Application of Selected Probiotic Bacteria in the Fermentation of Cowpea (Vigna unguiculata L. Walp) Milk

Abstract

There is mounting evidence that probiotic microorganisms improve human health in diverse ways, including suppressing the growth of pathogenic bacteria in the gastro-intestinal tract, and producing beneficial metabolites such as short chain fatty acids that reduce the risk for various non-communicable diseases. Fermented foods are a good vehicle for probiotics and their metabolites. Legumes, such as cowpeas, which are less expensive, are alternative sources of proteins especially for the economically vulnerable. Besides their traditional way of consumption as cooked whole seeds, they are also processed into legume ‘milks’ as a substitute for animal milks. The current study investigated the suitability of probiotic bacteria in the fermentation of cowpea milk, in terms of the extent of formation of beneficial short chain fatty acids in the fermented milks; the reduction of oligosaccharides, which contribute to the problem of flatulence upon consumption of legumes; the sensory acceptability of the fermented products; and changes in the physicochemical and microbial characteristics of the products during storage. Fermentation of cowpea milk was carried out using three mixed starter cultures containing (i) Lactobacillus acidophilus, Bifidobacterium spp, and Streptococcus thermophilus (ABT-5) (ii) Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus (YF-L 903) or (iii) Lactobacillus rhamnosus GR-1 and Streptococcus thermophilus (Yoba fiti). After fermentation at 45˚C for 14 hours, the population of the bacteria in cowpea milk attained more than 106 colony forming units (CFUs), which is the minimum accepted for a food product to be considered probiotic. There was a slight increase in microbial population during the first two weeks of storage (refrigeration at 4˚C) followed by a decline. However, the population did not drop below 106 CFUs by the 28th day of storage.The oligosaccharides and short chain fatty acids were determined by high performance liquid chromatography and gas chromatography, respectively. Cowpea milk fermentation caused 67-100% reduction in raffinose and 20-70% reduction in stachyose in a culture-dependent manner. The stachyose content of raw cowpea (1.39±0.23g/100g) was higher than raffinose (0.22±0.06 g/100g), while verbascose was not detected. All the cultures produced propionic acid, butyric acid and valeric acid in differing concentrations but only Yoba fiti and ABT-5 produced isovaleric acid. The product fermented with YF-L 903 attained 2430 ppm of propionic acid, which was four times and ten times higher than the concentrations produced by the ABT-5 and Yoba fiti cultures respectively. Proximate analyses of samples: ash, moisture, crude fiber and protein content were performed according to Association of Official Analytical Chemist (AOAC) official methods 923.03, 925.09, 978.10 and 979.09. Total lipids were determined by modified Bligh and Dyer method. The carbohydrate concentration was determined by difference between 100 and total sum of the percentage of ash, moisture, fiber, fat and protein. All the analyses were performed in triplicates. Crude fat decreased significantly (P<0.01) after fermentation except for Yoba fiti culture which led to 33.2% increase. Crude fiber was not detected in all the samples. Fermentation with Yoba fiti also led to increase in protein content, although this was not significant. A decrease was observed for carbohydrate content, after fermentation, with YF-L 903 culture leading to the highest decrease of 7.1%. Evaluation of the acceptability of the fermented products in terms of taste, texture, aroma and appearance was done on a nine-point hedonic scale where 1 = like extremely, 5 = neither like nor dislike and 9 = dislike extremely. The study demonstrates suitability of probiotic bacteria in the fermentation of cowpea milk. However, further work should be done to improve the sensory characteristics of these products.