International Journal of Food Science and Technology 2024, 59, 8825–8836 8825

Review
Valorisation of sweet potato leaves as a potential agri-food
resource: Assessing nutritional and nutraceutical values altered
by food processing—A review

Junpeng Yi,1,2,3* Luyao Li,1 Xin Li,4 Xu Duan,1* Junling Wang,1 Yuxin Han1 & Yan Gao1
1 College of Food and Bioengineering, Henan University of Science and Technology, Luoyang, Henan, China

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2 Luohe Vocational Technology College, Luohe, Henan, China
3 Food Laboratory of Zhongyuan, Luohe, Henan, China
4 School of Chemistry & Chemical Engineering, Henan University of Science and Technology, Luoyang, Henan, China
(Received 13 November 2023; Accepted in revised form 12 February 2024)

Summary Sweet potato leaves are rich in nutrients and bioactive substances. They are commonly utilised as human
foodstuff in some Asian, African and North American countries. Leafy vegetables are generally processed
through domestic cooking or industrial processing techniques before consumption. This paper reviews the
available literature on the nutritional composition and phytochemical profile, along health benefits of pro-
cessed sweet potato leaves. Discussion on the applications of sweet potato leaves as a valuable fortifying
ingredient in a variety of food formulations is included. Domestic cooking resulted in a substantial
increase in protein content at optimal conditions but led to a significant loss of bioactive compounds.
Vacuum freeze-drying retained most of nutrients and caffeoylquinic acid derivatives. Fermentation and
supplementation as fortifying ingredients in food formulations could improve the nutritional status and
sensory characteristics of the final products. This review can facilitate the development of an integrated
plant for the valorisation of sweet potato agro-industrial residues and aid the food industry in obtaining
fortified foods with sweet potato leaf loaded.
Keywords Domestic cooking, drying, fermentation, nutrients, polyphenols.

growing season with an annual yield of about 11 000

Introduction
pounds per acre, which is comparable to that of the
Sweet potato (Ipomoea batata L.) is a dicotyledonous roots, and additionally, at least two to three crops
plant and belongs to the family Convolvulaceae. It is could be raised in a year (Villareal et al., 1982; Le Van
seventh most important food crop worldwide and An et al., 2003). Most of them are utilised for feeding
is cultivated in tropical and subtropical regions. farm animals or discarded as waste. Faced with grow-
Around 88.97 million tonnes of sweet potato was pro- ing food scarcity, the use of sweet potato leaves as
duced in 2021 worldwide, with China being the largest food resource could increase the availability of food
producer, supplying about 53.52% of this total (FAO- substantially. In addition, sweet potato leaves contain
STAT, 2021). The tuberous root of sweet potato is abundant polyphenols so diets rich in them may pro-
rich in starch and has been widely utilised as staple vide additional benefits for human health. Reports on
food. However, the leafy part of the sweet potato, the medicinal values of sweet potato leaf polyphenols
which is a great source of dietary fibre, protein, vita- reveal that they have various biological activities,
mins, minerals and antioxidants, is often overlooked including anti-mutagenic, anti-tumour, anti-diabetic
and only taken as vegetables in China, the United and anti-inflammatory activities (Fu et al., 2016).
States, Japan and some African countries (Le Van An As green leafy vegetables, sweet potato leaves could
et al., 2003; Hong et al., 2020). Sweet potato leaves be served as side dishes like spinach, or as a supple-
could be harvested several times throughout the ment to make cookies, noodles, beverages and tea
drinks. Before being eaten by people, they have to be
*Correspondent: Fax: 0086-(0)379-64282342; e-mail: yijunpeng@126. processed in order to eliminate microorganisms or
com, yijunpeng@haust.edu.cn; duanxu_dx@163.com anti-nutritional factors, improve digestibility and

doi:10.1111/ijfs.17014
Ó 2024 Institute of Food, Science and Technology (IFSTTF).
8826 Valorisation of sweet potato leaves as a potential agri-food resource J. Yi et al.

extend shelf life. Examples of conventional processing be a potential way to improve the nutritional quali-
of sweet potato leaves involve domestic cooking, fer- ties of leafy vegetable products.
mentation, drying and so on (Luo et al., 2020a). Tra- Recently, sweet potato leaves were listed as a highly
ditional domestic cooking methods are thermal nutritious vegetable by the Asian Vegetable Research
treatment, including boiling, steaming, stir-frying, and Development Center (Tang et al., 2021). Previous
deep-frying and microwave treatment. Most domestic reviews mainly focused on the nutritional aspects,
cooking treatments could reduce oxalates, the most medicinal qualities and potential health benefits of
important anti-nutrients in sweet potato leaves, to a sweet potato leaves (Johnson & Pace, 2010; Nguyen
greater extent (Abong et al., 2020). In addition, et al., 2021). There is still a lack of information on the
domestic cooking could soften up fibres and improve changes in the nutrients of sweet potato leaves during
palatability and digestibility. However, thermal treat- the domestic cooking and industrial process, which
ment causes a series of changes in the nutrients of may impede the better utilisation of sweet potato

