 2019, 56, 7–17. [CrossRef]
13. Bamforth, C.W.; Cook, D.J. Food, Fermentation, and Micro-Organisms, 2nd ed.; John Wiley & Sons: Hoboken, NJ, USA, 2019.
[CrossRef]
14. Braga, A.; Guerreiro, C.; Belo, I. Generation of flavors and fragrances through biotransformation and de novo synthesis. Food
Bioprocess Techol. 2018, 11, 2217–2228. [CrossRef]
15. Sharma, R.; Garg, P.; Kumar, P.; Bhatia, S.K.; Kulshrestha, S. Microbial fermentation and its role in quality improvement of
fermented foods. Fermentation 2020, 6, 106. [CrossRef]
16. Moher, D.; Liberati, A.; Tetzlaff, J.; Altman, D.G.; The Prisma Group. Preferred reporting items for systematic reviews and
meta-analyses: The PRISMA statement. Phys. Ther. 2009, 89, 873–880. [CrossRef]
17. Cui, S.; Zhao, N.; Lu, W.; Zhao, F.; Zheng, S.; Wang, W.; Chen, W. Effect of different Lactobacillus species on volatile and nonvolatile
flavor compounds in juices fermentation. Food Sci. Nutr. 2019, 7, 2214–2223. [CrossRef]
18. Gao, H.; Wen, J.J.; Hu, J.L.; Nie, Q.X.; Chen, H.H.; Nie, S.P.; Xiong, T.; Xie, M.Y. Momordica charantia juice with Lactobacillus
plantarum fermentation: Chemical composition, antioxidant properties and aroma profile. Food Biosci. 2019, 29, 62–72. [CrossRef]
19. Wu, C.; Li, T.; Qi, J.; Jiang, T.; Xu, H.; Lei, H. Effects of lactic acid fermentation-based biotransformation on phenolic profiles,
antioxidant capacity and flavor volatiles of apple juice. LWT—Food Sci. Technol. 2020, 122, 109064. [CrossRef]
20. Li, T.; Jiang, T.; Liu, N.; Wu, C.; Xu, H.; Lei, H. Biotransformation of phenolic profiles and improvement of antioxidant capacities
in jujube juice by select lactic acid bacteria. Food Chem. 2021, 339, 127859. [CrossRef] [PubMed]
21. Chen, C.; Lu, Y.; Yu, H.; Chen, Z.; Tian, H. Influence of 4 lactic acid bacteria on the flavor profile of fermented apple juice.
Food Biosci. 2019, 27, 30–36. [CrossRef]
22. Ricci, A.; Cirlini, M.; Levante, A.; Dall’Asta, C.; Galaverna, G.; Lazzi, C. Volatile profile of elderberry juice: Effect of lactic acid
fermentation using L. plantarum, L. rhamnosus and L. casei strains. Food Res. Int. 2018, 105, 412–422. [CrossRef]
23. Chen, R.; Chen, W.; Chen, H.; Zhang, G.; Chen, W. Comparative evaluation of the antioxidant capacities, organic acids, and
volatiles of papaya juices fermented by Lactobacillus acidophilus and Lactobacillus plantarum. J. Food Qual. 2018, 2018, 9490435.
[CrossRef]
24. Di Cagno, R.; Filannino, P.; Gobbetti, M. Lactic acid fermentation drives the optimal volatile flavor-aroma profile of pomegranate
juice. Int. J. Food Microbiol. 2017, 248, 56–62. [CrossRef]
25. Liu, Y.; Cheng, H.; Liu, H.; Ma, R.; Ma, J.; Fang, H. Fermentation by multiple bacterial strains improves the production of bioactive
compounds and antioxidant activity of Goji juice. Molecules 2019, 24, 3519. [CrossRef]
26. Kaprasob, R.; Kerdchoechuen, O.; Laohakunjit, N.; Sarkar, D.; Shetty, K. Fermentation-based biotransformation of bioactive
phenolics and volatile compounds from cashew apple juice by select lactic acid bacteria. Process Biochem. 2017, 59, 141–149.
