Valorizing cabbage (Brassica oleracea L. var. capitata) and capsicum (Capsicum annuum L.) wastes: in vitro health-promoting activities

Abstract

The capsicum seed core and cabbage outer leaves are common wastes generated in the vegetable processing industry. We explored the in vitro health-promoting activity of these waste products for valorization. Freeze-dried and pulverized cabbage wastes had a high bile acid binding capacity and the capsicum wastes inhibited glucose dialysis more effectively. Methanolic extracts prepared with conventional solvent extraction and ultrasound-assisted extraction were analyzed to determine their 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging capacity, in vitro α-amylase inhibitory, in vitro lipase inhibitory, and prebiotic activity. Crude extracts of cabbage and capsicum wastes were screened using GC–MS analysis. The cabbage waste extracts showed high antioxidant activities but did not inhibit α-amylase. The capsicum waste extracts inhibited both lipase and α-amylase activities and supported the growth of the probiotic bacterium, Lactobacilli brevis. Volatile compounds of the vegetables consisted mainly of phenols and fatty acid esters. In all assays except the α-amylase inhibition assay, the extracts prepared with ultrasound-assisted solvent extraction showed higher activity than those prepared using the conventional method. The capsicum seed core and cabbage outer leaves are potential sources of phytochemicals and antioxidant fibers. Capsicum waste extract supported probiotic bacterial growth without a lag phase. These waste products may be processed into high-value functional ingredients.

Electronic supplementary material

The online version of this article (10.1007/s13197-019-03912-5) contains supplementary material, which is available to authorized users.

Introduction

As the world’s population increases, there are greater demands for food products and, consequently, more food waste. Fruit and vegetable wastes accounted for approximately 0.5 billion tons out of 1.3 billion tons of food waste in 2011 (Gustavsson et al. 2011). Vegetable processing chains generate seeds, peels, outer leaves, and unused biomass as waste products. Vegetable wastes can be converted to biofertilizer through vermicomposting (Pattnaik and Reddy 2010). However, these wastes can be made into higher-value products. The discarded parts of fruits and vegetables are often rich in phytochemicals such as flavonoids, essential oils, and glucosinolates that have hypoglycemic and hypolipidemic properties (Chang et al. 2017). Vegetable wastes also contain a large amount of dietary fiber (DF), comprising mainly cell wall constituents. The consumption of DF is related to the prevention of colon cancer, maintenance of cardio-vascular health, reduced serum lipid and cholesterol levels, and delayed absorption and digestion of carbohydrates (Dhingra et al. 2012; Thompkinson et al. 2014; Wang et al. 2015). DF may possess prebiotic activities (Yoo et al. 2012) which support the growth of probiotics from the colonic flora, that lead to risk reduction of both intestinal and systemic pathologies, thus maintain or improve the host’s health (Pandey et al. 2015).

In this study, we determined the in vitro health-promoting activities of capsicum (Capsicum annuum L.) seed cores and cabbage (Brassica oleracea L. var. capitata) outer leaves. Cabbage waste has been used as a substrate for microbial biomass production (Dong et al. 2015) and to produce soluble dietary fiber after an enzymatic treatment (Park and Yoon 2015). Research on capsicum seed cores and other functional properties of cabbage outer leaves is scarce. We tested the in vitro bile acid binding capacity (BABC) and in vitro glucose dialysis retardation index (GDRI) of the freeze-dried and pulverized vegetable wastes. We prepared crude phytochemical extracts from the wastes and analyzed their antioxidant activity, their ability to inhibit α-amylase and lipase in vitro; and their ability to support the growth of a probiotic bacterium, Lactobacilli brevis KCTC 3102. The activities of extracts prepared by conventional solvent extraction (SE) and ultrasound-assisted solvent extraction (UASE) were compared. This study on the functional properties of vegetable wastes will be useful for providing a ‘zero-waste’ option for vegetable producers, distributors, and processors to create high-value functional ingredients from wastes for the nutraceutical sector.