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green leafy vegetables, especially micronutrients, such leaves. This paper summarises the influences of differ-
as phenolics, vitamins and carotenoids (Mehmood & ent cooking methods and processing techniques on the
Zeb, 2020). Sch€ onfeldt & Pretorius (2011) reported nutritional composition and bioactive components
that after 15–40 min cooking in boiling water, the along with the health benefits of sweet potato leaves.
crude protein, crude fat, minerals and vitamin B2 con- Discussion on the applications of sweet potato leaves
tents of most dark green leafy vegetables decreased. as a valuable fortifying ingredient for staples is
Except for pumpkin leaves, cooking of the leaves included.
increased the b-carotene content significantly. Fresh
sweet potato leaves have high moisture contents rang-
Nutrients and phytochemicals of sweet potato
ing from 84.09% to 88.92%, resulting in rapid micro-
leaves
bial spoilage during storage (Sun et al., 2014a). Drying
treatments could prolong the shelf life of leafy vegeta- Unlike other leafy vegetables, sweet potato leaves are
bles and may retain their colour, fragrance and nutri- rich in protein and minerals. Sun et al. (2014a)
ents under properly controlled drying conditions. The reported that the crude protein content in the leaves
most common industrial drying technologies include from 40 sweet potato cultivars ranged between 16.69%
natural drying, forced convection drying, chemical dry- and 31.08% on dry basis (DW) and approximately
ing, microwave drying, freeze drying, radio-frequency 2.99% on fresh weight basis (FW). It is around eight
drying and so on. Drying methods and operating con- times higher than that of spinach (2.9% DW) and two
ditions have significant effect on the nutritional values times higher than that of sweet potato tubers (1.28%–
of leafy vegetables, especially vitamins, polyphenols, 2.13% FW) (Sun et al., 2014a; Murcia et al., 2020).
flavonoids and carotenoids. While the anti-nutrients, Besides, sweet potato leaves possessed a balanced
including phytate, tannin and soluble oxalates, were amino acid profile compared to FAO/WHO reference
usually reduced by the drying treatment (Abong et al., pattern while lysine is the only limiting amino acid
2020). (Ishida et al., 2000). Thus, sweet potato leaves could
Fermentation is a very ancient method of storing serve as a protein supplementary source for human
vegetables while retaining their nutrients, improving diets. The predominant mineral found in sweet potato
sensory properties and conveying probiotics. Indus- leaves is potassium (479–6843 mg/100 g DW), fol-
trial fermented leafy products include sauerkraut, lowed by phosphorus (131–2640 mg/100 g DW),
kimchi, pickles, gundruk, tea, lacto-juices and probi- calcium (230–1958 mg/100 g DW), magnesium (200–
otic beverages (Cagno et al., 2016). Fermentation not 727 mg/100 g DW) and sodium (8–832 mg/100 g
only alters the nutritional profile of leafy vegetables DW). This shows that a much higher potassium-to-
through conversion of macronutrients (i.e. dietary sodium ratio, ranging from 28.1 to 124.2, exists in
fibres, proteins, lipids and polysaccharides) but also sweet potato leaves as compared to spinach (Hong
bio-transforms vitamins, minerals and antioxidants et al., 2020; Tang et al., 2021). Daily intake of dietary
(Leonard et al., 2021). Terefe et al. (2022) reported potassium higher than 2.5 g day 1 with a controlled
an increase in protein content (34.91% increment), sodium intake of 2–4 g day 1 has been reported to be
along with improved in vitro protein digestibility strongly associated with reduction in blood pressure in
(increased by 28.07%) of cassava leaves during 48 h non-medicated hypertensive patients (Binia et al.,
of Lactobacillus plantarum fermentation. Furthermore, 2015). Thereby, sweet potato leaves could be utilised
two anti-nutrients cyanide and oxalate decreased by as dietary supplements for anti-hypertensive effect. The
97.17% and 86.44% respectively. Co-fermentation ferrum content in sweet potato leaves showed more
with Saccharomyces cerevisiae caused a significant than 40-fold variations among the genotypes, ranging
reduction in tannins (93.25%) and phytate (91.11%) from 1.92 to 91 mg/100 g DW (Hong et al., 2020;
in cassava leaves. Fermentation has been approved to Tang et al., 2021). It is comparable to that in spinach

International Journal of Food Science and Technology 2024 Ó 2024 Institute of Food, Science and Technology (IFSTTF).
 Valorisation of sweet potato leaves as a potential agri-food resource J. Yi et al. 8827

(4–35 mg/100 g DW), which was called ‘superfood’ potato leaves are phenolic acids and flavonoids
and recommended as an iron-rich vegetable in particu- (Table 1). Phenolic acids consist mainly of caffeic acid
lar (Murcia et al., 2020). Besides, the amount of oxa- and isomeric caffeoylquinic acids (CQA), including 1-
late present in leaves was reported to be in the range CQA, 3-CQA, 4-CQA, 5-CQA, 4,5-CQA, 3,5-CQA,
0.51–1.62 mg/100 g FW, which is much lower than 3,4-CQA and 3,4,5-CQA (Islam, 2006). Luo et al.
that in spinach (260.30–273.90 mg/100 g FW) (Alam, (2020b) utilised the UHPLC-hybrid quadrupole-
2021). Oxalate is the primary inhibiting factor for the orbitrap mass spectrometry to qualitative analysis of
bioavailability and absorption of iron in diets (Noo- phenolic acids in sweet potato leaves from cultivar
nan & Savage, 1999). It is suggested that vegetarians, Simon No. 1. Thirteen phenolic acids were identified,
especially vegans, could replace spinach with sweet of which esculin, protocatechualdehyde, 7-hydroxycoumarin
potato leaves to decrease the risk of iron deficiencies and ethyl caffeate were first detected in sweet potato
and kidney stone formation. In addition, sweet potato leaves. After quantitative analysis using UHPLC-

