Metabolite profile of Bambara groundnut malting process and characteristics of its malt extracts

Abstract

Bambara groundnut (Vigna subterranea) [BGN] is a hardy, drought-resistant crop, termed a complete food because of its high nutritional value, with an average protein, carbohydrate, and fat contents of 17.5%, 64.0% (dry matter), and 6.5%, respectively. BGN has been sprouted to know its nutritional content and functional characteristics in food application and product development. Sprouting facilitates the breakdown of macronutrients and antinutritional compounds in seeds and synthesis metabolites. Recently, metabolomics has been considered an effective tool for profiling metabolites. However, despite the studies on BGN seeds, there is little or no studies on the enzyme, total polyphenolics, antioxidant activities and metabolites of malted BGN as affected by steeping and sprouting times. Bambara groundnut was malted by steeping in distilled water at 25  3oC for 36 and 48 h. The steeped seeds were sprouted for 144 h at 30oC, and samples were drawn every 24 h for drying. The dried samples were analysed for physicochemical properties (sprout length, moisture, pH, colour, protein content), enzymes, antioxidant activities and metabolites. The base malt (BGN-BM) was produced by drying sprouted BGN (green malt) at 50oC for 24 h. The toasted malt (BGN-TM) was produced from the base malt (dried at 50oC) subjected to 170oC for 30 min. The caramel malt (BGN-CM) was produced from the green malt heated to 80oC for 1 h and gradually increased every 20 min to 120oC for 2 h. Roasted malt (BGN-RM) was produced by drying green malt at 180oC for 1 h. Syrups were produced isothermally from the speciality malts. The speciality malts and syrups were analysed for colour, pH, protein content, α and β-amylases, total polyphenols, antioxidants, and metabolites. Sprout length was measured using the vernier caliper, pH by the laboratory pH meter. The colour was measured by the colour Flex EZ 25 mm aperture set for daylight illumination D65 and 10° standard observer angle. The Dumas Nitrogen analysis was used to determine the protein content, and crude protein was calculated by multiplying the measured nitrogen by the protein factor of 6.25 expressed in percentage. The α- and β-amylase activities of the sprouted samples were analysed by the enzymatic Ceralpha Method (K-CERA, Megazyme) kit and enzymatic Beta-Amylase (Megazyme, K-BETA3) kit, respectively. The Folin–Ciocâlteu reagent (FCR), ferric reducing antioxidant power (FRAP), and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay methods were used to determine the polyphenolic and antioxidants activities. The metabolites were analysed using Gas Chromatography-Mass Spectrometry (GC-MS) for acids, sugars, sugar alcohols and amino acids. Gas Chromatography-Flame Ionization Detection (GC-FID) for Fatty acid methyl esters (FAME) and Headspace Gas Chromatography-Flame Ionization Detector (HS-GC-FID) for the volatiles. The moisture of the steeped seeds increased with steeping times. There was a significant difference (p = 0.004) between the 36 and 48 h steeped BGN seeds sprouts length, with 36 h steeped sprouts longer. The steeping and sprouting times significantly affected the BGN malt colour quality and pH. There was no significant difference in the protein content of the BGN seeds based on steeping and sprouting times. The activity of amylases differed significantly for the 36 and 48 h steeping time, with mean α-amylase of 0.16 and 0.15 CU/g, respectively, β-amylase of 0.22 and 0.23 BU/g for the two steeping times. The optimal condition for malting BGN seeds for an amylase-rich malt was 36 h steeping and 96 h sprouting. The total polyphenolic content increased significantly with increased steeping and sprouting times. There was a significant (p = 0.000) difference between FRAP and DPPH antioxidant concentration in the 36 and 48 steeping times; however, sprouting from 24 to 144 h did not show a significant difference. The metabolic components during the malting process of BGN seeds were affected by steeping and sprouting times. The metabolites showed different levels of fatty acid methyl esters (FAME), sugars and organic acids, amino acids, and volatiles in the raw and sprouted BGN seeds, as well as the speciality malts. Fatty acid methyl esters (FAME) were not significantly different for the two steeping times. Linoleic acids were high after 48 and 96 h sprouting for the two steeping regimes. Sugars and acids decreased as steeping time increased while myo-inositol increased. Sugars, acids, and sugar alcohol were significantly different (p ≤ 0.05) based on sprouting times for the two steeping regimes. The two steeping regimes differed significantly in the amino acid concentration while the essential amino acids increased with sprouting time. An increase in steeping time reduced the volatile concentration, and there were changes in its concentration throughout the sprouting process. Ten volatiles were identified in the raw BGN seeds, 9 volatiles in steeped BGN seeds (36 and 48 h), while 21 and 14 volatiles were detected in the sprouted 36 and 48 h steeped BGN. The volatiles detected consisted of hydrocarbons, organic, alkanes, ketones, and aromatics compounds. The colours of the speciality malts and syrups were significantly (p = 0.000) different. The BGN speciality malt showed differences in colour with a decrease in lightness (L*) and increased redness (a*) and yellowness (b*). The BGN speciality malts protein ranged from 6.30 to 6.52. The protein content of the BGN speciality malts was significantly different, while the protein content of the syrups was not significantly different. The amylase activities, total polyphenols and antioxidants of the BGN speciality malts differed for all the BGN speciality malt. Twenty-nine volatiles were detected in the speciality malts with the pyrazine, 2,5-dimethyl, more abundant. Fifteen amino acids consisted of seven essential amino acids, and eight non-essential amino acids were detected. Fatty acid methyl esters (FAME) identified were palmitoleic, oleic, linolelaidic, linoleic and arachidic acid. The sugars, organic acids and sugar alcohols consisted of lactic acid, fructose, sucrose, and myo-inositol. The steeped and sprouted BGN seeds, BGN speciality malts and their extracts exhibited physicochemical characteristics and metabolites that could be used in the household, industries and benefit health-conscious consumers.