Scientific African 8 (2020) e00443

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Scientific African
journal homepage: www.elsevier.com/locate/sciaf

Characterization of volatile compounds of liquid smoke

flavourings from some tropical hardwoods
Alphonse Sokamte tegang a,b, Pierre Desire Mbougueng c,∗,
Nakkarike Manjabhat Sachindra a, Nikaise Forestine Douanla Nodem b,
Leopold Tatsadjieu Ngoune d
a
CSIR-Central Food Technological Research Institute, Department of Meat and Marine Sciences, P.O Box 570020, Mysore, India
b
National School of Agro-Industrial Sciences, Department of Food Science and Nutrition, University of Ngaoundere P.O Box 455,
Ngaoundere, Cameroon
c
National School of Agro-Industrial Sciences, Department of Process Engineering, University of Ngaoundere, P.O Box 455, Ngaoundere,
Cameroon
d
University Institute of Technology, Department of Process Engineering and quality control, University of Ngaoundere, P.O Box 454
Ngaoundere, Cameroon

a r t i c l e i n f o a b s t r a c t

Article history: The aim of this study was to determine the chemical composition of the liquid smoke
Received 23 January 2020 flavouring obtained from the pyrolysis of five tropical kinds of wood. Chemical char-
Revised 13 May 2020
acterization was performed by FT-IR and GC/MS on liquid smoke fractions soluble in
Accepted 26 May 2020
dichloromethane. FT-IR spectroscopy revealed the presence of specific functional groups
and chemical bonds of certain volatile compounds in liquid smoke. Besides, the chem-
Keywords: ical composition of this volatile fraction was also studied by GC/MS technique. Fur-
Tropical hardwoods fural, phenol,2–methoxy-, creosol, and phenol,4-ethyl-2–methoxy- were the aromatic com-
Liquid smoke flavourings pounds detected in greater proportion in all samples. The analysed liquid smoke flavour-
Pyrolysis ings showed differences in terms of proportions of the major aromatic compounds and
GC/MS also as regards the absence or presence of certain volatile compounds detected. The results
FT-IR
suggest that all these woods are suitable for exploitation in the smoking food industry be-
volatile aromatic compounds
cause they produce smoke containing large proportions of aromatic substances classified
amongst the most flavour and odour-active compounds.
© 2020 The Author(s). Published by Elsevier B.V. on behalf of African Institute of
Mathematical Sciences / Next Einstein Initiative.
This is an open access article under the CC BY license.
(http://creativecommons.org/licenses/by/4.0/)

Introduction

Smoking is known as one of the oldest food preservation techniques. It consists of subjecting food to the action of smoke
resulting from the pyrolysis of wood [1]. The preservative effect of this technique is due to the reduction of the water ac-
tivity of the smoked product and to the antioxidant and antimicrobial compounds of smoke that permeate the products
[2]. With the development of modern preservation techniques, this process is increasingly used nowadays with the main

∗
Corresponding author.
E-mail address: pides_mbougueng@yahoo.fr (P.D. Mbougueng).

https://doi.org/10.1016/j.sciaf.2020.e00443
2468-2276/© 2020 The Author(s). Published by Elsevier B.V. on behalf of African Institute of Mathematical Sciences / Next Einstein Initiative. This is an
open access article under the CC BY license. (http://creativecommons.org/licenses/by/4.0/)
2 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443

purpose of providing to the products thus transformed, particular organoleptic characteristics [3]. Smoking is commonly ap-
plied to meat products, cheese, mushrooms, teas and mainly fish [4]. The consumption of these products is growing more
and more rapidly as they are highly appreciated for their pleasant smoky aroma and flavour. These organoleptic charac-
teristics depend on several factors which include, the type of wood, the moisture of wood, the pyrolysis temperature, etc.
[5,6]. All these factors and mainly the type of wood used, influence the chemical composition of the smoke that perme-
ates the products. Two varieties of wood are commonly used for smoking, softwoods and mostly hardwoods that produce
better smoke providing a more appealing aroma and flavour to the products, and which usually contains less toxic poly-
cyclic aromatic hydrocarbons [7]. Thus, many research works have been interested in the analysis of the chemical profile
of wood smoke, to identify the chemical compounds responsible for the organoleptic properties and to better understand
their relation [8,9,10,11,12,13]. More than 400 different chemical types of compounds have been reported in smoke [14]. Of
these compounds, the class of phenols and its derivatives have been considered as the main contributors to smoke flavour
and aroma [15]. 2-methoxyphenol, 4-methyl-2-methoxyphenol, 2,6-dimethoxyphenol, 4-(2-propenyl)−2-methoxyphenol, 4-
methyl-2,6-dimethoxyphenol, phenol, 2,6-dimethylphenol, 4-ethyl-2-methoxyphenol, 2-methylphenol are phenol derivatives
and were reported amongst the most odour-active compounds [12]. Other classes of compounds including acids, esters,
ketones, aldehydes, pyrans, furans and alcohols, and their derivatives have also been involved in the development of the
organoleptic characteristics of the products [16]. For example, furans impart fruity, caramel-like and sweet characteristics to
foods, and help soften the very pronounced smoke flavour associated with phenolic compounds [17]. In addition, following
a mechanism similar to the Maillard reaction, the carbonyl groups of ketones and aldehydes interact with the amine groups
of the food amino acids and thus contribute to the golden-brown colour of the smoked products [3]. On the basis of this
scientific knowledge related to the chemical composition of smoke and the particular role of certain specific compounds
on the organoleptic properties of smoked products, only a very small number of woods (oak, beech, hickory, alder, walnut,
birch, chestnut, etc.) are frequently used by the smoking food industry in the European Union [18]. However, the differ-
ence in climatic conditions causing varied vegetation from one region to another, results in the variability of woods used
as fuel for food smoking process in one zone to another. For example, unlike the EU, the woods used as fuel in the food
smoking process in Brazil are Bamboo (Bambusa vulgaris), Acacia (Acacia mearnsii and Acacia mangium), Bracatinga (Mimosa
scabrella), Teak (Tectona grandis), and mainly Eucalyptus (Eucalyptus spp.) [16]. In Cameroon, many hardwood species includ-
ing Okan (Cylicodiscus gabunensis), Azobe (Lophira alata), Red Padouk (Pterocarpus soyauxii), Tali (Erythrophleum suaveolens),
Ozouga (Sacoglottis gabonensis), are highly solicited for food smoking process and impart to the smoked products attractive
organoleptic properties [19,20]. Unfortunately, to the best of our knowledge, the volatile compounds responsible for their
organoleptic properties remain unknown unlike the woods used as fuel in the process of smoking food in the EU, Asia,
America, etc. This study aims to characterize these tropical wood species, to produce liquid smoke flavouring condensates
and to determine their chemical composition using GC/MS and FT-IR techniques.