[CrossRef]
27. Zhang, X.; Duan, W.; Zou, J.; Zhou, H.; Liu, C.; Yang, H. Flavor and antioxidant activity improvement of carrot juice by
fermentation with Lactobacillus plantarum WZ-01. J. Food Meas. Charact. 2019, 13, 3366–3375. [CrossRef]
Molecules 2023, 28, 3236 34 of 36

28. Xu, X.; Bi, S.; Lao, F.; Chen, F.; Liao, X.; Wu, J. Comprehensive investigation on volatile and non-volatile metabolites in broccoli
juices fermented by animal- and plant-derived Pediococcus pentosaceus. Food Chem. 2021, 341, 128118. [CrossRef] [PubMed]
29. Ricci, A.; Cirlini, M.; Maoloni, A.; Del Rio, D.; Calani, L.; Bernini, V.; Galaverna, G.; Neviani, E.; Lazzi, C. Use of dairy and
plant-derived lactobacilli as starters for cherry juice fermentation. Nutrients 2019, 11, 213. [CrossRef] [PubMed]
30. Lu, Y.; Putra, S.D.; Liu, S.Q. A novel non-dairy beverage from durian pulp fermented with selected probiotics and yeast. Int. J.
Food Microbiol. 2018, 265, 1–8. [CrossRef] [PubMed]
31. Kaprasob, R.; Kerdchoechuen, O.; Laohakunjit, N.; Thumthanaruk, B.; Shetty, K. Changes in physico-chemical, astringency,
volatile compounds and antioxidant activity of fresh and concentrated cashew apple juice fermented with Lactobacillus plantarum.
J. Food Sci. Technol. 2018, 55, 3979–3990. [CrossRef] [PubMed]
32. Liu, Y.; Chen, H.; Chen, W.; Zhong, Q.; Zhang, G.; Chen, W. Beneficial effects of tomato juice fermented by Lactobacillus plantarum
and Lactobacillus casei: Antioxidation, antimicrobial effect, and volatile profiles. Molecules 2018, 23, 2366. [CrossRef]
33. Chen, W.; Zhu, J.; Niu, H.; Song, Y.; Zhang, W.; Chen, H.; Chen, W. Composition and characteristics of yam juice fermented by
Lactobacillus plantarum and Streptococcus thermophilus. Int. J. Food Eng. 2018, 14, 20180123. [CrossRef]
34. Wei, M.; Wang, S.; Gu, P.; Ouyang, X.; Liu, S.; Li, Y.; Zhang, B.; Zhu, B. Comparison of physicochemical indexes, amino acids,
phenolic compounds and volatile compounds in bog bilberry juice fermented by Lactobacillus plantarum under different pH
conditions. J. Food Sci. Technol. 2018, 55, 2240–2250. [CrossRef]
35. Xu, X.; Bao, Y.; Wu, B.; Lao, F.; Hu, X.; Wu, J. Chemical analysis and flavor properties of blended orange, carrot, apple and Chinese
jujube juice fermented by selenium-enriched probiotics. Food Chem. 2019, 289, 250–258. [CrossRef]
36. Chen, Z.; Kang, J.; Zhang, Y.; Yi, X.; Pang, X.; Li-Byarlay, H.; Gao, X. Differences in the bacterial profiles and physicochemical
between natural and inoculated fermentation of vegetables from Shanxi Province. Ann. Microbiol. 2020, 70, 66. [CrossRef]
37. Goswami, R.P.; Jayaprakasha, G.K.; Shetty, K.; Patil, B.S. Lactobacillus plantarum and natural fermentation-mediated biotransfor-
mation of flavor and aromatic compounds in horse gram sprouts. Process Biochem. 2018, 66, 7–18. [CrossRef]
38. Chan, W.; Yi, X.; Liu, S. Solid-state fermentation with Rhizopus oligosporus and Yarrowia lipolytica improved nutritional and flavour
properties of okara. LWT—Food Sci. Technol. 2018, 90, 316–322. [CrossRef]
39. de Godoy Alves Filho, E.; Rodrigues, T.H.S.; Fernandes, F.A.N.; Pereira, A.L.F.; Narain, N.; de Brito, E.S.; Rodrigues, S.
