Study On The Characteristics Of Yogurt Prepared By Ultra-high Pressure Combined With Glutamine Transaminase Treatment

The sensory quality and physical and chemical indexes of yogurt are affected by many factors.Thermal sterilization can effectively kill pathogenic microorganisms in milk,but it will reduce its nutritional value and have a cooking taste.Ultra-high pressure is a cold sterilization technology,which can keep the original nutrition and flavor of milk,change the spatial structure of milk protein and increase digestibility.The appearance of milk after ultra-high pressure treatment is obviously different from that after heat treatment,and the treatment of dairy products with transglutaminase(TG enzyme for short)can catalyze the intramolecular or intermolecular cross-linking reaction of protein and promote the improvement of its structure and texture.In this paper,raw milk was treated by ultra-high pressure combined with TG enzyme,and yogurt was prepared by fermentation.The differences of sensory characteristics and physical and chemical indexes between raw milk and yogurt prepared by heat sterilization were compared.Phenolphthalein indicator method,rheometer,texture analyzer,centrifugal method and plate counting method were used to measure the changes of acidity,viscosity,hardness,water holding capacity and viable count of yogurt during fermentation.Use electronic eye,electronic tongue and electronic nose to measure the changes of color,taste and smell.The research results are as follows:(1)Ordinary yogurt(without TG enzyme)and yogurt with TG enzyme were prepared by heat sterilization.It was found that there was no significant difference in acidity,color,taste,smell and number of viable bacteria(P>0.05).After adding TG enzyme,the viscosity,hardness and water holding capacity of yogurt increased significantly(P<0.05).Therefore,adding proper amount of TG enzyme can significantly improve the texture of yogurt,and adding 2 U/(g protein)TG enzyme is more conducive to improving the quality of yogurt.(2)The optimum treatment conditions for improving TG enzyme activity are 400 MPa,holding time 40 min and temperature 32 ?.Under these conditions,the enzyme activity of treated TG was 147.87 U/g,which was 45.18% higher than that of the control group(the activity of untreated TG was 101.85 U/g).(3)Different ways were used to treat fermented milk,and it was found by comparison that:(1)The optimal technological conditions of ultra-high pressure treatment of TG enzyme were used to treat raw milk,which could achieve the ideal sterilization conditions.The raw milk was sterilized under this ultra-high pressure condition,and then fermented to prepare yogurt.(2)under three treatments: thermal sterilization,first adding TG enzyme and then ultra-high pressure sterilization,and first ultra-high pressure sterilization and then adding TG enzyme,the water holding capacity increased with the increase of TG enzyme addition,and the viscosity,hardness and water holding capacity of yogurt were significantly improved by adding a certain amount of TG enzyme(P<0.05).(3)the yogurt was treated by heat sterilization,heat sterilization plus 2 U/(g protein)TG enzyme,ultra-high pressure sterilization,first 2 U/(g protein)TG enzyme and then ultra-high pressure sterilization,first 4 U/(g protein)TG enzyme and then ultra-high pressure sterilization,then ultra-high pressure sterilization and then 2 U/(g protein)TG enzyme.Compared with yogurt without TG enzyme,the viscosity and hardness of yogurt with TG enzyme increased significantly(P<0.05).Compared with yogurt prepared by thermal sterilization,yogurt fermented by ultra-high pressure treatment has significant differences in color,taste and smell(P<0.05).The number of viable lactic acid bacteria was significantly higher than that of hot sterilized yogurt(P<0.05).Therefore,the viscosity,hardness and water holding capacity of yogurt were significantly increased by adding TG enzyme(P<0.05).There were significant differences in color,taste and smell between UHP sterilized yogurt and heat sterilized yogurt(P<0.05),and UHP sterilized yogurt significantly increased the number of viable bacteria in yogurt(P<0.05).Adding 4 U/(g protein)TG enzyme first,then ultra-high pressure sterilization treatment of milk fermented yogurt,and ultra-high pressure sterilization first and then adding 2 U/(g protein)treatment of milk fermented yogurt can significantly improve the quality of yogurt.