Materials and methods

Materials

Sawdust was collected from a certified timber company (TSC - Cameroon). The samples collected in July 2016 consisted of
Azobé (Lophira alata), Okan (Cylicodiscus gabunensis), Red Padouk (Pterocarpus soyauxii), Tali (Erythrophleum suaveolens) and
Ozouga (Sacoglottis gabonensis) known for their sensory performance for food smoking process in Cameroon. After sieving,
only sawdust less than 20 0 0 μm in diameter was collected and used to generate the smoke.

Methods

Physicochemical characterization of wood species

Elemental analysis and calculation of the gross calorific value. The total nitrogen, carbon, hydrogen and sulphur of wood were
determined using a CHNS (Carbon, Hydrogen, nitrogen, sulphur) analyser, model elemental analyser Vario EL III. For the
CHNS analysis, an oxidant (vanadium pentoxide [V2 O5 ]) is added to 2 mg of dry sample contained in a tin capsule, and the
whole is heated to 10 0 0 °C in a reactor. Under these conditions, the container melts with the mixture in an atmosphere
enriched in O2 . NO2 , SO2 , and CO2 which are products of combustion are carried by helium (carrier gas) and pass through
the column filled with a copper reducer and an oxidizer based on tungsten trioxide (WO3 ), both held at 10 0 0 °C. Under
these conditions, NO2 is reduced to N2 . At the outlet of the column, the gases are quantified using a thermal conductivity
detector (TCD) set at 290 °C.
The chromatographic responses are calibrated against previously analysed standards, and the content in CHNS elements
is indicated as a percentage. The formula below allows calculating the proportion of Oxygen element [21]:
%O = 100 − (%C + %H + %N + %S)
The gross calorific value (GCV) expressed in MJ/kg was then calculated using the formula below [21]:
GCV = 0, 3491xC + 1, 1783xH + 0, 1005xS − 0, 1034xO − 0, 0151xN − 0, 0211xAsh
Where C, H, N, O, and S are the percentages of carbon, hydrogen, nitrogen, oxygen, and sulphur, respectively.
 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443 3

Determination of moisture and ash content. The Association of Official Analytical Chemists [22] method was used to deter-
mine the moisture and ash content of wood firstly after drying in an oven at 105 °C / 24 h for moisture content and, on the
other hand, after incineration in a muffle furnace at 550 °C / 8 h for ash content.

Production and physicochemical characterization of liquid smoke flavouring

Production of liquid smoke flavouring. Dry distillation of sawdust samples was used for the production of liquid smoke
flavourings at the laboratory scale using the method described by Pino [12] with some modifications. 50 g of sawdust
was introduced into a 250 mL flat-bottom flask. A refrigerant and a thermometer positioned in the centre of the sawdust
were fitted on each of the flask ports. The flask was placed on a heater controlled by rheostat to start the pyrolysis process
whose total duration was 60 min after the appearance of smoke. The maximum temperature reached was 400 °C. The result-
ing smoke was trapped in 50 mL of distilled water in which one of the refrigerant ends was previously dipped. The aqueous
liquid smoke obtained was stored away from light in a brown bottle for 7 days to allow the mixture to equilibrate. Then it
was filtered through Whatman N°1 filter paper to retain the solid and oily particles before performing the characterization
of the liquid smoke.

Physicochemical characterization of liquid smoke (pH and titratable acidity). The pH was measured at room temperature using
a probe (ATC Probe model PH5TEMB01P) of a pH metre (EutechTM pH 700 metre, Thermo Fisher Scientific Inc.) introduced
in a volume of 10 mL of aqueous liquid smoke. Titratable acidity (TA) was determined by titrating 10 mL of aqueous liquid
smoke sample with 0.01 N NaOH to a pH of 8.3 [23]. The liquid smoke sample was previously diluted 1/10 (v/v) in distilled
water and the result was expressed as% acetic acid.

