Formula optimization of high autolytic complex microbial agents for yogurt

Abstract: Cell autolysis could be induced by bacterial peptidoglycan hydrolases (autolysins) degrading the peptidoglycan structure of cell walls. Autolysis of lactobic acid bacteria shows direct influence on the characteristics of fermented milk products. Three high autolysis rate lactic acid bacteria, Lactobacillus delbrueckii ssp. Bulgaricus LD3-A3, Streptococcus salivarius ssp. and Thermophilus GS1-A8, and Lactobacillus lactis S15-A3 were tested to obtain a good yogurt starter. The three-factors quadratic regression orthogonal rotational composite experiment was designed to determine the effects of the three high autolysis strains on yogurt characteristics during fermentation, and the effects of mixed symbiosis and interactions were also analyzed.Quadratic orthogonal rotation combination design was used to compare differences among 23 yogurt samples that were fermented by three strains of mixed starters; fresh milk was used as a culture medium. The titration acidities (Δ0T) and the organoleptic evaluation scores of the yogurts, which were stored at 10-15℃ were measured after 15 days. Linear correlation analysis was also used to clarify the main factors of the three strains. The interactions of the three strains were analyzed by response surface analysis. The optimum mixed formula of the three strains was selected according to the comprehensive models. The sensory quality and shelf life of yogurt fermented by the three high autolysis rate stains mixed starters was compared with the yogurt brought from the market and yogurt fermented by the DVS starter (Danisco). The yogurt manufacturing parameters were: Inoculation amount 10 percent, fermentation temperature 45℃ to pH 4.5-4, and storage for 15 days at 10-15℃. The titration acidities were determinate at days 0, 2, 6, 10 and 15 d.The most significant effect of post acidification was caused by LD3-A3, while S15-A3 could improve the flavor and organoleptic characteristics of yogurt. Results from the combination design showed that, while interaction between GS1-A8 and LD3-A3 was significant (p<0.01), the effects trend of yogurt Δ0T value and sensory evaluation scores on the three strains was almost the same. GS1-A8 and LD3-A3 have synergistic interaction. Taking the intersection at a confidence level of 95% of the processing, the best mixed formula of the three strains confirmed by the intersection of these two prediction intervals was: GS1-A8, 91.46 percent, LD3-A3, 2.44 percent, and S15-A3, 6.10 percent, with an approximate proportion of 45:1:3. Compared with yogurt brought from the market (26.27) and fermented by DVS starter (38.32), the new starter could significantly reduce the degree of post acidification yogurt to 14.76. The new starter made yogurt have better color, texture, and flavor. Especially when stored more than 15 days, the new starter yogurt organoleptic characteristics were 77, as compared with brought yogurt (58) and DVS starter yogurt (67).The results showed that high autolysis rate lactic acid bacteria starters could effectively reduce yogurt post acidification levels. Meanwhile, the interaction between different strains should be of concern when dairy fermentation starters are designed. The new starter obtained from this study could make the yogurt have favorable sensory properties, such as good flavor, better texture character, and higher shelf quality, and could be used in industrial production.