Chemometric evaluation of the volatile profile of probiotic melon and probiotic cashew juice. Food Res. Int. 2017, 99, 461–468.
[CrossRef]
40. Güneşer, O.; Yüceer, Y.K. Biosynthesis of eight-carbon volatiles from tomato and pepper pomaces by fungi: Trichoderma atroviride
and Aspergillus sojae. J. Biosci. Bioeng. 2017, 123, 451–459. [CrossRef]
41. Jin, X.; Chen, W.; Chen, H.; Chen, W.; Zhong, Q. Comparative evaluation of the antioxidant capacities and organic acid and
volatile contents of mango slurries fermented with six different probiotic microorganisms. J. Food Sci. 2018, 83, 3059–3068.
[CrossRef] [PubMed]
42. Markkinen, N.; Laaksonen, O.; Yang, B. Impact of malolactic fermentation with Lactobacillus plantarum on volatile compounds of
sea buckthorn juice. Eur. Food Res. Technol. 2021, 247, 719–736. [CrossRef]
43. Wu, B.; Liu, J.; Yang, W.; Zhang, Q.; Yang, Z.; Liu, H.; Lv, Z.; Zhang, C.; Jiao, Z. Nutritional and flavor properties of grape juice as
affected by fermentation with lactic acid bacteria. Int. J. Food Prop. 2021, 24, 906–922. [CrossRef]
44. Hadj Saadoun, J.; Ricci, A.; Cirlini, M.; Bancalari, E.; Bernini, V.; Galaverna, G.; Neviani, E.; Lazzi, C. Production and recovery of
volatile compounds from fermented fruit by-products with Lacticaseibacillus rhamnosus. Food Bioprod. Process. 2021, 128, 215–226.
[CrossRef]
45. Yi, C.; Li, Y.; Zhu, H.; Liu, Y.; Quan, K. Effect of Lactobacillus plantarum fermentation on the volatile flavors of mung beans.
LWT—Food Sci. Technol. 2021, 146, 111434. [CrossRef]
46. Hadj Saadoun, J.; Calani, L.; Cirlini, M.; Bernini, V.; Neviani, E.; Del Rio, D.; Galaverna, G.; Lazzi, C. Effect of fermentation with
single and co-culture of lactic acid bacteria on okara: Evaluation of bioactive compounds and volatile profiles. Food Funct. 2021,
12, 3033–3043. [CrossRef] [PubMed]
47. Pan, X.; Zhang, S.; Xu, X.; Lao, F.; Wu, J. Volatile and non-volatile profiles in jujube pulp co-fermented with lactic acid bacteria.
LWT—Food Sci. Technol. 2022, 154, 112772. [CrossRef]
48. Mandha, J.; Shumoy, H.; Devaere, J.; Schouteten, J.J.; Gellynck, X.; de Winne, A.; Athanasia, O.M.; Raes, K. Effect of lactic acid
fermentation of watermelon juice on its sensory acceptability and volatile compounds. Food Chem. 2021, 358, 129809. [CrossRef]
[PubMed]
49. Wang, Z.; Feng, Y.; Yang, N.; Jiang, T.; Xu, H.; Lei, H. Fermentation of kiwifruit juice from two cultivars by probiotic bacteria:
Bioactive phenolics, antioxidant activities and flavor volatiles. Food Chem. 2022, 373, 131455. [CrossRef] [PubMed]
50. Qi, J.; Huang, H.; Wang, J.; Liu, N.; Chen, X.; Jiang, T.; Xu, H.; Lei, H. Insights into the improvement of bioactive phytochemicals,
antioxidant activities and flavor profiles in Chinese wolfberry juice by select lactic acid bacteria. Food Biosci. 2021, 43, 101264.
