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

Background: 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.

Objective: 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.

Methods: 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.

Results: 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.

Conclusion: The obtained results provide a theoretical foundation for the future industrial production of milk-soy mixed yogurt products.