Nutritional, antioxidant and microbiological properties of finger millet-based beverages as influenced by starch source during lactic acid fermentation

Abstract

In Sub-Saharan Africa (SSA), where nutrient malnutrition is of concern, finger millet contributes substantially to the dietary protein and mineral intake of many communities. The quality of finger millet protein is, however, poor due to deficiency of essential amino acids like lysine and reduced digestibility due to complexation with antinutrients such as phytate which can also reduce mineral bioaccessibility. Despite the nutritional challenges, finger millet is rich in bioactive phenolic compounds which are gaining increased research focus due to the potential health benefits they can offer in terms of protection against diet-related non-communicable diseases (NCDs). Various fermented beverage products are consumed in SSA, where the fermentation process involves inclusion of an exogenous source of starch. Crucially, the fermentation process can modify the microbial population, nutrients, and bioactive compounds. This presents an opportunity for the improvement of cereal-based diets to leverage their nutritional and health benefits fully. In this study, the influence of starch source- maize and rice- on protein quality, mineral bioaccessibility, phenolic content, and antioxidant and microbiological properties of fermented finger millet-based beverages was determined. Finger millet slurries were prepared by spontaneous fermentation with or without addition of exogenous starch from maize or rice, whilst beverages were prepared by cooking finger millet slurries. Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) and 16S ribosomal ribonucleic acid (16S rRNA) amplicon sequencing were used to assess microbial diversity of slurries, whilst in vitro dialysability, in vitro protein digestibility (IVPD), and reactive (available) lysine assays were done on slurries and/or beverages to determine iron and zinc bioaccessibilities, and protein quality, respectively. Phenolics were extracted from slurries and beverages using acidified methanol [1% (v/v) conc. HCl in methanol] and the total phenolic content [TPC, Folin-Ciocalteu (F-C) method] and antioxidant properties of extracts against 2,2´-azinobis [3 ethylbenzothiazoline-6-sulphonic acid] (ABTS) diammonium salt were determined. MALDI-TOF MS revealed the dominance of Weissella confusa, Enterococcus casseliflavus, E. hermanniensis, Lactobacillus acidipiscis, L. salivarius, L. lactis, L. fructivorans, and L. lactis ssp. tructae at the end of finger millet fermentations. 16S rRNA sequencing revealed the dominance of Weissella at the end of fermentation followed by the genera Enterococcus and Lactococcus only in the formulation with added maize starch. Overall, there were no differences in microbial diversity and relative abundance within the fermentation groups. Starch source did not affect IVPD and reactive lysine content of beverages. Lactic acid fermentation improved IVPD with fermented and cooked samples showing higher IVPD than unfermented and cooked samples. This accentuates the role of fermentation in alleviating the negative effects of cooking on protein quality. The improvement in IVPD was attributed to pre-digestion of storage proteins and release of bound proteins at low pH which enhanced their availability to pepsin attack. Iron and zinc bioaccessibility were improved by the addition of exogenous starch. The improvement was attributed to complexation of starch hydroxyl groups to mineral ions whose release during digestion enhanced mineral bioaccessibility in vitro. Although fermentation reduced the phytate content of the samples, no improvement was observed in bioaccessible iron, however, zinc bioaccessibility was improved relative to unfermented samples. The reduction in phytate content was attributed to dephosphorylation of phytate by endogenous phytases activated at low pH. The reduction in iron bioaccessibility can be due to iron chelation by non-phytate organic compounds such as phenolic compounds. The addition of starch improved TPC and Antioxidant Activity (AA) of finger millet-based beverages. The increase was attributed to improved fermentation efficiency and better mobilisation of hydrolytic enzymes, which presumably helped release of bound phenolics. Although cooking did not affect TPC, AA was reduced. The decrease in AA suggested the presence of heat unstable antioxidants whose destruction during heat processing led to a decrease in AA of beverages. Lactic acid fermentation, with or without added exogenous starch, shows the potential to improve zinc bioaccessibility, protein quality (IVPD), and antioxidant properties of finger-millet beverages. Soured finger millet-based foods can potentially be used to reduce mineral and protein malnutrition, and oxidative stress-related NCDs in SSA.