| Lactobacillus plantarum is a kind of microorganism which can improve the microecological balance in the host’s gut and is beneficial to the health of the host.Therefore,Lactobacillus plantarum is often used in food production,especially in various dairy products.However,there are some main problems for probiotics,such as poor gastrointestinal fluid tolerance and poor storage stability.It has been reported that microbial immobilization technology can improve the stability of microbial to a certain extent.Nanofiber membranes possess high porosity and specific surface area,which can promote cell adhesion,differentiation and proliferation.Cellulose acetate is an inexpensive,wide source,non-toxic and biodegradable polymer.In this study,cellulose acetate nanofiber membranes were used as the immobilization materials of Lactobacillus plantarum.The influences of immobilization conditions on the immobilization efficiency were studied in detail.Vacuum freeze-drying technology was used to prepare immobilized Lactobacillus plantarum freeze-dried powder.The intestinal fluid tolerance of the immobilized Lactobacillus plantarum freeze-dried powder was also characterized.Finally,the immobilized Lactobacillus plantarum freeze-dried powder was used in yogurt fermentation to study its fermentation characteristics and repeated cycles of fermentation.The research results are as follows:1.Cellulose acetate nanofiber membranes were used as the carrier to immobilize Lactobacillus plantarum.Ultrasonic shock treatment was utilized to quantify the immobilized Lactobacillus plantarum.The optimal ultrasonic conditions are 100 W with 10min treatment.With this condition,the immobilized Lactobacillus plantarum can be fully released from the nanofiber membranes.The highest released Lactobacillus plantarum is about 1.01 × 1012 CFU/g.As seen from the results of viability counting,weighing method and scanning electron microscope observation,the optimum immobilization conditions were as follows:the immobilization time was 18 h,the inoculation amount was 1%,the number of the added membranes was 3.The number of bacteria immobilized on the cellulose acetate nanofiber membrane is 1.69 × 1012 CFU/g.In order to further determine the number of viable cells on the nanofiber membrane,the ultrasonic method,the enzymatic method,and the two combined methods were compared.The results show that the combined method can get the maximum count,2.72 × 1012 CFU/g.2.Through the pre-test and single-factor test,three kinds of protective agent and the best concentration range of protection were selected in the process of vacuum freeze-drying.The optimum freeze-dried protective agent formula was 11.63%skimmed milk powder,10.11%trehalose and 8.12%ascorbic acid.With this formula freeze-dried immobilized Lactobacillus plantarum,the survival rate of bacteria can reach 83.67%.3.The resistance of immobilized Lactobacillus plantarum freeze-dried powder,free Lactobacillus plantarum,and the undried immobilized Lactobacillus plantarum were determined by measuring the survival rate of Lactobacillus plantarum in artificial gastric juice(pH 2.5,3.5,4.5)and artificial intestinal juice(pH 6.8).The results showed that the longer immobilization time,the better the gastrointestinal fluid tolerance.The immobilized Lactobacillus plantarum freeze-dried powder has a sustained-release property in gastrointestinal fluids.The number of viable cells increased 0.2～0.34 log(CFU/g)after 3 h digestion.4.Immobilized Lactobacillus plantarum freeze-dried powder and Lactobacillus plantarum freeze-dried powder were used as the DVS to prepare yogurt.The results showed that the Immobilized Lactobacillus plantarum freeze-dried powder has stronger acidification capacity during the fermentation.The live bacterial in the yogurt with the immobilized Lactobacillus plantarum freeze-dried powder as DSV was about 2 log(CFU/g)higher than that with free bacterial powder as DSV after the yogurt was stored at low temperature for 21 days.The immobilized Lactobacillus plantarum freeze-dried powder can be reused for yogurt fermentation.It has been found that the pH,titration acidity,and viable count were maintained at a stable level after 6 time recycle. |