Characterization of volatile aromatic compounds.

Extraction of volatile compounds from liquid smoke. Liquid-liquid extraction was performed using analytical grade
dichloromethane (DCM) as the extraction solvent as described by Guillén and Manzanos [24] with some modifications.
15 mL of aqueous liquid smoke sample was introduced into a 100 mL separatory funnel and the volatile compounds were
extracted successively in two time by adding DCM (2 × 15 mL). Then, the extraction solutions were stripped of traces of
water through a column of anhydrous sodium sulphate. The column was rinsed with 10 mL of fresh DCM. All the extraction
solutions were combined and the volume was reduced to 5 mL using a rotavapor set to 30 °C and operating under vacuum.
The concentrated DCM fraction was filtered through a PTFE filter (Polytetrafluoroethylene) of 0.45 μm before characteriza-
tion by FT-IR and GC/MS.
Fourier transform infrared (FT-IR) analysis. FT-IR spectroscopy was performed using the BRUKER IFS 66 V spectrometer
equipped with a DTGS (deuterated triglycine sulphate) detector and using an attenuated total reflection (ATR) crystal acces-
sory on which a drop of the sample was deposited after recording the background spectrum. Then, the FT-IR spectrum of
the constituent compounds of the liquid smoke was recorded at room temperature, in transmittance mode in the 40 0 0–40 0
cm−1 region, with a nominal resolution of 4 cm−1 . The Nicolet OMNIC software was used to record and process the spec-
tra to estimate the area of the bands on which the corresponding qualitative data were based. The functional groups were
identified in the FT-IR spectra using the FT-IR table and previous work reported by Guillén et al. [8].
GC/MS analysis. The GC/MS technique was used to analyse the chemical composition of liquid smoke using a AutoSys-
tem XL gas chromatograph (Perkin Elmer, USA) equipped with an Elite 5 capillary column (30 m long, 0.25 mm diameter,
0.25 μm film thickness). The device was coupled to a TurboMass Gold Mass spectrometer detector (Perkin Elmer, USA) with
a scan speed (acquisition) of 65 scans / second and a mass average m / z of 40 – 400. After filtration through a PTFE filter of
0.45 μm, a volume of 1 μL of each sample was injected in split mode with a split ratio of 1/20 and an injection temperature
of 250 °C. The oven temperature program was set for a total analysis time of 55 min and was distributed as follows: oven
initial temperature 40 °C (held 2 min) and then increased to 150 °C at a rate of 3 °C / min and finally increased to 280 °C at
a rate of 10 °C / min (3 min holding). The carrier gas used was helium (1 mL / min). The mass spectrum detector allowed
the identification of the analytes at the exit of the column. The detection temperature was 280 °C. The electron impact
ionization mode was applied at 70 eV during mass spectral analysis, and after a 2 min solvent delay the data was acquired.
The identification of the compounds was made by comparing their mass spectrum with those available in the NIST library,
afterward by comparing the retention time with that of authentic compounds.

Statistical analysis
ANOVA analysis was performed at a 5% risk to see if there is a significant difference between samples and the Dun-
can test was performed to compare the averages between samples. These analyses were performed using STATGRAPHICS
Centurion XVI software version 16.01.0018 (StatPoint Technologies, Inc.) and the data were reported as mean ± standard
deviation.
4 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443

Table 1
Moisture content, ash content, elemental composition and gross calorific value of woods.

N (%) C (%) S (%) H (%) O (%) Moisture (%) Ash (%) Gross calorific value (MJ/kg)

C. gabunensis 0.38 49.78 0.07 4.77 45.00 13.90±0.17 c

0.76±0.01 d
18.33±0.08a
L. alata 0.28 48.42 0.04 4.88 46.40 11.61±0.24d 1.26±0.02c 17.83±0.05b
P. soyauxii 0.49 47.72 0.00 4.73 47.05 16.50±0.21a 1.98±0.07b 17.32±0.11c
E. suaveolens 0.38 47.48 0.00 4.95 47.19 13.89±0.13c 2.44±0.07a 17.47±0.09c
S. gabonensis 0.31 47.23 0.00 4.76 47.70 14.64±0.33b 1.17±0.07c 17.14±0.07d

The values in each column with different exponents are significantly different (p <0.05). Legends: N (nitrogen); C (carbon); S
(sulphur); H (hydrogen): O (oxygen).

Table 2
Physico-chemical analysis of liquid smoke.

Total acidity (%) pH

C. gabunensis 0.99±0.01 b
2.95±0.01b
L. alata 1.11±0.03a 2.88±0.01c
P. soyauxii 0.73±0.00c 2.97±0.05b
E. suaveolens 0.55±0.01e 3.04±0.01a
S. gabonensis 0.59±0.00d 3.08±0.03a

The values in each column with different super-

scripts are significantly different (p <0.05): Duncan
test.