Materials and methods

Sample preparation

Fresh cabbage outer leaves and capsicum seed cores were obtained from a vegetable processing plant, Veg Station (M) Sdn. Bhd., in Perak State, Malaysia. The sorted fresh vegetable wastes were washed, air-dried, and chopped into smaller pieces before freeze drying (Scanvac-Cool Safe 55-4, Labogene, Lynge, Denmark). The dried samples were pulverized, sieved, and stored at 4 °C until further analysis.

Determination of proximate composition

The moisture content of sample powders was determined by measuring weight loss 105 °C, using a moisture analyzer (A&D, MX-50, Chicago, IL, USA). The crude protein, crude fat, crude fiber and ash contents of the dried vegetable wastes were determined by the standard AOAC methods (AOAC 2000), expressed in percentage of dry weight basis (db).

Determination of bile acid binding capacity (BABC)

The hypocholesterolemic potential of the vegetable wastes was analyzed as described by Lim et al. (2018). Briefly, the ground sample was digested in HCl (0.01 M) at 37 °C for 1 h, and then neutralized with NaOH (0.1 M). The controls were prepared without the vegetable extract. The positive control contained bile acid and the blank control contained phosphate buffer (0.1 M) in place of the bile acid. A solution of bile acid (0.7 mM cholic acid, deoxycholic acid, or chenodeoxycholic acid) and pancreatin (activity, 6 × USP; 10 g/L, pH 7) was mixed with the digested sample and incubated at 37 °C for 1 h, followed by centrifugation. The supernatant was collected and analyzed to determine the amount of free bile acid. The supernatant was mixed with H₂SO₄ (70% v/v), and then incubated and reacted with furfural solution (25% v/v) before measuring absorbance at 490 nm using a Genesys 20 spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). The BABC was calculated using Eq. (1):

$$\text{BABC}(\%)=1-\left(\frac{Ab{s}_{positivecontrol}-Ab{s}_{\mathit{sample}}}{Ab{s}_{positivecontrol}}\right)\times 100.$$ 1

Determination of glucose dialysis retardation index (GDRI)

The hypoglycemic potential of the vegetable wastes was evaluated for solid samples obtained by removing most of the soluble carbohydrates with 80% ethanol (Lim et al. 2018). The sample was hydrated with glucose solution (10 mg/mL) at a ratio 2:75 for 1 h. The hydrated sample was transferred to a 13-cm dialysis tube (10,000 molecular weight cut-off) and then placed in a 200-mL distilled water reservoir and incubated at 37 °C in a shaking water bath. At 15-min intervals, 0.1 mL dialysate was sampled and the glucose concentration was determined colorimetrically. The GDRI was calculated for each sample using Eq. (2):

$$\text{GDRI}(\%)=\left(1-\frac{Totalglucoseindialysate,sample}{Totalglucoseindialysate,control}\right)\times 100.$$ 2

Conventional solvent and ultrasound-assisted solvent extraction of vegetable wastes

Aqueous methanol (60%) was added to each sample (dried weight basis, db) at a solid to liquid ratio of 1:20 (w/v). For conventional solvent extraction (SE), the mixture was shaken with an orbital shaker incubator at 250 rpm at 37 °C for 1 h. For ultrasound-assisted solvent extraction (UASE), the samples were disrupted with a 40-kHz ultrasonic treatment 30 min (5510 Ultrasonic Cleaner, Branson Ultrasonics Corp., Danbury, CT, USA) at 37 °C. The mixture was vacuum-filtered, and then the extraction was repeated. The collected filtrates were combined and then concentrated using a rotary evaporator at 40 °C before being further dried at 50 °C. The extracts were stored at 4 °C until further analysis.