[CrossRef]
51. Li, H.; Huang, J.; Wang, Y.; Wang, X.; Ren, Y.; Yue, T.; Wang, Z.; Gao, Z. Study on the nutritional characteristics and antioxidant
activity of dealcoholized sequentially fermented apple juice with Saccharomyces cerevisiae and Lactobacillus plantarum fermentation.
Food Chem. 2021, 363, 130351. [CrossRef]
52. Chen, C.; Zhao, S.; Hao, G.; Yu, H.; Tian, H. Role of lactic acid bacteria on the yogurt flavour: A review. Int. J. Food Prop. 2017, 20,
316–330. [CrossRef]
Molecules 2023, 28, 3236 35 of 36

53. Akhtar, M.K.; Dandapani, H.; Thiel, K.; Jones, P.R. Microbial production of 1-octanol: A naturally excreted biofuel with diesel-like
properties. Metab. Eng. Commun. 2015, 2, 1–5. [CrossRef]
54. Aguedo, M.; Ly, M.H.; Belo, I.; Teixeira, J.A.; Belin, J. The use of enzymes and microorganisms for the production of aroma
compounds from lipids. Food Technol. Biotechn. 2004, 42, 327–336.
55. Pugh, S.; McKenna, R.; Halloum, I.; Nielsen, D.R. Engineering Escherichia coli for renewable benzyl alcohol production. Metab.
Eng. Commun. 2015, 2, 39–45. [CrossRef]
56. Valera, M.J.; Boido, E.; Ramos, J.C.; Manta, E.; Radi, R.; Dellacassa, E.; Carraua, F. The mandelate pathway, an alternative to the
phenylalanine ammonia lyase pathway for the synthesis of benzenoids in ascomycete yeasts. Appl. Environ. Microbiol. 2020.
[CrossRef]
57. Marilley, L.; Casey, M.G. Flavours of cheese products: Metabolic pathways, analytical tools and identification of producing strains.
Int. J. Food Microbiol. 2004, 90, 139–159. [CrossRef]
58. McSweeney, P.L.H.; Sousa, M.J. Biochemical pathways for the production of flavour compounds in cheeses during ripening: A
review. Le Lait 2000, 80, 293–324. [CrossRef]
59. Rapp, A.; Mandery, H. Wine aroma. Experientia 1986, 42, 873–884. [CrossRef]
60. Corre, M.; Lubachevsky, G.; Rankin, S.A. A study of the volatile composition of Minas cheese. LWT—Food Sci. Technol. 2005, 38,
555–563. [CrossRef]
61. Suomalainen, H. Yeast esterases and aroma esters in alcoholic beverages. J. Inst. Brew. 1981, 87, 296–300. [CrossRef]
62. De Carvalho, B.T.; Holt, S.; Souffriau, B.; Brandão, R.L.; Foulquié-Moreno, M.R.; Theveleina, J.M. Identification of novel alleles
conferring superior production of rose flavor phenylethyl acetate using polygenic analysis in yeast. MBio 2017, 8, e01173-17.
[CrossRef]
63. Beresford, T.P. Lactic acid bacteria: Citrate fermentation by lactic acid bacteria. In Encyclopedia of Dairy Sciences, 2nd ed.; Academic
Press: Cambridge, MA, USA, 2011; pp. 166–172. [CrossRef]
64. Laëtitia, G.; Pascal, D.; Yann, D. The Citrate Metabolism in Homo- and Heterofermentative LAB: A Selective Means of Becoming
Dominant over Other Microorganisms in Complex Ecosystems. Food Nutr. Sci. 2014, 5, 953–969. [CrossRef]
65. Quintans, N.G.; Blancato, V.; Repizo, G.; Magni, C.; López, P. Citrate metabolism and aroma compound production in lactic acid
bacteria. In Molecular Aspects of Lactic Acid Bacteria for Traditional and New Applications; Mayo, B., López, P., Pérez-Martínez, G., Eds.;
Research Signpost: Thiruvananthapuram, India, 2008; pp. 65–88. ISBN 978-81-308-0250-3.