Results and discussion

Physicochemical characteristics of wood

The moisture content, ash content and elemental composition are often realized and are part of the characterizations
carried out to know the properties or the performances relating to the use of woods as fuel for food smoking or as fuel for
an energy purpose. The elemental analysis expressed as a percentage of C, H, O, N and S is presented in Table 1.
The results show that the dry matter of the wood consists essentially of carbon, hydrogen and oxygen which repre-
sent more than 99% in all the samples analysed. The elements C, H, and O are the basic atoms mainly involved in thermal
processes. Table 1 shows that carbon content values range from 47.23 - 49.78%, indicating that all sawdust samples anal-
ysed belong to the hardwood species (gymnosperm) because of their carbon content less than 50% [25]. The hydrogen and
oxygen content is between 4.73 to 4.95% and 45.00 to 47.70%, respectively. The nitrogen content is less than 0.5%, with P.
soyauxii, E. suaveolens and C. gabunensis having the highest levels, 0.49, 0.38 and 0.38%, respectively. Sulphur is present in C.
gabunensis and L. alata at a lower proportion, 0.07 and 0.04%, respectively. However, sulphur was not detected in P. soyauxii,
E. suaveolens and S. gabonensis. These low values found for nitrogen and sulphur are favourable because, during the pyrolysis
of biomass, there will be a very weak release of atmospheric emission of toxic gas mixtures (SOx , NOx , NH3 ) derived from
these two atoms [26].
The results of the moisture and ash content of woods are also presented in Table 1. The moisture content ranged from
11.61 to 16.50%, respectively for L. alata and P. soyauxii. The non-combustible solid material of sawdust consists of ash. It
mainly contains alumina (Al2 O3 ), silica (SiO2 ), oxide ions (Fe2 O3 , FeO), MgO and CaO, etc. The ash content varies from 0.76 to
2.44% (Table 1). E. suaveolens has the highest value (2.44%), followed by P. soyauxii (1.98%) and L. alata (1.26%). C. gabunensis
has a significantly lower value (p <0.05) compared to all other samples. Mitchual et al. [27] found ash content ranging from
0.61 to 5.04% for some tropical hardwood species. A high ash content is appropriate for the use of biomass as a fuel during
food smoking process as it causes a reduction in the gross calorific value of the wood. This is explained by the reduction of
the heat transfer rate and the diffusion of oxygen on the surface of the biomass [27]. From Table 1 it can be seen that the
gross calorific values range from 17.14 to 18.33 MJ/kg. The gross calorific value is one of the most important parameters for
assessing the quality of biomass. These low values (17.14 - 17.83 MJ/kg) found in this study suggest that L. alata, P. soyauxii,
E. suaveolens, and S. gabonensis wood species are more suitable for food smoking compared to C. gabunensis which presented
the highest value (18.33 MJ/kg). This is correlated with the low ash content of C. gabunensis wood species, compared with
other hardwoods studied (Table 1).

Characterization of liquid smoke

Physico-chemical analysis
The characteristics of liquid smoke generated by pyrolysis of sawdust from different woods are summarized in Table 2.
The measured pH values vary from 2.88 to 3.08; S. gabonensis with the highest value (3.08), but no statistically significant
difference was observed with E. suaveolens (3.04). With a pH of 1.5 - 5.5, the liquid smoke of the wood is usually acidic
[28]. The pH of the samples studied is inversely proportional to the titratable acidity (r = −0.93, p = 0.02, r2 = 0.87,
 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443 5

AT = - 2.93 pH +9.54), which is similar to results reported by Montazeri et al. [11]. Thus, L. alata having the lowest pH
has a titratable acidity value of 1.11, which would indicate a presence of organic acids in large proportion, the latter being
derived from the partial pyrolysis of cellulose and the hemicellulose of wood. The total acidity of the liquid smoke is mainly
influenced by the presence of acetic acid, formic acid, propionic acid and butyric acid [29]. The organic acids generally affect
the flavour, colour, texture and microbiological stability of foods [11]. In addition, organic acids contribute to the chelation
of metal ions involved in the lipid oxidation phenomenon.

Volatile aromatic compounds of liquid smoke flavourings

FT-IR analysis. FT-IR technique can reveal the presence of specific functional groups and chemical bonds of a complex mix-
ture of chemical compounds or that of a pure molecule [8]. The IR spectra generally consist of an area characteristic of the
functional groups and an imprint area. The IR spectrum of the liquid smoke flavourings is shown in Fig. 1 for each of the
samples analysed.
It is clear from these figures that the analysed samples generally have the same functional groups because they have
similar absorption bands on their IR spectrum. The stretching vibration of the intermolecular hydrogen bond O–H of alcohols
and phenols is represented on the spectrum by a broad absorption band (3200 - 3500 cm−1 ) with an absorption maximum
at 3400 cm−1 . Interconversion by keto-enol tautomerism may also lead to the appearance of an absorption band at this
wavenumber by the stretching vibration of the O–H bonds of the carbonyl compounds [8].
At 3100 cm−1 , a low-intensity absorption band is observed due to the aromatic C–H stretching vibration. This absorption
may also be due to the stretching vibration of the olefinic C–H bond (C - H alkene).
The two peaks observed at 2980 - 2925 cm−1 correspond to asymmetric stretching vibration characteristic of the alkane
C–H bond. The symmetrical stretching vibration observed at 2850 cm−1 may correspond to several classes of compounds,
notably the class of ethers whose group - CH3 absorbs with a wavenumber of about 2850 cm−1 . The stretching vibration
of the aldehyde C–H bond can also absorb in this region with an average intensity and a low wavenumber (2734 cm−1 )
probably due to the Fermi resonance [30].
The intense and broadband observed at 1710 cm−1 is characteristic of the C = O carbonyl function of ketones, aldehy-
des, diketones, phenones, and carboxylic acids. However, the intense peak observed at 1674 cm−1 could correspond to the
stretching vibration of the C = C bond of an asymmetric alkene.
The stretching vibrations of aromatic C = C bonds are the main cause of the multiple absorption bands observed at 1610,
1516 and 1464 cm−1 . This is correlated with the GC/MS analysis as phenol, phenol, 2–methoxy-, phenol, 2,6-dimethoxy-,
and their derivatives were reported in the liquid smoke samples analysed.
As reported by Guillén et al. [8], the C–H scissoring vibrations of alkenes and / or asymmetric C–H deformation vibrations
of alkanes could result in an absorption band at 1463 cm−1 . The symmetric deformation vibration of alkane C–H bond
could contribute to the absorption band observed at 1363 cm−1 . These vibrations could be explained by the presence of
alkanes and alkenes detected in the liquid smoke samples analysed (Table 3). Some IR absorption bands can also be used to
distinguish the hardwoods and softwoods. Hergert [31] associated the difference in the band at 1277 cm−1 (weak) and 1224
cm−1 (high) with the hard characteristic of wood. This hypothesis seems to be verified in this study.

