Screening High Acid-producing And Acid-resistant Lactobacillus Casei Strains And Optimization Of The Fermentation Condition

Lactobacillus casei is recognized as one of a probiotic strain by the Ministry of Health of China,which is often used as a starter and auxiliary starter for dairy products such as yogurt,milk,and cheese.It is widely used in the development of dairy products because of its good health care function and the ability of acid resistance,bile resistance.In this paper,Lactobacillus casei LC2 W was subjected by Ultraviolet(UV),Nitrosoguanidine(NTG),Diethyl sulfate(DES)respectively,and then treated with simulated gastric fluid to explore its acid-producing and acid-resistant changes.In addition,the green tea was added to the fermentation process to explore the optimization of fermantation conditions.This is aimed to lay the foundation for the further application of Lactobacillus casei.The results showed that the optium time of UV was 30 s;the optium time and concentration of NTG was 30 min,0.3 g/L;the optium time and concentration of DES was 30 min,0.12%,respectively.The mutation strain LC2W-NTG-12 and LC2W-UV-11 exhibited high acid-producing and acid-resistant.In the same amount of inoculation,the acidity of the fermented by the parent strain and mutation strain were 76.6 °T and 112.7 °T,98.2°T after 24 h,respectively.The acid-production rate of mutation strain was improved 47.13%,28.20%.In addition,when the strains were treated with the acid(pH=1.5),the mortality rate of parent stain and mutation strain were 76.44% and 52.06%,56.36% after 2 h,respectively.The acid-resistant rate of mutation strain was improved 24.38%,20.08%.But the effect of mutation strain LC2W-DES-33 which was subjected by 0.16% DES for 30 min was not obvious.The acid-production rate of mutation strain was improved only 14.10% compared with th the parent strain.This implied that UV and NTG mutation can effectively improve the acid-producing and acid-resistant of the L.casei LC2 W.2% green tea was added to the process of fermented milk,The results showed that the pH value of the 2% green tea group was 5.95 and the control group was 5.39,the pH value of the green tea supplement group was lower than that of the control group,Therefore,the addition of 2% green tea had a stimulatory effect on the fermentation,and the stimulatory effects on the growth of L.casei LC2 W occurred during the early stages of fermentation.Chinese green tea was added to the fermentation of milk with different concentration.The results showed that the concentration of 5% green tea supplement exhibited the most obvious effects on the growth of L.casei LC2 W.During storage,cell loss in fermented milk supplemented with green tea was slower than in control group during storage,suggesting that the green tea supplement can extend the shelf life of fermentation milk.The contents of the flavor compounds in the fermented milk were significantly increased with green tea supplement,indicating that green tea could promote the flavor compounds in fermentation milk.The incremental content of the total free amino acid in fermented milk with green tea supplement was significantly increased to 232.01 mg / kg,which was 137.95 higher than that in the control group,suggesting that the addition of green tea stimulated the proteolytic activity of L.casei LC2 W,resulting in a large number of free amino acids released,providing the nitrogen source for the growth of L.casei LC2 W.Therefore,the addition of green tea could shorten the fermentation time,achieved the optimization of fermentation conditions,increased probiotic function of yogurt,and not only accepted by people more easily,but also more conducive to industrial production.