Study On The Thermal Processing Characteristics Of Milk And Its Salt Equilibrium

Thermal treatment is a key procedure in dairy productos preparation, as it can ensure productsquality. However, as a heat-sensitive food, thermal treatment inevitably may lead to some negativeeffects on dairy products, especially on liquid milk, such as flavor, color, odor and nutritional value andso on. In addition, liquid milk quality will change as the storage temperature and storage period changes.In this study, we focused on the different thermal treatment intensity and different storage conditionsand evaluated the sensory quality, physicochemical and nutritional quality, biological quality of liquidmilk, such as pasteurized milk, ultra-pasteurized milk and ultra high temperature sterilized milk. Inaddition, we studyed the salt equilibrium in view of the importance of it for the stability of milk. Themain findings were as follows:(1). Thermal treatment could effectively reduce the number of bacteria and somatic cell count inmilk, and decreased the brightness and ionized calcium content, but increased the colloidal caseinparticle size. By different heat treatment, soluble phase transformed to the colloidal phase and alsoincreased the lipid oxidation reaction extent, but decreased the degree of hydrolysis of milk protein. Thecontent of furosine and lactulose increased as the improvement of thermal treatment. In contrast, thecontent of vitamin B1, vitamin B2and niacin decreased as the improvement of thermal treatment. Theloss of vitamins was higher as the higher and the longer the thermal treatment. Meanwhile, the plasminactivity decreased gradually along with the thermal treatment level improvement, and it still existed inthe ultra-pasteurized milk and ultra high temperature sterilized milk. The evaluation of the quality ofliquid milk should be comprehensive analysis and not rely solely on one aspect.(2). The optimal microwave digestion parameters were determined by orthogonal test. The bestconditions for the digestion were gained as following volume of nitric acid and hydrogen peroxide wererespectively0.9ml,4.0ml and digestion for28min. Meanwhile, the highest pressure was1.5MPa andthe largest power was1000W for microwave digestion. The recovery was in range of96.0%to98.1%for calcium in milk,and the precision expressed as RSD values was0.50%. The results showed thatmicrowave digestion was rapid, simple with large sample and accurate for determining elements inmilk.(3). The treatment like heating and adding calcium and chelating agents could change the contentof ionized calcium and pH value. In addition, the properties of colloidal casein was influenced as thetransformation between soluble and colloidal phase salt. The calcium salt could reduce the stability ofmilk. However, the thermal processing and chelating agents could improve the stability of milk and theinfluence was related to the degree of the heat treatment and chelating agent types closely. There weremany factors that affect the salt equilibrium in milk, e.g. temperature, pH value and adding salt. Thecalcium and pH value were more important factors.(4). Many indexes changed for UHT milk during storage period. The pH value, L*value, colloidalcasein particle size and the content of ionized calcium decreased as the storage temperature and time, but the b*value, the degree of hydrolysis of milk protein and the lipid oxidation reaction extent showedthe upward trend. Meanwhile, the change level of indexes for UHT milk was significantly higher thanthe low temperature and room temperature. In addition, the quality and the shielf life of UHT milk wasclosely related to the storage temperature and time.