GC/MS analysis. The liquid smoke fractions soluble in dichloromethane were also analysed by gas chromatography coupled
to mass spectrometry (GC/MS) with the aim to identify chemical compounds responsible for their organoleptic characteris-
tics. Fig. 2 shows the chromatograms of the chemical compounds of the liquid smoke flavourings of the different samples
studied.
It can be observed that all woods have partially similar chromatograms gaits which may indicate that they contain certain
types of identical volatile compounds but in varying proportions. In addition, these chromatograms also indicate differences
with regard to the absence or presence of certain types of volatile compounds of a sample to another. The observed dif-
ferences can be explained by the species difference and the moisture content of sawdust [9]. The type of soil, the harvest
period, the climatic conditions of the region of the harvest, the age of the plant could also explain these differences. All
these factors affect the composition of the wood including its lignin, cellulose, hemicellulose and extractives content. The
proportions of the various volatile compounds detected in the liquid smoke flavourings are presented in Table 3, as well as
their chemical names. A total of 100 compounds were identified.
Generally, the thermal degradation of the basic wood components (lignin, hemicellulose, and cellulose) is at the origin
of the volatile compounds detected in the analysed liquid smoke flavourings (Table 3). Aldehydes, ketones, diketones, esters,
acids and furans are the main groups of compounds derived from hemicellulose and cellulose. On the other hand, the pyrol-
ysis of lignin mainly involves the formation of phenols, phenol, 2–methoxy-, phenol, 2,6-dimethoxy-, and their derivatives.
Carbonyl compounds. Carbonyl compounds have been reported in literature as the most diverse group of chemical com-
pounds in liquid smoke and include lactones, ketones, aldehydes and diketones. The major carbonyl compounds found
(Table 3) in the liquid smoke samples analysed in this study are: 2-methyl-2-cyclopen-1-one (0.77 - 2.57%), 1 - (acety-
loxy) −2- propanone (0.95 - 2.54%) and 5,9-Dodecadien-2-one, 6,10-dimethyl-, (E, E) (0.86 - 2.31%); the highest values of 2-
methyl-2-cyclopen-1-one were found in E. suaveolens, followed by S. gabonensis and P. soyauxii. 3-methyl-1,2-cyclopentadione
or cyclotene is also present in high proportion in the liquid samples analysed (1,68 - 3,16%), with the exception of E.
suaveolens and S. gabonensis. The smell and taste of cyclotene have been described in literature with very pronounced
sensory notes similar to that of maple syrup and to a lesser extent that of liquorice [32]. 2-methyl-2-cyclopen-1-one and
6 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443

Fig. 1. FT-IR profile of the aromatic compounds of the liquid smoke flavourings of C. gabunensis ; L. alata ; P. soyauxii ; E. suaveolens and S. gabonensis.
A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443 7

Fig. 1. Continued
8 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443

Table 3
Volatile aromatic compound profile detected (%) in liquid smoke flavourings of wood species.

Volatile compounds C. gabunensis L. alata P. soyauxii E. suaveolens S. gabonensis