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leaves are superior source of dietary fibres, ranging DAD, three di-CQA were prominent and accounted
from 37.28 to 51.8 g/100 g DW (approximately 3.85– for 60.6% of phenolic acids. Flavonoids in sweet
7.23 g/100 g FW), similar to spinach (3.5 g/100 g FW) potato leaves were reported mainly composed of
and contain the highest amount of soluble dietary fibre anthocyanins, rutin, hyperoside, isoquercitrin, astra-
among each part of sweet potato (Hong et al., 2020; galin, quercetin, tiliroside, kaempferol, luteolin, api-
Ishida et al., 2000; Su
arez et al., 2020). These findings genin, myricetrin, diosmetin, jaceosidin, chrysin and
suggest that sweet potato leaves could be considered pectolinarigenin (Liu et al., 2019; Luo et al., 2020b).
comparable or superior to the traditional vegetable Fifteen kinds of anthocyanin compounds were deter-
spinach in terms of nutrient profile. mined in sweet potato leaves from different varieties
Sweet potato leaves contain considerable amounts while the major anthocyanin composition is cyanidin
of water-soluble vitamins, especially B complex vita- type (Vishnu et al., 2019). Since the cyanidin-type
mins and vitamin C (Isabelle et al., 2010). In recent anthocyanins possess superior anti-mutagenic activity
studies, higher content of vitamin C (77.69– than peonidin ones, the anthocyanin extracts of sweet
511 mg/100 g) has been reported in sweet potato potato leaves showed a powerful anti-carcinogenic
leaves as compared to cucumber, white radish and effect against multiple cancer cell types (Vishnu et al.,
spinach (Isabelle et al., 2010; Tang et al., 2021). As a 2019).
dietary bioactive compound, vitamin C is important Phenolic acids and flavonoids in sweet potato leaves
for the human body’s function. Consumption of 18– showed diverse beneficial bioactivities, including
100 g of sweet potato leaves (per serving) could meet antioxidant activity, anti-LDL oxidation activity, anti-
the requirements as per dietary reference intakes cytotoxic activity, anti-proliferation activity and hypo-
(DRI). The vitamins B2 (1.02–4.69 mg/100 g DW) and glycaemic effect. Both sweet potato leaf phenolic acids
B3 (0.55–0.58 mg/100 g DW) content in sweet potato and sweet potato leaf flavonoids were better inhibitors
leaves was higher than those in wheat, which are of a-glucosidase and a-amylase than acarbose (Luo
known as a good source of B vitamin (Shewry et al., et al., 2020b). 3,4,5-CQA in the sweet potato leaves
2011; Luo et al., 2020a). Sweet potato leaves contain exhibited the strongest a-glucosidase inhibition while
considerable amounts of carotenoids, ranging between the more caffeoyl groups bound to quinic acid and
47.92 and 119.28 mg/100 g, which is 32.69 times lower methoxylation of flavonols resulted in stronger
higher than spinach (Bunea et al., 2008; Hong a-glucosidase inhibition. Islam (2016) reported that
et al., 2020). Sweet potato leaves contain a substantial caffeoylquinic acid derivatives in sweet potato leaves
amount of chlorophyll a (89.11–518.6 mg/100 g DW) were capable of inhibiting reverse mutations while
and chlorophyll b (26.08–193.6 mg/100 g DW) which 3,4,5-COA showed the strongest anti-mutagenicity. In
was 7.6–14.7 times higher than that of stems and 4.02– addition, flavonoid extracts of sweet potato leaves pos-
29.84 times higher than that of raw spinach (Chen & sessed good antioxidant activity compared to soy iso-
Chen, 1993; Li et al., 2017; Murcia et al., 2020). flavones, ginkgo biloba extract and propolis flavone
Polyphenols are involved in the protective mecha- (Liu et al., 2019). For sweet potato leaf polyphenols,
nism against oxidative stress (Islam, 2006). Several their O2 -scavenging activity was 5.84 and 6.20 times
health and pharmacological functions of sweet potato higher than tea polyphenols and grape seed polyphe-
leaves are attributed to the presence of polyphenols in nols respectively. The oxygen radical absorption
it. The total phenolic compounds are found in great capacity was 1.28 and 1.27 times higher than tea poly-
amounts in sweet potato leaves, ranging from 87.74 to phenols and grape seed polyphenols respectively (Sun
13 113 mg/100 g DW, varied greatly among cultivar et al., 2016). Besides, it was reported that oral admin-
genotypes and between growth stages (Sun et al., istration of sweet potato leaf polyphenols can effec-
2014a; Islam, 2016; Krochmal-Marczak et al., 2020). tively decrease fasting blood glucose levels, reverse
The predominant phenolic compounds present in sweet dyslipidaemia, relieve liver inflammation and maintain

Ó 2024 Institute of Food, Science and Technology (IFSTTF). International Journal of Food Science and Technology 2024
8828 Valorisation of sweet potato leaves as a potential agri-food resource J. Yi et al.

Table 1 Major polyphenolic compounds identified in sweet the islet structure to b-cell apoptosis of the type 2 dia-
potato leaves betes mellitus mice (Luo et al., 2021).
Harvesting sweet potato leaves at different vegeta-
Chemical
Phenolic compounds structures Contents (mg/g DW)
tive stages and frequencies significantly affects the con-
tents of nutrients and phytochemicals. Su arez
Phenolic Caffeic acid 0.15–0.04 (Fu et al. (2020) compared the nutritional and phenolic
acid et al., 2016) composition of sweet potato leaves harvested at differ-
16 (Luo et al., 2020b)
ent vegetative stages. It was reported that sweet potato
4.62 (Sun
et al., 2014b)
leaves harvested in the third stage (21 September),
6.1 (Liu et al., 2019) 1 month after the initial harvest, had the highest levels
22.2 (Islam, 2016) of vitamin C, vitamin E, polyphenols and antioxidant
5-Caffeoylquinic 57 (Luo et al., 2020b) activity. But they also exhibited a l5% reduction in the

