Effect of storage temperature on shelf-life of yogurt-like product from soybean (Glycine max)
Soy yogurt was produced from soy milk using commercially freeze-dried samples of Streptococcus thermophilus and Lactobacillus bulgaricus at a ratio of 1:1 as starter culture. Evaluation of nutritional quality of the soy yogurt showed that the fat content (3.27%), moisture (89.42%) and ash content (0.96%) were higher than those of a popular brand of dairy yogurt (1.78%, 82.66% and 0.93%) respectively. However, the protein content (3.20%) and carbohydrate content (3.15%) were lower than values of (3.51%) and (11.12%) respectively for dairy yogurt. Soy yogurt had significantly lower acceptability (P ≤ 0.05) than the dairy yogurt. Shelf life study was carried out at room temperature (27±3oC) and refrigeration temperature (7±2oC). A product shelf life of four days at 27±3oC and ten days at 7±2oC was obtained without additives. There was decrease in pH from 3.8 to 3.3 and increase in titratable acidity from 1.21% to 1.93% during storage. Total aerobic bacterial and yeast counts reached 6.3× 103 cfu/ml and 4.5× 105 cfu/ml at 27±3oC respectively compared to 3.4 × 102 cfu/ml and 1.1× 103 cfu/ml at 7±2oC. The counts for the starter cultures at 107 cfu/ml dropped two log cycles at 27±3oC and a log cycle at 7±2oC. The spoilage yeasts in soy yogurt were mainly Saccharomyces sp. and Candida sp. Soy yogurt spoilage was evident from the curdling of the yogurt, increased moisture and yeasty odour.
Keywords: Soy yogurt, soy milk, shelf-life, spoilage, temperature