1- (1,2-Dimethyl-cyclopent-2-enyl) -ethanone nd nd nd Nd 0.76

1,2,3-Trimethoxybenzene nd 0.50 nd Nd nd
1,2,4-Trimethoxybenzene nd 1.35 nd Nd nd
1,2-Cyclopentanedione nd nd 1.03 nd nd
1,2-Cyclopentanedione, 3-methyl 1.68 3.16 2.06 nd nd
1,2-Ethanediol, monoacetate nd 1.94 1.41 1.48 2.25
1,4-Dioxin, 2,3-dihydro- nd nd nd 0.79 nd
1-Heneicosanol 0.65 nd nd nd nd
2 (5H) -Furanone nd 1.09 0.89 0.57 0.46
2 (5H) -Furanone, 3-methyl- nd nd 0.50 0.55 0.61
2- Furanol, tetrahydro-2-methyl- nd nd 1.26 0.48 nd
2- Methoxy-6-methylphenol nd nd 0.37 nd nd
2- Methoxytetrahydrofuran nd nd 0.46 nd nd
2(3H)-Furanone, 5-methyl nd 0.57 0.68 0.58 1.04
2(5H)-Furanone, 5-methyl- nd 0.46 nd nd nd
2,3-Dimethoxytoluene nd 0.18 nd nd nd
2,3-Pentanedione nd nd nd 1.17 0.31
2,4-Dimethoxyphenol 0.30 0.22 nd nd nd
2,5-Dimethoxybenzyl alcohol nd 0.24 nd nd nd
2,5-Hexanedione nd 0.69 nd nd 0.24
2-Butanone, 1-(acetyloxy)- 0.50 1.17 0.68 1.30 1.51
2-Cyclopenten-1-one, 2,3-dimethyl- nd 0.36 1.07 0.66 0.52
2-Cyclopenten-1-one, 2–hydroxy- 0.88 1.78 nd 0.373 0.33
2-Cyclopenten-1-one, 2–hydroxy-3-methyl nd nd nd 1.75 1.57
2-Cyclopenten-1-one, 2-methyl- 0.77 1.18 2.04 2.57 2.33
2-Cyclopenten-1-one, 3-methyl- 0.46 0.44 0.60 0.53 nd
2-Furancarboxaldehyde, 5-methyl- 0.95 2.22 1.15 1.08 3.56
2-Furanmethanol, tetrahydro- 0.50 0.39 nd nd 0.484
2-Furanone, 2,5-dihydro-3,5-dimethyl nd 0.84 0.49 0.46 0.39
2-Methoxy-4-vinylphenol nd 0.68 0.35 nd nd
2-Methoxy-5-methylphenol 0.42 0.41 nd 0.33 0.16
2-Oxo-n-valeric acid 0.47 0.74 1.22 nd 0.46
2-Propanone, 1-(4–hydroxy-3-methoxyphenyl)- 0.55 nd nd nd nd
2-Propanone, 1-(acetyloxy)- 0.95 2.54 1.98 2.41 nd
3,4-Dimethoxytoluene nd 0.24 0.23 nd 0.22
3-Allyl-6-methoxyphenol nd nd nd nd 0.11
3-Ethenyl-3-methylcyclopentanone nd nd nd nd 0.23
3-Furaldehyde nd 0.60 nd 0.45 1.37
3-Hepten-2-one, (Z)- nd nd nd nd 0.61
3-Penten-2-one, (E)- nd 0.57 0.95 1.05 0.90
4-Hydroxy-2-methylacetophenone nd 0.25 nd nd 0.16
4-Methyl-5H-furan-2-one nd 0.99 nd nd nd
5,9-Dodecadien-2-one, 6,10-dimethyl-, (E,E)) 0.86 2.31 2.10 1.12 0.93
5-Eicosene, (E)- nd nd 0.36 0.31 0.33
Acetyl eugenol nd nd 0.21 nd nd
Benzaldehyde, 4-ethoxy-3–hydroxy- nd nd nd nd 0.12
Benzene, 1,2-dimethoxy- nd nd 0.32 nd nd
Benzene, 1,4-dimethoxy-2-methyl- 0.14 0.44 nd nd 0.19
Benzene, 4-ethenyl-1,2-dimethoxy- nd nd 0.56 nd nd
Butanoic acid nd nd nd 0.22 nd
Butanoic acid, methyl ester nd nd nd 0.36 nd
Butyrolactone 0.85 1.03 1.57 0.78 0.54
Creosol 5.85 11.64 6.87 5.86 6.87
Cyclopentanone nd 0.37 1.16 1.27 0.76
Cyclopentanone, 2-methyl- nd nd 0.43 nd nd
Cyclopentanone, 3-methyl- nd nd nd nd 0.32
Diisooctyl phthalate 1.00 nd nd nd nd
Eicosane, 2-methyl nd nd 0.98 nd nd
Ethanone, 1-(2-furanyl)- 0.41 0.54 0.98 1.03 0.92
Ethanone, 1-(2–hydroxy-5-methylphenyl)- nd nd 0.15 0.36 nd
Ethyl-1-propenyl ether nd 0.50 0.84 0.78 0.76
Furan, 2-propyl- nd 0.27 nd nd 0.32
Furfural 7.59 16.62 13.82 15.30 24.52
Hexadecanoic acid, methyl ester 0.43 nd nd nd nd
Hexanoic acid nd nd 0.45 nd nd
Methyl stearate 1.93 nd nd nd nd
n-Hexadecanoic acid 0.65 0.64 1.39 1.03 0.91
Nonane, 2,6-dimethyl- nd 0.26 nd nd 0.33
(continued on next page)
 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443 9

Table 3 (continued)

Volatile compounds C. gabunensis L. alata P. soyauxii E. suaveolens S. gabonensis

