The effect of malting and fermentation on the nutritional and potential health-promoting properties of finger millet [Eleusine coracana (L.) Gaertn] grain

Abstract

Finger millet (FM) [Eleusine coracana] is an underutilised cereal grain used as a food source in South Africa. Increased research interest in FM has span over the years owing to its unique nutritional and bioactive composition. Following the recent interest in natural curative substances over their synthetic counterparts in the treatment of food dependent diseases, FM has shown potential nutraceutical effects. Some important health effects like antidiabetic, antioxidative, anti-inflammatory and antimicrobial properties have been reported in recent trials with FM. In view of the increasing utilisation and application of FM in the region of Thulamela Municipality, Vhembe District of South Africa, two common indigenous FM varieties (brown and dark brown) were obtained and analysed for their physicochemical properties, levels of minerals, phytic acid, phenolic compounds and antioxidant activities. For this process, malted non-alcoholic beverages were produced and analysed for their physicochemical properties, levels of phenolic compounds, and total phenolics and antioxidant activities. FM grains were soaked, germinated and kilned at an interval of 24 h for 96 h, using sorghum as an external reference. Mineral composition of the FM and sorghum samples were analysed using an inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mass spectroscopy (ICP-MS), and atomic absorption spectrometer (AAS). Identification and quantification of phenolic compounds were performed using ultra-performance liquid chromatography mass spectrometer (UPLC-MS). All experiments were performed in triplicate except for the UPLC-MS analysis of the malted non-alcoholic beverages that was done in duplicate. Data were analysed by one way analysis of variance, and the mean values were separated by Duncan’s multiple comparison test using SPSS version 24.0. Data showed that the FM varieties were rich in macro- and micro- or trace elements. The macro-elements calcium, magnesium, potassium, phosphorus and sulphur were found in high amounts ranging from 1597.37 mg/ kg – 6775.03 mg/ kg; iron, zinc, strontium and silicon were found in significant amounts in the range 21.47 mg/ kg – 55.67 iii mg/ kg, copper and boron were found in low amounts (2.2 mg/ kg – 7.7 mg/ kg), along with selenium and cobalt (0.02 mg/ kg – 0.05 mg/ kg). Heavy metals, barium and aluminium were found in the FM varieties. Varietal difference was found to play an important role in the mineral content of the grains during malting. Malting for 24 h reduced mineral content except for sodium. Beyond 48 h of malting, mineral content increased, particularly, for 96 h in FM grain malt. Significant (p < 0.05) increases in the mineral content of FM varieties were noted at 48 h and 96 h of malting. Increase occurred at 72 h of malting for potassium, iron and boron. Malting did not have any effect on the manganese content of the dark brown FM; however, it increased the manganese content at 48 h of malting for brown FM. Malting for 96 h significantly (p < 0.05) reduced sodium content. Consecutive decrease in phytic acid content of the grains was not recorded with durations in malting time. Although statistically significant differences (p < 0.05) were observed, malting did not result in too much change in the physicochemical properties of the grains. Several flavonoids, catechin, epicatechin, quercetin, taxifolin, and hesperitin were isolated, whilst protocatechuic acid was the only phenolic acid detected in the unmalted and malted FM. Increases in catechin, epicatechin and protocatechuic acid were observed for 72 and 96 h malt of brown FM with similar observations recorded for sorghum. Complete loss of taxifolin, catechin, and hesperitin were noted with malting time. FM grains exhibited 2, 2-diphenyl-1-picrylhydrazyl (DPPH), 2, 2՛-azinobis-3-ethylbenzthiazoline-6-sulfonic acid (ABTS) radical scavenging action and iron reducing activities. Increased iron reducing activity alongside ABTS radical scavenging activity was recorded with malting time. A fermentation-time dependent decrease in the pH of the non-alcoholic beverages, with a corresponding increase in sugar content were recorded. A similar decrease was also recorded for the viscosities of the beverages. The FM malt beverages were found to contain a higher amount of citric acid compared to the sorghum malt beverage. A decrease in the citric acid content with fermentation time was noted in the grain malt beverages fermented with Lactobacillus fermentum, particularly for the iv FM beverage. The phenolic compounds detected in the FM malt beverages fermented with the grain microbial flora and Lactobacillus fermentum were protocatechuic acid, catechin and epicatechin. Taxifolin and kaempferol along with the earlier mentioned compounds were detected in the sorghum malt beverage. Catechin was found in higher amount compared to other phenolic compounds in the FM and sorghum malt beverages. FM malt beverages were found to contain a higher amount of total phenolics compared to the beverage prepared from sorghum malt. Fermentation with the grains’ microbial flora and L. fermentum resulted in reduced total phenolics of FM and sorghum malt beverages, particularly after 24 h of fermentation. A fermentation-time dependent decrease in total phenolics of FM beverages fermented with L. fermentum was noted. Fermentation within 24 - 48 h using the grain microbial flora showed higher total individual phenolic compounds for the dark brown FM and sorghum, compared to other fermentation periods. Fermentation of the beverages for 24 h retained a higher amount of the total phenolics compared to other fermentation periods, especially for the L. fermentum beverages. Reduced total phenolic content and antioxidant activity of the beverages were noted at 24 h of fermentation for the two microbial sources. Significant (p < 0.05) increases in total phenolics were observed within 72 – 96 h of fermentation of the brown FM malt beverage with the grains’ microbial flora. Fermentation for 72 h and 96 h with L. fermentum increased the total phenolic content of the brown FM. Increase in total flavonoid content (TFC) of brown FM malt beverage was noted at 72 h fermentation for both microbial sources. Unlike with L. fermentum, no significant (p > 0.05) change in TFC was observed for the dark brown FM beverage after 24 h fermentation with the grains’ microbial flora. Beverages exhibited DPPH, ABTS radical scavenging action and iron reducing activities, which were significantly (p < 0.05) reduced at 96 h fermentation for both microbial sources. The 24 h fermented beverage retained a higher amount of total phenolic and flavonoid contents, and had higher antioxidant activity compared to other fermentation periods for both microbial sources. The study shows that FM is a rich source of essential minerals and v phenolic compounds, and demonstrates that 72 to 96 h of malting has a positive effect on minerals and certain phenolic compounds over the 48 h malting period widely used for preparation of FM malt. The presence of hesperitin in FM grain was established. A new method was developed for the production of FM non-alcoholic beverage with measurable amounts of health-promoting compounds. An ideal fermentation period (24 h) for FM malt non-alcoholic beverage production with enhanced health-promoting compounds, using Lactobacillus fermentum was demonstrated. Fermentation limit (96 h) for production of FM malt beverage using either the grain microbial flora or L. fermentum was confirmed. These findings provide a rationale for increased utilisation of FM as a functional food grain, and its use as malt in production of non-alcoholic beverage for health promotion and wellness.