Evaluation of bambara groundnuts (Vigna subterrenea (L.) Verdc.) milk fermented with lactic acid bacteria as a probiotic beverage

Abstract

The aim of this study was to evaluate bambara groundnut milk (BGNM) subjected to fermentation with lactic acid bacteria (LAB) as a probiotic beverage with a view to developing value-added product. Central Composite Rotatable Design (CCRD) was used to optimise the hydration time and temperature of BGN flour for optimum BGN milk (BGNM) production. The optimum time and temperature was 2 h at 25oC. The effect of variety was assessed on the quality and consumer acceptability of BGNM prepared from five varieties of BGN (black, red, brown, brown-eye, and black-eye) which were representatives of the BGN available in South Africa. BGNM from the five varieties differed significantly (p<0.05) in, lightness, chroma, redness, yellowness, hue and antioxidative activity, while the pH were not significantly different. The four BGNM samples were significantly different (p < 0.05) in appearance, colour, mouthfeel and overall acceptability but not in aroma and taste. A three factor design (4 x 3 x 3) consisting of probiotics (Lactobacillus acidophilus, L. bulgaricus, L. casei and L. plantarum), temperature and fermentation time, were used to estimate the optimal conditions for the production of BGN probiotic beverage (BGNPB). The optimal condition for the production of BGNPB was estimated to be 35oC for 24 h with a desirability of 0.854 for L. bulgaricus. The next promising probiotic was L. plantarum that could be fermented at 35oC for 24 h with 0.843 desirability. BGNM from the red variety were fermented with L. bulgaricus and L. plantarum and L bulgaricus (in combination), making plain and sweetened BGNPB which were evaluated for their quality and consumer acceptability. The four BGNPB samples were significantly different (p < 0.05) in aroma, taste, mouthfeel and overall acceptability but not in appearance and colour. The plain BGNPB were assessed for their proximate composition, antioxidant activity, in vitro probiotic tolerance to simulated gastric juices and bile and a 28 days shelf life study at 5, 15 and 25oC. The protein, total dietary fibre (TDF), ash and antioxidative activity of the BGNPB were significantly different while the fat and carbohydrates were not significantly different. Time and concentration of the gastric juice and bile had significant effects on the percentage bacterial survival of probiotics in the BGNPB. However, the probiotics did survive, in low numbers, in the simulated gastric juice and bile after 180 and 240 minutes of incubation. Titratable acidity, pH, microbial load and colour of the BGNPB were significantly affected by the storage time and temperature during the shelf life study. At the 5oC storage temperature the BGNPB had a right censored shelf life on day 28. At 15oC the shelf life was 18 and 10 days for L bulgaricus and L. plantarum and L. bulgaricus respectively. The outcome of this research showed that a novel BGNPB product can be made from fermenting BGNM with LAB.