n-Tetracosanol-1 2.90 nd nd nd nd
Octadecanoic acid nd nd 1.05 nd nd
Oxazolidine, 2,2–diethyl-3-methyl- nd 0.28 nd nd nd
Oxazolidine, 2-ethyl-3-methyl nd nd 0.43 nd nd
Pentanoic acid nd 0.40 0.47 0.34 0.87
Phenol 1.69 1.71 1.29 1.49 1.97
Phenol, 2,3-dimethoxy-, acetate nd nd 0.49 nd nd
Phenol, 2,6-dimethoxy- 1.57 1.11 0.39 nd nd
Phenol, 2,6-dimethoxy-, acetate nd nd 0.47 0.36 nd
Phenol, 2–methoxy- 12.55 16.48 15.85 12.87 12.49
Phenol, 2–methoxy-3-(2-propenyl)- nd nd nd 0.33 0.33
Phenol, 2–methoxy-3-methyl- 0.26 0.36 0.66 nd 0.29
Phenol, 2–methoxy-4-(1-propenyl)- 0.90 nd 0.33 0.65 nd
Phenol, 2–methoxy-4-(2-propenyl)-, acetate nd nd nd 0.37 nd
Phenol, 2–methoxy-4-propyl- nd 0.25 nd 0.44 nd
Phenol, 2–methoxy-6-(2-propenyl)- nd 0.28 nd nd nd
Phenol, 2-methyl- 0.52 nd nd nd nd
Phenol, 3,4-dimethoxy- 0.37 0.16 0.37 0.56 nd
Phenol, 3–methoxy-2-methyl- nd nd nd 0.06 nd
Phenol, 3-methyl- nd nd 0.60 nd 0.36
Phenol, 4-ethyl-2–methoxy- 2.45 3.77 2.44 2.01 2.15
Phenol, 4–methoxy-3-methyl- nd nd nd 0.36 nd
Propan-2-one, 1-(4-isopropoxy-3-methoxyphenyl)- 0.17 nd nd 0.32 nd
Propanoic acid 0.38 nd 0.51 nd nd
Propanoic acid, 2-oxo-, methyl ester nd 1.93 1.57 1.61 1.56
Propanoic acid, ethenyl ester 0.59 1.83 0.79 1.01 2.59
Succindialdehyde nd 0.79 1.28 0.50 0.25
Tetracosane nd nd 3.56 nd nd
trans-13-Octadecenoic acid, methyl ester 0.37 nd nd nd nd
trans-4-Oxo-2-pentenoic acid nd nd 0.57 nd nd
trans-Isoeugenol 0.19 1.02 nd 0.54 0.30
Vinyl butyrate nd 0.77 1.10 nd nd

nd : not detected.

3-methyl-1,2-cyclopentadione have also been reported amongst the major carbonyl compounds of aqueous fractions of oak
and hickory liquid smoke [10]. Other ketone compounds present in low proportion are 3-methyl-2-cyclopenten-l-one and
cyclopentanone. Unlike ketone and diketone derivatives, the lactone and aldehyde derivatives are present in low numbers
and the most widespread are succindialdehyde (0.25 - 1.28%) and butyrolactone (0.54 - 1.57%). Butyrolactone provides a
sweet roasted flavour to foods [33].
Another important group of carbonyl compounds are furans and are also present in the analysed liquid smoke samples.
These compounds play an important role in the overall aroma and flavour conferred by smoke to foods and also help to
soften the very heavy smoked sensory notes of which the phenolic compounds are responsible [17]. The major furan com-
pounds found in the liquid smoke samples analysed were: furfural (7.59 - 24.52%), 2-furancarboxaldehyde, 5-methyl- (0.95
- 3.56%) and 2 (5H) - furanone (0.46 - 1.09%). The highest rate of furfural was found in S. gabonensis, followed by L. alata
and E. suaveolens. The highest value of 2-furancarboxaldehyde, 5-methyl- was found in S. gabonensis. The highest proportion
of 2 (5H) -furanone was detected in L. alata. The higher abundance of furfural observed in this study is consistent with
that observed in the liquid smoke condensate of Quercus sp. in which it was also detected as the major compound [10].
This high proportion could be explained by the fact that furfural is one of the major products resulting from the thermal
decomposition of hemicellulose and cellulose [34]. These two polysaccharides account for more than 50% of the lignocellu-
losic biomass. Guillén and Ibargoitia [9] found a positive correlation between the yield of furfural obtained from cellulose
and the increase in the pyrolysis temperature, which also indicates that this compound is stable at high temperature. Be-
cause of their low odour and taste threshold values, furfural and 2-furancarboxaldehyde, 5-methyl- significantly contribute
to the overall flavour of smoked samples. Both compounds have some similar sensory characteristics (almond odour and
sweet taste). Besides, furfural has a bread odour and its taste descriptors have been described as mild, bread-like, woody,
sweet, nutty caramellic, while 2-furancarboxaldehyde, 5-methyl- taste descriptors are maple-like and caramellic [3]. Nu-
merous other compounds derived from furan were detected, amongst which: 3-furaldehyde; 5-methyl-2 (3H) -furanone,
3-methyl-2 (5H) -furanone, 5-methyl-2 (5H) -furanone, 2-propyl furan etc.
Phenolic compounds. Phenolic compounds are primarily responsible for the smoke aroma and flavour of liquid smoke
and in food products subjected to smoking [15]. These phenolic compounds come mainly from the thermal decomposition
of lignin [6]. In all the samples analysed (Table 3), phenol, 2–methoxy- (12.49 - 16.48%) is the most dominant pheno-
lic compound, followed by creosol (5.85 - 11.64%) and 4-ethyl-phenol, 2–methoxy- (2.01 - 3.77%). The highest levels were
observed in L. alata, followed by P. soyauxii. This high content is explained by the fact that phenol, 2–methoxy- is one
10 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443

Fig. 2. Total ion chromatogram of aromatic compounds from liquid smoke flavourings of different wood species.
 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443 11

