Chemical characterisation and microbiological quality of naturally fermenting soy milk

Abstract

Soymilk is the rich creamy liquid extract of soybean and is the most available soy products. Changes that occur during natural fermentation of soymilk to soy-nono were evaluated. Clean soybean seeds were soaked, blanched and blended in order to obtain soymilk. The soymilk was filtered and allowed to undergo natural fermentation in order to obtain soy-nono. Soy-nono samples were taken at 6 h intervals from 0-72 h and analysed for their pH, titrable acidity and proximate composition. Also, the microbiological quality of the fermenting soymilk was carried out by enumerating the microbial population (total aerobic count, Lactobacillus spp., yeast and mould) and the isolated microorganism were characterised and identified. The pH of the fermenting soymilk decreased from 6.90 to 4.09 while the total titrable acidity increased from 0.42 to 1.82% (lactic acid equivalent) during the fermentation period. The crude fat and carbohydrate content decreased from 2.18 to 0.87% and 1.52 to 0.60%, respectively while total ash and crude protein content increased from 0.23 to 0.74% and 2.62 to 5.09%, respectively. Changes in the magnesium content, zinc content, total solid, moisture content, protein content, energy value of the soy-nono throughout the fermentation period were not significantly different at 5% confidence level. The increase observed in the calcium content of the soy-nono increased significantly after 36 h fermentation period. Based on the microbiological quality of soy-nono, the total aerobic, lactic acid bacteria and yeast count increased throughout the fermentation period from 5.60 to 7.30, 3.48 to 5.90 and 1.30 to 3.63 log cfu/ml, respectively. Mould counts increased from 1.00 to 3.07 log cfu/ml between 0 and 42 h and then decreased till the end of the fermentation period. The microorganisms present in the soy-nono were found to be Lactobacillus fermentum, Lactobacillus acidophilus, Lactobacillus farciminis, Lactobacillus alimentarius, Saccharomyces cerevisiae and Candida pseudotropicalis. This study shows that several microorganisms are involved in the natural fermentation of soymilk to soy-nono and lactic acid bacteria were the dominant microbial group in the soy-nono. Also, the protein and calcium content of the soy milk increased as a result of its conversion to soy-nono by natural fermentation.