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acid 2.58 (Sun protein content compared to leaves harvested in the
et al., 2014b) first stage. The accumulation of caffeic acid, 3,4,
3-Caffeoylquinic 0.03–4.11 (Fu 5-CQA, 4,5-CQA and anthocyanin in sweet potato
acid et al., 2016)
leaves with maturity was detected in the leaves of both
30.1 (Luo
et al., 2020b)
orange-fleshed and white-fleshed sweet potatoes (Su
3.06 (Sun et al., 2019; Suarez et al., 2020). According to the find-
et al., 2014b) ings of grey relational analysis, the third stage was
4-Caffeoylquinic 24.8 (Luo considered the optimal period for leaf harvesting
acid et al., 2020b) because leaves in this stage had the maximum compre-
13.55 (Sun hensive nutritional value (Suarez et al., 2020).
et al., 2014b)
Repeated harvesting would lead to a reduction in pro-
4,5- 3.56–8.12 (Fu
Dicaffeoylquinic et al., 2016)
tein content and an increase in carbohydrate levels
acid 68 (Luo et al., 2020b) (Pace et al., 1988). However, the cumulative leaf pro-
27.23 (Sun tein yield over harvest stages (120 days), with a 20-day
et al., 2014b) harvest interval, reached as high as 797 kg ha 1. It
76.4 (Liu et al., 2019) was more than twice the protein yield obtained when
432.3 (Islam, 2016) leaves were harvested only once at the final stage (Le
3,5- 0.05–11.21 (Fu Van An et al., 2003). Sasaki et al. (2015) studied the
Dicaffeoylquinic et al., 2016)
variation of CQA profiles in the sweet potato leaves
acid 450 (Luo
et al., 2020b)
harvested monthly. They found that repeated harvest-
25.02 (Sun ing significantly decreased the content of 4-CQA and
et al., 2014b) 5-CQA while there was no notable impact on the
845.6 (Islam, 2016) levels of CA, di-CQAs and tri-CQA. Furthermore,
3,4- 0.36–2.01 (Fu during the spring–summer growing season (April to
Dicaffeoylquinic et al., 2016) August), the tuber productivity of sweet potatoes
acid 84.8 (Luo
would not be affected by the harvesting intervals of
et al., 2020b)
14.18 (Sun
leaves. However, during the winter–spring season
et al., 2014b) (December to April), longer harvesting intervals
163 (Islam, 2016) resulted in lower tuber yield. Leaf harvesting every
3,4,5- 25 (Luo et al., 2020b) 30 days with the removal of 50% of the sweet potato
Triaffeoylquinic 10.68 (Sun vines only caused a reduction of approximately 20%
acid et al., 2014b) in tuber yield (Le Van An et al., 2003).
6.5 (Liu et al., 2019)
2.21 (Islam, 2006)
49 (Islam, 2016) Effect of processing on the nutritional value of
Flavonoid Rutin 47.2 (Luo sweet potato leaves
et al., 2020b)
There are plenty of domestic and industrial processing
Quercetin 49 (Luo et al., 2020b) methods to improve the sensory quality, nutrient bio-
12.5 (Liu et al., 2019) availability, shelf life and safety of sweet potato leaves
Isoquercetin 48 (Luo et al., 2020b) in order to meet novel consumer demands. Various
62.4 (Liu et al., 2019) processing treatments to which sweet potato leaves are
Anthocyanin 0.00107–0.00365 (Fu subjected can be classified as domestic cooking, fer-
et al., 2016) mentation, drying treatment and other multiple pro-
cesses. They could alter the composition of nutrients

International Journal of Food Science and Technology 2024 Ó 2024 Institute of Food, Science and Technology (IFSTTF).
 Valorisation of sweet potato leaves as a potential agri-food resource J. Yi et al. 8829

and health-promoting compounds in sweet potato phenolic retention in sweet potato leaves that are con-
leaves (Onyimba et al., 2010; Sun et al., 2014b; Sui sistent with steamed red radish, kale, broccoli and
et al., 2019). white cabbage (Sß eng€
ul et al., 2014).
The total content of caffeoylquinic acid derivatives
was initially found in low concentration in raw sweet
Effect of domestic cooking process on the nutritional
potato leaves. Processing of sweet potato leaves by
composition of sweet potato leaves
steaming, baking and frying led to the formation of
Commonly, sweet potato leaves are consumed after these compounds in large quantities (Sun et al.,
domestic cooking to improve their digestibility, nutri- 2014b). Notably, the highest total content of caffeoyl-
ent bioavailability and palatability. Thermal degrada- quinic acid in sweet potato leaves was observed
tion, oxidation, polymerisation and Maillard reaction during baking (Sun et al., 2014b). However, content of
are involved in the domestic treatment and would 5-CQA in the baked leaves was significantly lower