Fig. 2. Continued

of the basic constituents of lignin and is produced mainly by depolymerisation during the thermal degradation of lignin
[35]. Phenol, 2–methoxy- is the main compound responsible for the smoked flavour of smoked food products [16]. Phe-
nol and phenol, 2,6-dimethoxy- are also present in high proportion: 1.29 - 1.97% and 0.39 - 1.57%, respectively. However,
phenol, 2,6-dimethoxy- is not detectable in E. suaveolens and S. gabonensis. Phenol, 2–methoxy- and phenol, 2,6-dimethoxy-
and their derivatives are natural chemical compounds with excellent antioxidant properties and are constituents frequently
detected in smoke and smoked foods. The following sensory notes have been described for these powerful aromatic com-
pounds: heavy, cresolic, smoke, pungent, sweet and burnt [10]. Against all expectations, the phenol, 2,6-dimethoxy-/phenol,
2–methoxy- ratio is very low and zero for some samples, compared to the 3.3/1 ratio reported in literature when a hard-
wood is subjected to pyrolysis [8]. This low phenol, 2,6-dimethoxy-/phenol, 2–methoxy- ratio is associated with the higher
proportion of phenol, 2–methoxy- and its derivatives in the liquid smoke obtained by pyrolysis of the wood used in this
study. Similar findings were recently published by Pimenta et al. [13] who found that the pyrolysis of Eucalyptus urogran-
dis wood originating from Brazil gives a liquid smoke which contains mainly guaiacyl units with a very high proportion
of phenol, 2–methoxy- (13.33%) compared to that of phenol, 2,6-dimethoxy- (3.01%). Moreover, it were also observed that
Erythrina cristagalli which is a hardwood produces only guaiacyl units due to the absence of the ferulate 5-hydroxylase en-
zyme as suggested by Higuchi [36]. This enzyme is involved in the metabolic pathway of phenylpropanoids and catalyses
in several steps the formation of sinapoyl CoA which is the precursor of sinapyl alcohol, which is finally polymerized into
syringyl-lignin units [37].
12 A. Sokamte tegang, P.D. Mbougueng and N.M. Sachindra et al. / Scientific African 8 (2020) e00443

Organic acids. Organic acids originate from the pyrolysis of wood carbohydrates and play an important role in the
flavour, colour, texture and microbiological stability of foods impregnated during smoking [38]. The organic acids found
in the liquid smoke samples analysed are butanoic acid, propanoic acid, pentanoic acid, hexanoic acid, octadecanoic acid,
n-hexadecanoic acid and their derivatives (Table 3). Of the short-chain organic acids, pentanoic acid (0.34–0.87%) is the most
common, followed by propanoic acid (0.38–0.51%). Butanoic acid (0.22%) is present only in E. suaveolens, while hexanoic acid
(0.45%) is present only in P. soyauxii. Propanoic acid contributes to the overall odour of liquid smoke by providing a slightly
pungent note and has antimicrobial properties against sporulating bacteria and food spoilage moulds [24]. The presence of
propanoic acid (1.16%), butanoic acid (0.91%) and pentanoic acid (0.22) were also detected by Pimenta et al. [13] in the liquid
smoke of Eucalyptus urograndis. For long-chain organic acids, all samples analysed in this study contain n-hexadecanoic acid
(0.64 - 1.39%) with the highest content found in P. soyauxii (1.39%), followed by E. suaveolens (1.03%). In addition, only P.
soyauxii contains octadecanoic acid (1.05%). Most organic acids have a high odour threshold values and have such a negligi-
ble contribution to the overall odour of smoked products. The most common organic acid derivatives (with ketone or ester
function) found in greater proportion are: 2-oxo-n-valeric acid (0.46 - 1.22%); propanoic acid, 2-oxo-, methyl ester (1.56 -
1.93%) and propanoic acid, ethenyl ester (0.59 - 2.59%).

Conclusion

In conclusion, the results obtained in this study reveal that the woods analysed contain a low proportion of sulphur and
nitrogen, precursors of toxic gases in the smoke. In addition, they have low gross calorific values, which is interesting for
the production of dense smoke during food smoking. Preliminary analysis liquid smoke flavouring using GC/MS technique
indicates the presence in large quantities of certain phenolic and carbonyl compounds that have been reported in the liter-
ature as the most active aromatic compounds, which may be useful in explaining the effect of these woods on the sensory
properties of smoked products. Besides, the GC/MS analysis demonstrates that the assumption that hardwoods contain more
phenol, 2,6-dimethoxy- compared to phenol, 2–methoxy- does not apply to the tropical woods analysed in this study. For
a better understanding of the chemical composition of the liquid smoke flavourings of the analysed woods, it would be in-
teresting in future studies to make a chemical characterization of the constituents of the wood and to evaluate the activity
of the enzymes involved in the process of lignin biosynthesis of the wood. Finally, the chemical composition of the liquid
smoke flavourings analysed demonstrates that they are suitable for the food smoking industry. As the analysed woods show
differences regarding the presence or absence of certain compounds and in terms of the proportion of predominant active
aromatic compounds, the choice will depend on the expectations on the taste qualities of the smoked foods products.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have
appeared to influence the work reported in this paper.

Acknowledgements

Authors are grateful to Director, CSIR-Central Food Technological Research Institute (India) for support in the form of
infrastructural facilities made available for undertaking the present study. The first author is grateful to The World Academy
of Sciences (TWAS) / Department of Biotechnology (DBT) for the post-graduate fellowship to carry out this project under
the grant numbers 3240287117.

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