66. Wang, Y.; Wu, J.; Lv, M.; Shao, Z.; Hungwe, M.; Wang, J.; Bai, X.; Xie, J.; Wang, Y.; Geng, W. Metabolism Characteristics of Lactic
Acid Bacteria and the Expanding Applications in Food Industry. Front. Bioeng. Biotechnol. 2021, 9, 612285. [CrossRef] [PubMed]
67. Le Bars, D.; Yvon, M. Formation of diacetyl and acetoin by Lactococcus lactis via aspartate catabolism. J. Appl. Microbiol. 2007, 104,
171–177. [CrossRef] [PubMed]
68. Ardö, Y. Flavour formation by amino acid catabolism. Biotechnol. Adv. 2006, 24, 238–242. [CrossRef]
69. Kranenburg, R.V.; Kleerebezem, M.; van Hylckama Vlieg, J.; Ursing, B.M.; Boekhorst, J.; Smit, B.A.; Eman, H.E.A.; Smit, G.;
Siezen, R.J. Flavour formation from amino acids by lactic acid bacteria: Predictions from genome sequence analysis. Int. Dairy J.
2002, 12, 111–121. [CrossRef]
70. Zaunmüller, T.; Eichert, M.; Richter, H.; Unden, G. Variations in the energy metabolism of biotechnologically relevant heterofer-
mentative lactic acid bacteria during growth on sugars and organic acids. Appl. Microbiol. Biotechnol. 2006, 72, 421–429. [CrossRef]
[PubMed]
71. Tsvetanova, F.; Petrova, P.; Petrov, K. Microbial production of 1-butanol—Recent advances and future prospects (review). J. Chem.
Technol. Metall. 2018, 53, 683–696.
72. Cheon, Y.; Kim, J.S.; Park, J.B.; Heo, P.; Lim, J.H.; Jung, G.Y.; Seo, J.H.; Park, J.H.; Koo, H.M.; Cho, K.M.; et al. A biosynthetic
pathway for hexanoic acid production in Kluyveromyces marxianus. J. Biotechnol. 2014, 182–183, 30–36. [CrossRef]
73. Lee, S.M.; Oh, J.; Hurh, B.S.; Jeong, G.H.; Shin, Y.K.; Kim, Y.S. Volatile compounds produced by Lactobacillus paracasei during oat
fermentation. J. Food Sci. 2016, 81, C2915–C2922. [CrossRef] [PubMed]
74. Tan, Z.; Yoon, J.M.; Chowdhury, A.; Burdick, K.; Jarboe, L.R.; Maranas, C.D.; Shanks, J.V. Engineering of E. coli inherent fatty acid
biosynthesis capacity to increase octanoic acid production. Biotechnol. Biofuels 2018, 11, 87. [CrossRef]
75. Zhang, Y.; Nielsen, J.; Liu, Z. Engineering yeast metabolism for production of terpenoids for use as perfume ingredients,
pharmaceuticals and biofuels. EMS Yeast Res. 2017, 17, fox080. [CrossRef] [PubMed]
76. Moser, S.; Pichler, H. Identifying and engineering the ideal microbial terpenoid production host. Appl. Microbiol. Biotechnol. 2019,
103, 5501–5516. [CrossRef]
77. Rannou, C.; Laroque, D.; Renault, E.; Prost, C.; Sérot, T. Mitigation strategies of acrylamide, furans, heterocyclic amines and
browning during the Maillard reaction in foods. Food Res. Int. 2016, 90, 154–176. [CrossRef]
Molecules 2023, 28, 3236 36 of 36

78. Amann, A. Characterization and Pathway Investigation of Off-Flavor Formation in Stored Commercial Apple and Orange Juice
Products. Retrieved from the University of Minnesota Digital Conservancy. 2016. Available online: https://hdl.handle.net/1129
9/178913 (accessed on 23 November 2021).
79. Rauhut, D. Usage and formation of sulphur compounds. In Biology of Microorganisms on Grapes, in Must and in Wine; König, H.,
Unden, G., Fröhlich, J., Eds.; Springer International Publishing AG: Cham, Switzerland, 2017; pp. 255–291. [CrossRef]

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