The Effect Of Milk Compositions And Protein Polymorphism On The Heat Stability Of Yak Milk

With the accelerate development of yak milk and yak dairy products, the effect of processing characters of yak milk on product quality presents prominent problems. Physico-chemical properties, yak milk compositions and contents can provide important basis to determine the technological parameters of yak milk. Heat stability of yak milk is an important index used to access rationality of manufacturing technique and dairy product quality. Heat stability of yak milk may be affected by many factors, such as milk composition, breeds, growing environment, processi ng conditions, and so on. These interior and exterior factors can influence the yak milk composition, its physico-chemical properties, and its heat stability. In this paper, the research focuses on the yak milk composition, heat dissociation and aggregation of milk proteins, protein polymorphism of milk proteins, relationship between milk compositions, and heat stability. The aim of this research is particularly to discuss heat stability of yak milk from milk compositions and physico-chemical properties in order to provide the theoretical information for further utilization and process of dairy products.The total of 78 yak milk samples, obtained from different lactation periods and different seasons, including their basic physico-chemical properties, buffer capacity, heat-induced acidity and heat coagulation time(HCT) were studied. The experimental results showed that the contents of casein, whey protein, non protein nitrogen and fat in yak milk were significantly higher than those in bovine milk. The physico-chemical properties of yak milk and bovine milk were significantly different. Under maximum buffer condition of p H 5.1, the maximum buffer index of acidified yak milk was 0.049; the buffer capacity of yak milk was higher than those of bovine milk. Yak milk belongs to the type A milk. Yak milk maximum HCT at p H 6.8 is 17.5 min when yak milk was heated at 140℃. The p H value of yak milk decreased by 0.45 p H units when it was heated at 140℃ for 180 min. However, the p H value of bovine milk only decreased 0.31 p H units under same heating condition. The degree of heat-induced acidity of yak milk was higher than that of bovine milk. The heat stability of yak milk was found to be in direct correlation to p H. If yak milk contained higher contents of κ-CN, β-Lg, and non protein nitrogen, it had a longer HCT. With an increase of soluble calcium content, the heat stability of yak milk decreases. Therefore, based on the results, the heat stability of yak milk is significantly different from that of bovine milk.The relationship of denaturation of whey protein, dissociation and aggregation of casein micelle, protein polymorphism of milk proteins and heat stability at different p H and heat treatments were also studied. The contents of whey proteins, β-Lg and κ-CN in natural yak milk indicated positive correlation with heat stability, whereas the content of αs1-CN in yak milk indicated negative correlation with heat stability. The denaturation characteristic of β-Lg and α-La in yak milk was different from that in bovine milk, the denaturation degree of β-Lg was higher than that of α-La at p H 6.4- 6.6, while the denaturation degree of α-La was higher than that of β-Lg at p H 6.8-7.6. The denaturation degree of β-Lg and α-La in natural yak milk was lower than that in bovine milk. The dissociation degree and speed of four caseins at p H 6.4-7.6 was as follows: κ-CN>β-CN>αs1-CN>αs2-CN. Casein dissociation increased with the increase of heating temperature. At p H 6.4-7.6, the aggregation of casein micelle did not occur at high temperature(140oC 2 min,120 oC 10 min)treatment.Based on the analyzed results of two-dimensional electrophoresis and HPLC-TOF, αs2-CN、αs2-CN、κ-CN and β-Lg had three protein variants. β-CN only had A2 protein variant and α-La had B protein variant. All of four caseins contain phosphorylation group yak milk proteins that contained β-Lg A and κ-CN A variants, which can improve the heat stability of yak milk. Heat stability of yak milk was high because more phosphorylation groups of caseins could be able to combine more calcium and form stable casein micelle structures.The content of soluble calcium had negative correlation with p H and HCT at natural p H of yak milk p H(6.51-6.70), while total phosphorus and soluble phosphorus had positive correlation with p H and HCT. 32% of calcium and magnesium present ion state in serum, and others present citrate and phosphate state in casein micelle. 90% of phosphorus in yak milk present in HPO42- and H2PO4- form. High temperature treatment enables soluble calcium, citrate and phosphate in serum to decrease and form a new balance. However, the effect of high temperature treatment on salt balance was lower than that of bovine milk. The results showed that heat stability of yak milk was better thanthat of in bovine milk under the same heat treatment conditions.The relationship of the content of non protein nitrogen, urea, fat, and lactose in yak milk with HCT(140 oC), and the effect of the content changes of urea on heat stability of yak milk were also discussed. The results showed that the contents of non protein nitrogen and urea in yak milk had positive correlation with heat stability of yak milk. There was no correlation between contents of lactose and fat and milk heat stability. According to principal component analysis, the content of β-Lg, κ-CN, whey protein, non protein nitrogen, urea, total phosphorus, soluble phosphorus and p H had positive correlation with heat stability of yak milk. However, t he content of soluble calcium and αs2-CN had negative correlation with heat stability. The internactions were fitted between heat stability(HCT) and four variables(p H, β-Lg, NPN and soluble calcium), based Gauss mathematical model and Lorenz function. The results indicated that their interaction of four variables can significiantly effect the heat stability of yak milk.