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modify the nutritional profile of sweet potato leaves compared to that in the raw. Ferracane et al. (2008)
(Table 2). discovered that the intramolecular ester exchange reac-
Sun et al. (2014b) found that steaming, boiling, tion of 5-CQA is enhanced by heat treatment. There-
microwaving and baking could reduce the contents of fore, the substantial decrease in the content of 5-CQA
crude fat and crude fibre in processed sweet potato in baked sweet potato leaves can be attributed to this
leaves, but the crude protein content increased signifi- phenomenon. A significant decrease in the content of
cantly in the processed leaves. It was attributed to the the major isomer (3,5-diCQA) compared to raw sweet
partial loss of water-soluble fatty acids and dietary potato leaves was only observed in boiled sweet potato
fibres or destroyed volatile fats and fibres during the leaves. Furthermore, the content of other major CQA
heat treatment. Fried leaves had a much higher level isomers, such as 3,4-diCQA and 4,5-diCQA, demon-
of crude fat which was around 10 times as much as strated a greater susceptibility to leaching rather than
that of the raw. This is due to the fact that frying oils heat exposure during the cooking process (Sun et al.,
penetrate into the leaf matrix and replace water. The 2014b).
increase in the fibre content of fried leaves was attrib-
uted to the significant loss of liposoluble components
Effect of drying treatment on the nutritional composition
caused by cell damage during frying. Comparable
of sweet potato leaves
results were also found in the fried bitter melon and
fried red cabbage (Donya et al., 2007; Wu et al., As leafy vegetables, sweet potato leaves are susceptible
2021a). to aflatoxin contamination and microbial spoilage at
Phenolics are the major health-promoting com- ambient temperatures due to its high moisture content.
pounds in sweet potato leaves (Islam, 2006). Cooked Drying treatment, one of the oldest methods for food
by boiling, microwaving and frying led to a significant preservation, could inactivate deterioration and
decrease in the total phenolic content of sweet potato pathogenic-related enzymes and microorganisms by
leaves, with a loss of 30.51%, 25.70% and 15.73%, removal of water and reduction in water activity that
respectively, compared with the raw. On the contrary, improves the stability of vegetables against biochemi-
phenolics in sweet potato leaves retained over 100% cal degradation. Several drying methods such as hot
after steaming and baking (Sun et al., 2014b). Heat- air drying, microwave vacuum drying, vacuum freeze
treatment-induced inactivation of polyphenol oxidase drying and airflow drying have been applied to dehy-
enzymes and collapse of the cellular structure would drate sweet potato leaves (Sugiura & Watanabe, 2011;
release more polyphenol compounds in the food Jeng et al., 2015; Sui et al., 2019). Sui et al. (2019)
matrix (Yamaguchi et al., 2003). Improved accessibility reported that vacuum freeze drying retained most of
caused higher extraction efficiency and substantial loss the carbohydrates, proteins, fats, dietary fibres, vita-
of phenolics that diffused into boiling water and frying mins (C, E, B1 and B2), minerals (Mg, P and Zn) and
oil due to the lixiviation phenomenon. Besides, micro- polyphenols compared to hot air drying and micro-
wave energy absorption led to an increase in the tem- wave vacuum drying. The vacuum-freeze-dried sweet
perature inside the leaf cells, which may cause the potato leaves showed higher antioxidant activity, being
degradation of some thermo-labile phenolics (Sun 1.09 and 2.56 times higher than microwaves-dried
et al., 2014b). Highest loss in caffeoylquinic acid con- products and hot-air-dried products respectively. But
tent of blueberries (23.4%–29.0%) after being micro- microwave-dried sweet potato leaf powder showed the
waved has been reported, with comparison of baking highest water absorb index, oil absorption capacity,
and boiling treatment (Zhao et al., 2017). It could be swelling power and solubility, as well as the lowest
responsible for the significant decrease in the total phe- activity of polyphenol oxidase. Destruction of leaf
nolic content of sweet potato leaves cooked by micro- matrix and loosening of the fibre microstructure
waving (Table 2). Steaming led to positive results for caused by microwave irradiation could lead to cavity

Ó 2024 Institute of Food, Science and Technology (IFSTTF). International Journal of Food Science and Technology 2024
 8830

Table 2 Effects of domestic cooking methods on the nutritional profiles of sweet potato leaves

Cooking
methods Cooking conditions Protein Fat Fibre Lutein b-Carotene Phenolics Flavonoids Oxalates Reference

Boiling Material–water ratio 1/1 g mL 1, 20 min NA NA NA 12.77%– 81.89%– 5.99%– 20.10%– 0.45%%– Abong et al.
133.85%↑ 88.03%↑ 72.53%↓ 42.51%↑ 23.12%↓ (2020)
Material–water ratio 1/5 g mL 1, 2 min 6.32%↑ 27.33%↓ 7.59%↓ NA NA 30.51%↓ NA NA Sun
et al. (2014b)
Material–water ratio 3/5 g mL 1, 7.93 min 35.94%↓ NA 23.26%↓ NA 69.31%↓ 27.68%↓ 23.52%↓ NA Gouekou
et al. (2021)
Material–water ratio 4/5 g mL 1, 10 min 6.95%↓ NA 17.88%↑ NA 55.08%↓ 9.68%↓ 20.88%↓ NA Gouekou

International Journal of Food Science and Technology 2024

et al. (2021)
Material–water ratio 4.41/5 g mL 1, 15 min 8.52%↓ NA 6.97%↑ NA 52.36%↓ 3.51%↓ 13.69%↓ NA Gouekou
et al. (2021)
Steaming Weight 100 g, 2 min 19.22%↑ 24.37%↓ 8.12%↓ NA NA 9.44%↑ NA NA Sun
et al. (2014b)
Microwaving Microwave power 10/1 W g 1, 2 min 16.93%↑ 3.49%↓ 11.30%↓ NA NA 25.70%↓ NA NA Sun
et al. (2014b)
Valorisation of sweet potato leaves as a potential agri-food resource J. Yi et al.

Microwave power 14/1 W g 1, 2 min NA NA NA 30.48%↓ 25.26%↓ NA NA NA Chen &

Chen (1993)
Microwave power 14/1 W g 1, 4 min NA NA NA 41.60%↓ 52.48%↓ NA NA NA Chen &
Chen (1993)
Microwave power 14/1 W g 1, 8 min NA NA NA 55.97%↓ 71.08%↓ NA NA NA Chen &
Chen (1993)
Baking Weight 100 g, 204°C, 2 min 22.96%↑ 23.00%↓ 3.27%↓ NA NA NS NA NA Sun
et al. (2014b)
Frying Oil–material ratio 1/10 mL g 1, 2 min, NA NA NA NA NA 53.56%↓ NA 55.82%↓ Mwanri
cooked with lemon et al. (2011)
Oil–material ratio 1/10 mL g 1, 2 min, NA NA NA NA NA 19.13%↓ NA 53.54%↓ Mwanri
cooked without lemon et al. (2011)
Oil–material ratio 1/2 mL g 1, 180°C, 2 min 28.83%↓ 877.68%↑ 81.02%↑ NA NA 15.73%↓ NA NA Sun
et al. (2014b)

The ↑ and ↓ indicate an increase and decrease respectively. NA, not available; NS, not significant.

Ó 2024 Institute of Food, Science and Technology (IFSTTF).

