Effect Of Heat Treatment On The Properties Of Casein Form Yak Milk

Heat treatment is a widely-used method to sterilize in dairy processing, but heating canalso influence the functional properties of protein. Casein is the main protein in milk. Butthere are still many problems in its industrial production. Especially in the process of heattreatment, a series of physical and chemical properties change will happen, which causes thechange of the molecular structure, affecting the quality of the product. The fresh yak milk inTianzhu, Gansu province was used as the raw material in the experiment. The casein, caseinin skim milk and changing tendency of the casein structure and properties have been studiedunder the conditions of different temperatures (30、40、50、60、70、80、90℃) and differentheating time(5、10、15、20、25min) along with the condition of adding whey protein andlactose in yak milk. The results are as follows.1. In the experiment, ultra-centrifuge process which has little influence on the caseinstructure and properties has been applied to obtain casein. By measuring nitrogen content inthe casein, make sure the casein in the prepared fresh yak milk accounts for98%in the totalprotein content.2. The properties of the casein, casein in skim milk and the casein after adding wheyprotein and lactose change like this. With the temperature rising, the heat stability andemulsibility of casein lowered, especially when the temperature was above70℃. The EAI ofcasein was reduced to4.5140m~2/g when the temperature was90℃. To some extent, the longerthe heating time is, the more dramatic the declining tendency will be. The heat treatmentlasting longer than15min will have fairly great effect.3. With the temperature rising, the surface hydrophobicity and average particle size ofthe casein, casein in skim milk and the casein after adding whey protein and lactose changed alot, and the turbidity of the solution changed accordingly. The average particle size of caseinbecame smaller and then larger. When the temperature was70℃, the particle size wasreduced to155.8nm. And the particle size changed slightly after adding lactose. The change inparticle size was the same after adding whey protein with the temperature rising, it becamelarger, especially when the temperature was above70℃, and when it’s90℃, the particle sizeof casein in skim milk reached267.3nm. The longer the heating lasted, the greater the changewould be. It’s the most obvious that the particle size of casein reduced to138.8nm when thetemperature was70℃. The surface hydrophobicity of casein all increased with the temperature rising, especially when the temperature was above60℃. The longer the heattreatment lasted, the greater the change would be.4. The surface structure of yak casein changed according to the observation during theheat treatment by scanning electron microscope. While heating, whey protein and lactoseaffected the surface structure of casein obviously. This result is identical to the changingtendency of turbidity and heat stability, and confirms the results of hydrophobicity andaverage particle size to some degree.Therefore, it’s of great necessity to study the change of the casein structure in heattreatment. The changes of the yak casein structure and properties under different heattreatment conditions have been discussed in this thesis, which can provide the theoreticalreference for the processing of yak milk as well as other dairy products.