Determination of radical scavenging activity

The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity of the extract (0.5–6 mg/mL) was determined as described by Lim et al. (2018), with slight modifications. The DPPH solution (0.15 mM) was added to the sample at a 2:1 ratio and the mixture was left for 30 min in the dark before measuring absorbance at 517 nm. The control contained methanol (99.9%) instead of extract. The radical scavenging activity of each extract was calculated using Eq. (3). Then, the effective concentration of sample extract to scavenge half of the free radicals (EC₅₀) was determined through linear regression analysis.

$$\text{DPPH radical scavenging activity}(\%)=\left(\frac{Ab{s}_{\mathit{control}}-Ab{s}_{\mathit{sample}}}{Ab{s}_{\mathit{control}}}\right)\times 100$$ 3

GC–MS screening analysis

Agilent Technologies gas chromatography–mass spectrometry (GC–MS) system Model 7890A (Agilent Technologies, CA, USA) equipped with HP5-MS capillary column (30 m × 0.25 mm ID × 0.25 µm film thickness) was used to screen the metabolic profiles of cabbage and capsicum extracts. The chromatography protocols were performed according to the method of Hong and Kim (2013). Briefly, 2 µL of samples were injected at an injector temperature of 250 °C and detector temperature of 280 °C in split mode (10:1). The initial temperature of GC oven was at 50 °C for 2 min, was programmed to 280 °C at a rate of 5 °C/min, and was finally held at 280 °C for 2 min. Purified helium was used as the carrier gas at the flow rate of 1 mL/min. The mass spectra analysis was carried out at ionization energy of 70 eV and the samples were analyzed with three replicates each. The volatile compositions detected were confirmed by comparison of spectra with NIST library database.

Lipase inhibition assay

The lipase inhibition assay was conducted as described by Reshmi (2009) and Roh and Jung (2012). A para-nitrophenol palmitate (p-NPP) working solution (16.5 mM p-NPP containing 0.01 M phosphate buffer, pH 8.0, 4% Triton X-100, and 0.1% gum arabic) was mixed with sample extract (2–10 mg/mL) at a 9:1 (v/v) ratio. Porcine pancreatic lipase (5 mg/mL) at the same volume as the sample extract was added, and the mixture was incubated at 37 °C for 25 min before measuring absorbance at 400 nm. Orlistat (1 mg/mL) was used as the inhibition control. The lipase inhibitory capacity was calculated using Eq. (4):

$$\text{Lipase inhibitory capacity}(\%)=1-\left(\frac{Ab{s}_{\mathit{sample}}-Ab{s}_{inhibitioncontrol}}{Ab{s}_{Noinhibitor}-Ab{s}_{inhibitioncontrol}}\right)\times 100.$$ 4

α-Amylase inhibition assay

The α-amylase inhibitory capacity of the crude extract was determined as described by Xiao et al. (2006). Sample extracts (2–10 mg/mL) were prepared using sodium phosphate buffer (20 mM, containing 6 mM NaCl; pH 6.9). Each sample was supplemented with an α-amylase solution (5 U/mL) at a ratio of 1:1 then incubated at 37 °C for 10 min. The same volume of starch solution (1%, w/v) was added and the mixture was re-incubated at 37 °C for 15 min. The reaction was terminated by adding 20 µL HCl (1 M). Then, 100 µL Gram’s iodine reagent (1% w/v) was added before measuring absorbance at 620 nm. Acarbose (50 µg/mL) was used as the inhibition control. The α-amylase inhibitory capacity was calculated using Eq. (5):

$$\mathrm{\alpha }\text{-Amylase inhibitory capacity}(\%)=\left(1-\frac{Ab{s}_{\mathit{sample}}-Ab{s}_{inhibitioncontrol}}{Ab{s}_{Noinhibitor}-Ab{s}_{inhibitioncontrol}}\right)\times 100.$$ 5

Determination of prebiotic activity: survival of Lactobacillus brevis in the crude extract