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 Valorisation of sweet potato leaves as a potential agri-food resource J. Yi et al. 8831

creation that enables better physical entrapment of detected for polyphenol oxidase below 15 °C. The bet-
water and oil. Exposure of the lipophilic groups on ter retention of polyphenols in dehydrated sweet
the surface of the fibre improved the functional prop- potato leaves after microwave drying compared to hot
erties of dehydrated sweet potato leaves as well (Sui air drying could be attributed to the more effective
et al., 2019; Zhao et al., 2023). Microwaves-dried inactivation of polyphenol oxidase activity and less
product has appreciable functional properties that degradation caused by short heating duration (Sui
could be exploited in food formulations such as cook- et al., 2019).
ies, noodles and breads (Kaur et al., 2021).
Polyphenols are the main functional compounds in
Effect of fermentation on the nutritional composition of
sweet potato leaves and most of them are thermally
sweet potato leaves
sensitive and reactive. Isomerisation and enzymatic
oxidation have been considered primarily responsible Fermented vegetables have been a part of human

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for the degradation of phenolic compounds (Machei- diet for centuries. During fermentation, vegetable
ner et al., 2021). Jeng et al. (2015) reported a signifi- undergoes a series of biochemical changes, such as oxi-
cant loss of dicaffeoylquinic acids in 30 °C and 70 °C dation, polymerisation, decomposition and transforma-
hot-air-dried sweet potato leaves, whereas an increase tion, induced by controlled microbial digestion and
in the contents of 4-caffeoylquinic acid and 5- enzymatic conversion. It produces multiple end prod-
caffeoylquinic acid. It may relate to the decomposition ucts, such as organic acids, alcohols, peptides, free
of dicaffeoylquinic acids and following isomerisation amino acids and other metabolites for improvement of
of mono-caffeoylquinic acids caused by heat treatment. the nutritional, sensory and preservation characteristics
However, 100 °C hot air drying of sweet potato leaves of vegetables (Boekel et al., 2010; Xiang et al., 2019).
led to massive loss in all of the caffeoylquinic acid Beyond these characteristics, fermented vegetables have
derivatives and flavonoids (Jeng et al., 2015; Liu been found to be important in the promotion of human
et al., 2020). It appears that both of them are sensitive health in ways attributable to the bioactive components
to heat treatment and the suggested specific level for forming upon fermentation. Besides, fermented vegeta-
the heat stability of polyphenols is 75 °C (Liu et al., bles could serve as carriers for probiotics that are bene-
2020). Sweet potato leaves treated by freeze-drying ficial to health of an individual by improving the
maintained higher contents of total caffeoylquinic acid balance of the gut microbiota (Ajibola et al., 2023).
derivatives and corresponding higher antioxidant activ- In the 24-h nature-fermented sweet potato leaves,
ity than those treated with hot air drying (Jeng L. plantarum was identified, along with another probi-
et al., 2015). That could be explained by the inactiva- otic Enterococcus durans, accounting for 1.19% and
tion of polyphenol oxidase under freezing temperature 14.9% of relative abundance respectively (Su arez
(Torres et al., 2021). Torres et al. (2021) found that et al., 2022). They could be served as potentially desir-
caffeoylquinic acid derivatives are the favourable sub- able probiotic foods (Fig. 1). Due to the short fermen-
strates for polyphenol oxidase while no activity was tation time, no significant difference in both fat and

Figure 1 Effects of fermentation on the nutrient profile and antioxidant activity of sweet potato leaves.

Ó 2024 Institute of Food, Science and Technology (IFSTTF). International Journal of Food Science and Technology 2024
8832 Valorisation of sweet potato leaves as a potential agri-food resource J. Yi et al.

protein contents has been found in the fermented However, probiotic fermentation could produce much
leaves compared to the non-fermented leaves. Nature higher contents of 5-caffeoylquinic (72% increase) and
fermentation after 20 days, a substantial reduction in chlorogenic acid (17% increase) compared to the
the free amino acid contents, was detected in the sweet unfermented leaves. It was attributed to the intercon-
potato leaves that were consumed by microbes and version between di-caffeoylquinic acid into mono-
resulted in the formation of volatile compounds from caffeoylquinic acid (Suarez et al., 2022).
microbial amino acid metabolism. Secretion of organic
acids by the bacterial cells during fermentation led to
Food formulation
an increase in the lactic acid and citric acid contents,
more than twice that of the non-fermented leaves (Cui Because of their rich nutritional profile, sweet potato
et al., 2011). Sweet potato leaf meal has been incorpo- leaves were utilised at domestic or commercial level as
rated into the production of fermented animal feed a valuable fortifying ingredient in many food formula-

