The Effects of Compound Starter Culture and Sugar and Soy Milk on the Quality and Probiotic Activity of Milk-Soy Mixed Yogurt

Yogurt has emerged as an essential nutritional food in contemporary diets, and the development of new multi-component yogurt formulations has become a focal point of current research. In this study, the effects of fermentation compounds and the addition of sugar, soy milk on the quality and probiotic activity of milk-soy mixed yogurt were studied to determine the optimal formation of mixed yogurt. The various fermentation compounds (YO-MIX 883, Lactobacillus Casei complex starter cultures, Lactobacillus Paracasei compound starter cultures), different concentrations of milk-soy additions (0, 25, 50, 75, 100%) and sugar (2, 4, 6, 8%) were tested within each experimental group, and the pH, appropriate acidity, and total viable bacterial count of the fermented milk-soy mixed yogurt were determined throughout the fermentation and refrigeration processes. The obtained results showed that the Lactobacillus Paracasei complex was particularly effective for the fermentation of soy milk. The mixed yogurt formulation, comprising 50% soy milk and 4∼6% sucrose, exhibited enhanced acidity, superior sensory evaluation scores, and overall improved product quality. It was observed that during refrigeration, an increase in the milk content of yogurt corresponded to a more pronounced post-acidification effect. The optimal formulation for the milk-soy mixed yogurt identified in this research consisted of 0.3% Lactobacillus Paracasei compound fermenter, 6% sucrose, and 40% soy milk. Under these optimal conditions, the mixed yogurt achieved an acidity level of 76°T, a sensory score of 92 points, and a survival rate index of 1.25. Additionally, the yogurt exhibited a distinctive soybean aroma in its aftertaste, contributing to its overall quality. Furthermore, the probiotic survival index of the mixed yogurt containing 40% soy milk, following simulated gastrointestinal fluid digestion, was recorded at 0.767, indicating that the probiotic activity in this yogurt was significantly higher than that of other yogurts. The obtained results provide a theoretical foundation for the future industrial production of milk-soy mixed yogurt products.