A single colony of Lactobacillus brevis KCTC 3102 was cultured in de Man, Rogosa, and Sharpe (MRS) broth overnight, in an orbital shaker. The culture was centrifuged at 5 °C, and the pellet containing bacterial cells was washed and diluted with sterile saline (0.9%) to an OD₆₂₀ of 0.6 (López de Lacey et al. 2014) and then further diluted 10⁴ times. Plant extracts (0.1% and 10% w/v) were prepared, passed through a 0.20 µm filter, and then mixed with the bacterial suspension at a ratio of 20:1 (v/v). The mixture was incubated at 37 °C for 48 h. Approximately 50 µL culture was sampled at 24-h intervals, inoculated onto MRS agar using the spread plate method, and then viable cells were quantified after 24 h of incubation (Online Resource Fig. 1). The viable cell count was determined using Eq. (6):

$$\text{Number of bacteria}(\text{CFU/mL})=\frac{Numberofcolonies(\text{CFU})}{Dilution\times Volumeplated(\text{mL})}.$$ 6

Experimental design and statistical analysis

Freeze-dried and pulverized samples were used for BABC and GDRI determinations, and triplicate samples were analyzed. Similarly, methanolic extractions (SE and UASE) were carried out twice and mixed thoroughly and triplicate samples were analyzed. Data were tested by t test or ANOVA where appropriate. Differences were considered significant at P < 0.05.

Results and discussion

The sieved capsicum seed core powder was dark green and the cabbage outer leaf powder was greenish-yellow. The color of the powders was more intense than that of the fresh samples because the water loss increased the compactness of macromolecules. Freeze-drying removed 93.6–94.4% of the initial weight from both vegetable wastes. The capsicum waste contained 22.2% crude protein, 31.3% crude fiber, and 15% ash content (db) with extremely low amount of fat while the cabbage waste contained 19.1% crude protein, 10.1% crude fiber, 0.5% fat and 9.2% ash content (db).

Solid vegetable wastes: potential hypocholesterolemic and hypoglycemic effects

In vitro bile acid binding capacity (BABC)

The vegetable waste extracts had BABC values ranging from 65 to 80%. The BABC of cabbage waste extract was higher than that of capsicum waste extract. This was probably due to differences in the types of phytochemicals, physical binding sites, porosity, adsorption forces, and hydrophobicity between the cabbage and capsicum extracts. Both types of vegetable wastes showed the highest BABC values for cholic acid: 78.39% for cabbage waste, and 73.72% for capsicum waste. The BABC values of cabbage waste for chenodeoxycholic acid and deoxycholic acid (67.7–69.34%) were higher than those of capsicum waste extract (65.44–67.96%). Cabbage outer leaves contain a large lignin fraction with few hydroxyl groups connected to the benzoyl rings. This may increase the BABC through greater adsorption forces and hydrophobic interactions (Cornfine et al. 2010; Rubio-Senent et al. 2015).

Glucose dialysis retardation index (GDRI)

Cabbage contains about 3.92% of total sugars (Li et al. 2002). Thus, we used ethanol to remove most soluble carbohydrates from the samples to avoid interference with the GDRI analysis.

Both vegetable wastes lowered the movement of glucose across the dialysis membrane into the external solution, as shown by the increase in GDRI up to 45 min. The GDRI of capsicum wastes (21.31–22.47%) within 90 min were higher than those of cabbage wastes (13.92–14.92%) (Fig. 1), probably because of differences in the complex dietary fiber structure between the two vegetable wastes. Insoluble DF blocks glucose adsorption while soluble DF delays glucose diffusion and adsorption because of its high viscosity. For both vegetable wastes, the maximum GDRI was observed at 45 min. This could be caused by the higher glucose diffusion rate in the control after 45 min. During the prolonged osmosis process, some glucose molecules would begin to dissociate from the adsorbed fibers and diffuse into the external solution. The combined effect of weakening entrapment of DF together with the continual increase in the glucose diffusion rate in the control may have led to the decrease in GDRI (calculated using Eq. 2) after 45 min.