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through agro-industrial byproducts and agricultural tions. Like common green vegetables, such as spinach,
residues. Natural fermentation of a 1:1 mixture of broccoli, Amaranthus leaf and Swiss chard, sweet
spent sorghum grain and sweet potato leaves over a potato leaves could be added to the traditional staple
period of 3 weeks effectively increased protein, lipid, food as dietary fibre supplements or natural pigment
calcium and phosphorus contents of the mixed sub- and sensory additives (Silva et al., 2013; L
opez-Nicolas
strate by 25.43%, 81.29%, 114.42% and 195.45% et al., 2014; Qumbisa et al., 2022). Ishida et al. (2003)
respectively (Onyimba et al., 2010). Solid-state fermen- studied the sensory characteristics and preservation
tation of sweet potato leaf meal by fungus Chaeto- stability of homemade cookies with 2% replacement of
mium globosum for 120 h also maximised the efficient wheat flour with sweet potato leaf powder. Addition
use of feed protein (Meshram et al., 2018). The crude of sweet potato leaves with high content of chlorophyll
protein and lipid content in fermented sweet potato produces cookies with a green colour and green-leaf
leaf meal increased from 23.92% and 5.55% to odour. Besides, chlorophyll exhibits high antioxidant
30.34% and 7.05% respectively. Dietary replacement and anti-mutagenic activities when applied in foods
of de-oiled rice bran with fermented sweet potato leaf but its sensitive nature to light and heat limited its
meal significantly increased the growth performance applications (Nabi et al., 2023). After 8 months of
and feed utilisation of Labeo rohita fingerling. A 100% storage in a dark room at 37 °C, the retention in chlo-
replacement led to the highest weight gain of 98.1%, rophyll in the sweet potato leaf cookie was 62.3%
specific growth rate of 1.13 and protein efficiency ratio ~ 68.9% which is acceptable. However, the peroxide
of 1.82, after a 60-day feeding trial, compared to the value of the lipid fraction in sweet potato leaf cookies
feed of de-oiled rice bran, 50% replacement and 100% increased more compared to the leaf-free cookies,
replacement of raw sweet potato leaf meal. Further- whereas the increased rate of acid value was compara-
more, the anti-nutritional factors in the sweet potato ble (Ishida et al., 2003). Hu et al. (2022) produced
leaves, including phytate, oxalate, alkaloid, tannins steamed bread by replacing wheat flour with 2%
and hydrogen cyanide, were reduced after fermenta- ~ 16% sweet potato leaf powder. The colour of bread
tion by 37.55%, 30.88%, 46.32%, 62.96% and 40.62% became greener and darker as replacement level was
respectively (Meshram et al., 2018). increased because of degradation of chlorophyll and
No matter whether it is natural fermentation or pro- formation of magnesium chlorophyll after fermenta-
biotic fermentation, loss of phenolic compounds in the tion and cooking. A 16% replacement led to an
sweet potato leaves was detected and led to a decrease increase in the contents of protein, fat and dietary
in the antioxidant activity of methanolic extracts fibre of 15.06%, 27.16% and 49.90%, respectively,
(Suarez et al., 2022). Degradation of phenolic com- whereas the glycaemic index of bread significantly
pounds present in some plant-derived foods by decreased from 95.25% to 87.51%. Higher amount of
L. plantarum has been found in the fermentation of dietary fibre and resistant starch, along with abundant
avocado leaves, apple juice, olive products and so on polyphenols which could form complex with starch
(Li et al., 2018; Landete et al., 2021; Montijo-Prieto and bind to amylolytic enzymes, in the steamed bread
et al., 2023). Hydroxycinnamic acids, including caffeic with sweet potato leaves addition could effectively
acid and chlorogenic acid (3-caffeoylquinic acid), could modulate the starch digestion. A 14% replacement
be released from the naturally esterified forms in the bread has the highest polyphenol content which is
plant by esterase action and subsequently metabolised seven times higher than that of plain bread and
by decarboxylation during L. plantarum fermentation showed vigorous antioxidant activity which is four
(Landete et al., 2021). In the nature-fermented sweet times as compared with the plain one. Besides, the
potato leaves, contents of caffeic acid, tricaffeoylquinic addition of sweet potato leaf powder brings high levels
acid, dicaffeoylquinic acids, chlorogenic acid and 4- of B vitamins and minerals (Na, P, Ca, K, Mg, Fe,
caffeoylquinic acid were decreased significantly. Zn, Cu and Mn). It reveals that dietary replacement

International Journal of Food Science and Technology 2024 Ó 2024 Institute of Food, Science and Technology (IFSTTF).
 Valorisation of sweet potato leaves as a potential agri-food resource J. Yi et al. 8833

by sweet potato leaves could effectively improve the processing. However, contents of carbohydrates, solu-
nutritional values of steamed bread and its health ben- ble fibres, amino acids, vitamin B2, vitamin E,
efits for humans. b-carotene, Na and Se increased significantly which
Apart from its rich nutritional profile, sweet potato was attributed to the dissolution of the hydrophobic
leaves contain high amounts of phytochemicals, waxy layer on leaf surfaces and disruption of cell
including lutein, caffeic acid, quercetin, chlorogenic membranes after blanching. There are significant losses
acid, anthocyanins and caffeoylquinic acid derivatives in water-soluble nutrients, including fatty acids, K, Mg
(Islam, 2006). Marques et al. (2022) reported that b- and vitamin C, caused by leaching and diffusion. The
caryophyllene, a-caryophyllene and d-cadinene were total phenolic content in blanched sweet potato leaf
the characteristic aroma of the essential oils extracted powder was reduced by 23.52% due to its thermal sen-
from sweet potato leaves. They are the major scent sitivity, resulting in a significant drop in antioxidant
components present in many species of mountain teas activities (Luo et al., 2020a).

