The Study Of The Strains And The Fermentation Process Optimization Of Soy Yoghurt

Soybean contains a variety of active substances such as soybean isoflavone, soy oligosaccharides, soy peptide, etc., which has many benefits on health. Soy milk was made from soybean-grinding which could fermented with probiotics which called vegetable protein drinks.The technology of this product and market remains to be further research and develop. The paper studied the fermentation processing of soy yoghurt, which including the study of screening of strains and nutrition factors, and the relationship between the processing methods and the beany smell, and the optimization of the fermentation process, and property changes during the storage period.The study chose Lactobacillus bulgaricus and Streptococcus thermophilus as culture strains. The domestication of strains was performed by raising the proportion of soymilk in the mixture of milk and soymilk gradually. The study also did a research about the influence of five kinds of nutritional factors which were tomato juice, carrot juice, fructo-oligosaccharides, soybean peptide and amino acid on these two probiotic stains during their fermentation. After the domestication, viable counts and acidity of fermented soymilk with single strain or mixed strains were both increased. The effect of increasing acid degree of soybean followed as:peptide> amino acid> carrot juice> oligofructose> tomato juice. By adding4%(v/v) concentration of3%soybean peptide, and the acid degree increased the most, the acidity of the soy yoghurt increased from47.5°T to60°T. Total number of living bacteria increased from3.7×109to6.83×109.The study used HS-SPME-GC-MS method to measure the volatile substances in the soy milk produced with different processings as well as the flavor components of soy yoghurt. There were48kinds of volatile substances detected in the soy milk. The major content of beany smell in the control groups was much higher than those in the group2and group3and group4. Known from the analysis of the statistics, the volatile substance in the soy milk processed by different methods that most kinds of the beany flavor were appeared in the pulping section. Using8590℃hot water for grinding could get the best effect of removing the beany smell. By adding a reducing cysteine while soaking the bean, and separating seriflux after the boilingcould also help reducing the relative content of beany smell. Flavour compounds and content of fermented soy yogurt varied from those in the soy milk.The organic acids in soy yogurt were determined by HPLC. The major kinds of organic acids were actic acid, pyruvic acid and succinic acid, while the formic acid, acetic acid, propionic acid were lower. Both of the organic acid content fermented by single or mixed bacteria strains was increased during fermentation period, and the lactic acid increased the most. By the end of the storage time, the lactic acid content of soy yoghurt fermented with Thermophilic streptococcus, Bulgaria bacillus were0.4045g/L,0.3025g/L,and lactic acid content of soy yoghurt was0.7164g/L fermented with mixed strains. The organic acids in soy yoghurt fermented by mixed bacteria strains were higher than the organic acid content of soy yogurt fermented by single bacteria strain. The content of succinic and propionic acid were higher in soy yogurt fermented by lactobacillus bulgaricus than that fermented by thermophilic streptococcus, while the content of other organic acids were on the contrary.According to the principle of Box-Benhnken center combination experiment designing and the response surface optimization with three factors(fermentation temperature, fermentation time and inoculation quantity) and fuzzy evaluation as the response value, the study concluded that the optimum process conditions of immobilized acetic acid fermentation were with5.02%of the inoculation quantity, fermentation temperature at41.82℃and5.37hours of fermentation time, then the fuzzy sensory score was82.15points. Using the above experimental conditions, fuzzy sensory evaluation score was81.3±1.06. There was no significant difference in predicted and actual values so that it showed the reliability of this model.The study measured the variation in properties of soy yogurt in the storage of19days. Changes of rheological property of soy yogurt were evaluated by rheometer. The elastic modulus was the maximum on the first day of storage period. The elastic modulus gradually decreased with the increase of storage time, and the viscous modulus also showed a similar trend. The hydraulic (WHC) was in a downward trend in the process of storage, which decreased from69.7%to57.6%. There was little change of the springiness during the time of storage, and the final springiness of the system was0.9945. The resilience decreased from0.0571to0.03. The hardness and adhesiveness increased after the first reducing during the period of storage. On the storage time of11days, the hardness reached the maximum which was313.4N, and then decreased rapidly. Till then the system gel system had changed, so it was recommended to eat the soy yoghurt before13days in storage.