Fig. 1.

Glucose dialysis retardation index (GDRI) against time. Values are mean ± SD (n = 3). Different letters on bars indicate significant difference (P < 0.05)

Methanolic extraction yield

We obtained four different types of extracts: cabbage wastes SE extract (ScabL), cabbage waste UASE extract (UcabL), capsicum waste SE extract (ScapC) and capsicum waste UASE extract (UcapC), with extraction yields of 33.0, 47.6, 37.0, and 42.0%, respectively. The yields of UASE extracts were higher than those of SE extracts. The high-frequency vibrations of the sonicator not only break the cell wall but also separate essential bioactive phytochemicals that are tightly bound to the cell wall or associated with other macromolecules. The higher yield of ScapC than ScabL and lower yield of UcapC than UcabL indicate that the cell wall components in capsicum waste may have higher polarity and/or be bound more loosely than those in cabbage waste, making them more accessible to the polar solvent.

Vegetable waste extracts: potential hypolipidemic and hypoglycemic effects

In vitro lipase inhibitory capacity

All extracts (2.5–10 mg/mL) except for ScabL inhibited lipase activity. Compared with cabbage extracts, capsicum extracts had stronger lipase inhibitory effects (70.0–76.0% for ScapC; 81.9–88.4% for UcapC; Fig. 2). UcabL showed the weakest inhibitory effect on pancreatic lipase (6.21–27.66%). These results implied that DF- and phytochemical-containing extracts with high BABC may not have strong lipase inhibitory effects, and vice versa. One possibility is that secondary metabolites present in cabbage wastes had antagonistic effects that canceled the lipase inhibition effects (Milugo et al. 2013).

Fig. 2.

Lipase inhibitory capacity of cabbage waste ultrasound-assisted solvent extract (UcabL); capsicum waste conventional solvent extract (ScapC); and capsicum waste ultrasound-assisted solvent extract (UcapC) at different concentrations. Values are mean ± SD (n = 3). Different letters on bars indicate significant difference (P < 0.05)

In vitro α-amylase inhibitory capacity

The cabbage extracts did not inhibit α-amylase activity within a concentration range of 2–10 mg/mL. Capsicum extracts weakly inhibited α-amylase activity (10.0–24.7% for ScapC; and 6.9–21.9% for UcapC; Fig. 3). The SE extracts inhibited α-amylase more strongly than did the UASE extracts, whereas the opposite trend was observed in all other analyses. The phytochemicals that inhibited α-amylase probably differed from those that inhibited lipase. The ultrasonic treatment of capsicum wastes may have dissociated specific functional groups that lowered its capacity to block α-amylase activity.

Fig. 3.

α-Amylase inhibition by capsicum waste conventional solvent extract (ScapC); and capsicum waste ultrasound-assisted solvent extract (UcapC) at different concentrations. Values are mean ± SD (n = 3)

DPPH radical scavenging capacity

Almost all the extracts at 4 mg/mL scavenged more than 90% of DPPH radicals except for ScapC, for which 5 mg/mL was required to scavenge 90% of DPPH radicals (Fig. 4). The EC₅₀ values of ScabL, UcabL, ScapC, and UcapC were 2.04, 1.95, 2.20, and 2.01 mg/mL, respectively. Cabbage extracts showed higher antioxidant activity than did capsicum extracts, and the antioxidant activity was not correlated with either lipase inhibition or α-amylase inhibition. Compared with the conventionally prepared extracts, the extracts prepared by ultrasonication had higher scavenging activity. This observation agrees with other research in which ultrasound-assisted extraction increased the yield of antioxidant compounds such as rutin, hydroxytyrosol glucoside and oleuropein from olive (Jerman et al. 2010) and lycopene from tomato processing wastes (Kumcuoglu et al. 2014).

Fig. 4.

