Study On Cross-linking Of Qula Casein From Yak Milk By Microbial Transglutaminase

Qula are dry milk slag that nomads make yak milk degreasing separation, boil,inoculation lactic acid bacterias which ferment milk protein and make them clotting in theqinghai-tibet plateau region. Qula are the unique Chinese resources. Caseins arephosphoproteins and approximately80%of the total protein content in milk proteins. Caseinsare at the core of dairy industry because of their desirable emulsifying properties and foamingproperties. Foreign scholars almost have no research to Qula, while the domestic scholarsmainly study on concentrated in the improving process and the nutrition value of Qula.Therefore, it is very necessary to study modification of Qula casein in order to expand itsapplication in industry.The study optimizes modification conditions by single factor test, Plackett-Burmandesign and Response Surface experiment, and explore the changes and mechanism on thestructure and functional properties of casein after cross-linking. The results showed that:1. The optimal condition as follows: mass concentration of casein5mg/mL, additiveamount of transglutaminase15U/g, reaction time135min, temperature47℃, pH7.0. In theseconditions, the research obtained maximum crosslinking degree of kuruts casein20.12%.2. After transglutaminase cross-linking, the content of free amino groups reduced from77.20μg/mg to61.73μg/mg. It showed that casein molecules cross-linked and generatedpolymers; the content of turns,-sheet and irregular structures decreased, α-helix were formed,and secondary structural components changed; the fluorescence intensity andhydrophobicity of Qula casein increased; the average particle size increased; the turbidityincreased.3. Transglutaminase cross-linking improved the thermal stability, calcium ion stability,alcohol stability of Qula casein, but the pH stability decreased, increased in pH4.5and5.After transglutaminase cross-linking, emulsifying activity and emulsion stability increasedsignificantly (p<0.05); the foaming and the foam stability increased significantly (p<0.05);water holding capacity increased significantly (p<0.05), oil holding capacity was not affected,viscosity increased significantly (p<0.05).In summary, transglutaminase cross-linking of casein induced secondary structures, tertiary structures and fourth structures changing, and the change of spatialstructure caused by changes in functional properties of casein. Transglutaminasecross-linking is an effective method to improve the properties of casein and a certainsignificance to expand application of casein.