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and provide an important contribution to the woody In addition, other traditional regional food formulae
and herbal aroma attributes of sweet potato leaves. incorporated with sweet potato leaves that are being
Therefore, sweet potato leaf could be a potential ingre- consumed worldwide include bread, pickle, vinegar,
dient for herbal teas and functional beverages. dairy beverage and so on (Wu et al., 2021b; Wei
Mbouche et al. (2019) produced specific green teas et al., 2016; Jiwung et al., 2022; Zhou, 2016). The
after withering, fixing, rolling and drying two varieties nutritional and functional characteristics of sweet
of sweet potato leaves cultivated in Cameroon. Sen- potato leaves in these food formulae should be
sory properties and phytochemicals content of the explored in the future.
sweet potato leaf tea infusions were analysed and com-
pared to two commercial green teas. The results
Conclusions and perspectives
revealed that the infusion of sweet potato leaf tea has
therapeutic potential since it contains various second- Sweet potato is an important non-cereal staple food
ary metabolites, namely flavonoids, tannins, alkaloids, crop with drought tolerance cultivated worldwide. Its
quinones, saponins and phenols, similar to commercial leaves could be safely consumed as a leafy vegetable in
green tea. Cytotoxicity test using brine shrimp model dishes, soups and stews. Considering their high yield
confirmed that sweet potato leaf tea had no or few and great nutritive value, sweet potato leaves offer
effects on cellular metabolism while the LC50 value great potential in the light of imminent challenges
was over 1000 lg mL 1 (Manasathien & Khanema, associated with increasing demand for vegetables.
2022). So daily intake of sweet potato leaf tea infu- Therefore, assessment of the nutritional loss or
sions for physiological and pharmacological benefits enhancement of sweet potato leaves in food processing
was considered safe. Both phenolic content and flavo- is necessary in the pursuit of sustainable food produc-
noid content in sweet potato leaf teas varied signifi- tion and healthy diets. Commonly, domestic cooking
cantly among varieties, along with tannins, chlorophyll methods lead to great loss of vitamins and bioactive
and anthocyanins. However, there was no significant compounds. Drying and low-temperature treatment
difference in colour, odour, taste and overall accept- could effectively extend the shelf life and better pre-
ability between sweet potato leaf teas and commercial serve nutritional factors of sweet potato leaves but
green teas. Due to withering treatment in tea proces- does decrease their flavour and palatability. Fermenta-
sing, contents of protein and phenolic compounds tion and supplementation as fortifying ingredients in
were significantly reduced compared to the fresh sweet food formulations could improve the nutritional status
potato leaf, ranging from 0.19 to 0.13 g/100 g DW for and sensory characteristics of the final products. How-
protein and 343.27 to 1260.92 mg GAE/100 g DW ever, there are few studies concerning metabolites and
for phenolics (Mbouche et al., 2019). It was caused by microbial characteristics of sweet potato leaf fermenta-
breakdown of protein by peptidase and oxidation of tion and resulting flavour and aroma characteristics.
polyphenols by polyphenol oxidase during withering Furthermore, the interaction between leaf additives
under humid and oxygen conditions (Fu et al., 2024). with food matrix components and the effect of leaf
Luo et al. (2020a) developed a nutritional beverage additives on the rheological properties of dough was
using blanched sweet potato leaf powder, with addi- not revealed so far. Processing technologies will be
tion of 2.5% xanthan gum, 1% calcium lactate, 2% improved based on advances in these understandings.
ascorbic acid, 12% maltodextrin, 20% xylitol and Further exploration is needed to enhance the bioavail-
0.9% apple essence. The ultrafine leaf powder had ability of micronutrients and phenolic compounds in
high suspension stability in the beverage and provided sweet potato leaf-based food with targeted therapeutic
consumer-acceptable flavour and texture. No loss of effects. Overall, there are research opportunities for
protein, vitamin C, vitamin B3, folic acid and some better valorisation of sweet potato leaf in food
minerals (P, Ca and Cu) was detected during beverage formulation.

Ó 2024 Institute of Food, Science and Technology (IFSTTF). International Journal of Food Science and Technology 2024
8834 Valorisation of sweet potato leaves as a potential agri-food resource J. Yi et al.

Cui, L., Liu, C., Li, D. & Song, J. (2011). Effect of processing on
Ethical guidelines taste quality and health-relevant functionality of sweet potato tips.
Agricultural Sciences in China, 10, 456–462.
Ethics approval was not required for this research. Donya, A., Hettiarachchy, N., Liyanage, R., Lay, J., Chen, P. &
Jalaluddin, M. (2007). Effects of processing methods on the proxi-
mate composition and momordicosides K and L content of bitter
Acknowledgments melon vegetable. Journal of Agricultural and Food Chemistry, 55,
5827–5833.
This work was financially supported by the Leading FAOSTAT. (2021). Crops and livestock products. Food and Agri-
Talents of Science and Technology in the Central Plain culture Organization of the United Nations. Retrieved from:
of China (NO. 234200510020). https://www.fao.org/faostat [Accessed 12th August 2023]
Ferracane, R., Pellegrini, N., Visconti, A. et al. (2008). Effects of dif-
ferent cooking methods on antioxidant profile, antioxidant capac-
Conflict of interest statement ity, and physical characteristics of artichoke. Journal of
Agricultural and Food Chemistry, 56, 8601–8608.

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The authors declare no competing interests. Fu, Z., Chen, L., Zhou, S., Hong, Y., Zhang, X. & Chen, H. (2024).
Analysis of differences in the accumulation of tea compounds
under various processing techniques, geographical origins, and har-
Author contributions vesting seasons. Food Chemistry, 430, 137000.
Fu, Z., Tu, Z., Zhang, L., Wang, H., Wen, Q. & Huang, T. (2016).
Junpeng Yi: Supervision; resources; funding acquisition. Antioxidant activities and polyphenols of sweet potato (Ipomoea
Luyao Li: Writing – original draft; writing – review batatas L.) leaves extracted with solvents of various polarities.
and editing; supervision. Xin Li: Writing – review and Food Bioscience, 15, 11–18.
Gouekou, D.A., Guede, S.S., Gbogbo, M., Agbo, E.A., N’dri, D.Y.
editing; supervision; funding acquisition. Xu Duan: & Gbogouri, A.G. (2021). Impact of cooking conditions of sweet
Supervision; resources. Junling Wang: Supervision. potato leaves (Ipomoea batatas) on the hematological and biochem-
Yuxin Han: Supervision. Yan Gao: Supervision. ical parameters of the rats (Wistar). Journal of Food and Nutrition
Research, 9, 23–30.
Hong, J., Mu, T., Sun, H., Richel, A. & Blecker, C. (2020). Valori-
Data availability statement zation of the green waste parts from sweet potato (Impoea batatas
L.): nutritional, phytochemical composition, and bioactivity evalu-
The authors declare that the data supporting the find- ation. Food Science & Nutrition, 8, 4086–4097.
ings of this study are available within the article. Hu, Y., Sun, H. & Mu, T. (2022). Effects of sweet potato leaf powder
on sensory, texture, nutrition, and digestive characteristics of steamed
bread. Journal of Food Processing and Preservation, 46, 1–10.
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