DPPH radical scavenging activity of cabbage waste conventional solvent extract, ScabL (open diamond); cabbage waste ultrasound-assisted solvent extract, UcabL (filled diamond); capsicum waste conventional solvent extract, ScapC (open circle); capsicum waste ultrasound-assisted solvent extract, UcapC (filled circle); and ascorbic acid (filled rectangle). Values are mean ± SD (n = 3)

Volatile components of vegetable waste extracts

Cabbage and capsicum are vegetables that have been reported with phytochemical compounds like phytosterols, fatty acids, sugars (Hong and Kim 2013; Aranha et al. 2017). In this study, we analyzed the volatile components in the extracts of these vegetable wastes using GC–MS. The major volatile compounds identified in the extracts of both samples are shown in Table 1. Presence of volatile compounds was higher in the cabbage waste extract as compared to capsicum waste extract and this can be related to the strong antioxidant activity of cabbage waste as compared to capsicum waste as mentioned in the DPPH radical scavenging results (Fig. 4). Compounds like phenols and esters detected in both vegetable waste extracts were similar to previous reports on Korean cabbage, Hungarian red paprika and Habanero pepper (Hong and Kim 2013; Kocsis et al. 2002; Sosa-Moguel et al. 2017). Phenols and fatty acid esters from vegetable oils are known to possess antioxidant activity that promotes pharmacological effects such as anti-inflammatory, hepatoprotective, and anti-cancer (Toorani et al. 2019).

Table 1.

Major volatile compounds identified in vegetable waste extracts

No.	RT	Area (%)	Compound name	Molecular formula	Nature of compound
Cabbage
1	11.59	4.78	4H-Pyran-4-one, 2,3-dihydro-3,5-dihydroxy-6-methyl	C6H8O4	Phenol
2	12.99	0.78	2-Dodecene, (Z)	C12H24	Alkene
3	18.29	1.18	2-Tetradecene, (E)	C14H28	Alkene
4	21.12	12.78	Phenol, 2,4-bis(1,1-dimethylethyl)	C14H22O	Phenol
5	23.07	0.90	1-Hexadecene	C16H32	Alkene
6	34.32	0.56	Linoleic acid, ethyl ester	C20H36O2	Fatty acid ester
7	34.44	0.97	9,12,15-Octadecatrienoic acid, ethyl ester, (Z,Z,Z)	C20H34O2	Fatty acid ester
8	45.67	8.50	Nonacosane	C29H60	Alkane
9	48.56	1.03	Vitamin E	C29H50O2	Methylated phenol
10	49.76	5.03	Campesterol	C28H48O	Phytosterol
Capsicum
1	12.99	2.87	2-Dodecene, (Z)	C12H24	Alkene
2	18.29	3.27	2-Tetradecene, (E)	C14H28	Alkene
3	21.15	5.16	Phenol, 2,6-bis(1,1-dimethylethyl)	C14H22O	Phenol
4	23.10	5.17	3-Hexadecene, (Z)	C16H32	Alkene
5	27.45	2.29	3-Octadecene, (E)	C18H36	Alkene

Survival of a probiotic bacterium in the vegetable waste extract

Lactobacillus brevis showed interesting survival trends after incubation in 10% cabbage waste extract and 10% capsicum waste extract for 24 h and 48 h at 37 °C. The increase in the bacterial colony count on MRS agar after 24 h of incubation indicated that bacteria were able to grow in 10% vegetable waste extract (Table 2). In contrast, 0.1% of vegetable waste extract could not support probiotic bacterial growth, with reduced viable cell count (data not shown). Bacteria incubated in distilled water (negative control) lost viability and stopped growing. Bacteria incubated with 10% capsicum waste extract showed an increase in viable cell count, indicating that the capsicum waste extract supported and enhanced L. brevis, a type of probiotic bacterial growth (Online Resource Fig. 2). Similarly, capsicum waste extract has been used to grow Candida utilis 1769 with high yields in a single-cell protein system (Zhao et al. 2010). Bacterial growth showed a different pattern with 10% cabbage waste extract. The viable counts of bacteria decreased after 24 h of incubation but increased after 48 h of incubation (Online Resource Fig. 3). The bacteria were probably in an adaptation stage in the first 24 h of culture. Cabbage extracts might contain complex phytochemicals to which L. brevis must adapt before proliferating. The UASE extracts stimulated the growth of L. brevis (UcabL:1130 × 10⁴ CFU/mL; UcapC: uncountable) more than did the SE extracts (ScabL: 1110 × 10⁴ CFU/mL; ScapC: 1140 × 10⁴ CFU/mL) after 48 h of incubation.

Table 2.

Viable cell counts of Lactobacilli brevis after incubation in 10% vegetable waste extract for 0, 24, and 48 h and cultureon MRS agar for 24 h

Incubation time (h)	Negative control	Viable cell count (× 104 CFU/mL)
		Cabbage outer leaves		Capsicum seed cores
		ScabL	UcabL	ScapC	UcapC
0	142 ± 12	166a ± 7.2	136c ± 5.3	140ac ± 5.3	174ab ± 19
24	0	46a ± 9.2	40a ± 11	158b ± 19	906c ± 25
48	0	1110a ± 4.2	1130a ± 42	1140a ± 26	Uncountableb

Values are mean ± SD (n = 3)

ScabL, cabbage waste conventional solvent extract; UcabL, cabbage waste ultrasound-assisted solvent extract; ScapC, capsicum waste conventional solvent extract; UcapC, capsicum waste ultrasound-assisted solvent extract

Within each row, different letters indicate significant difference (P < 0.05)

In previous studies, probiotic survival was correlated with the presence of phenolic compounds and certain oligosaccharides (Hansawasdi and Kurdi 2017; Hervert-Hernández et al. 2009). Molan et al. (2009) and Shah et al. (2010) suggested that the anti-radical and oxygen-scavenging properties of polyphenols may modulate the oxidative stress caused by metabolic activities in the medium, thus conferring a more favorable environment for probiotic bacterial growth. In addition, the probiotic bacteria may convert certain oligosaccharides enzymatically to make them more bioavailable, which in turn would further stimulate bacterial growth (Hansawasdi and Kurdi 2017; Shah et al. 2010). The probiotic potential of vegetable wastes should be confirmed in additional in vivo and in vitro studies.

Conclusion

Cabbage and capsicum wastes contain a significant amount of DF, which binds to bile acids and retards glucose dialysis. Cabbage wastes showed higher BABC while capsicum wastes exerted greater in vitro hypoglycemic effects based on their higher GDRI and α-amylase inhibitory capacity. Compared with the conventional extraction method, extraction using an ultrasonic treatment resulted in higher yields and stronger activities in most assays i.e., DPPH radical scavenging and lipase inhibition assays. Compared with capsicum waste extracts, cabbage waste extracts exhibited higher antioxidant capacity but did not inhibit α-amylase. The antioxidant activities of both vegetable wastes are relevant to the presence of phenols and fatty acid esters. Capsicum waste extract contained essential nutrients to support the growth of L. brevis without a lag phase. In most of the assays, capsicum waste extracts displayed a wider spectrum of functional properties as compared to cabbage waste extracts. Both vegetable wastes have great potential for development into health-promoting functional ingredients. On the basis of the positive results obtained in this study, vegetable producers, distributors, and processors can diversify their wastes by transforming them into high-value functional ingredients.

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Abbreviations

BABC

Bile acid binding capacity

DF

Dietary fiber

DPPH

2,2-diphenyl-1-picrylhydrazyl

EC₅₀

The effective concentration of the sample extracts to scavenge half of the free radicals in the mixture (EC₅₀)

GDRI

Glucose dialysis retardation index

MRS

de Man, Rogosa and Sharpe

SE

Solvent extraction/extracted

UASE

Ultrasound-assisted solvent extraction/extracted