| Heat-treatment of milk proteins have many significant properties that are widely applied in dairy products. In the last few decades, extensive research has been concentrated on the effect of proteins interaction during heat treatment of milk on process quality of milk products. The research selects yak milk as material, protein compositions were analyzed by RP-HPLC. Heat-induced aggregation of κ-casein(κ-CN)/β-lactoglobulin(β-Lg) were separated by size-exclusion fast protein liquid chromatography(SEC). At the same time, the protein composition of heat-induced aggregation of κ-CN/β-Lg was analyzed by RP-HPLC. The formation pathway of soluble heat-induced aggregation of κ-CN/β-Lg was discussed by simulating the heat-induced aggregation between κ-CN and β-Lg. Finally, the effect of heat-induced aggregation between κ-CN and β-Lg on acid gel properties of yak milk was studied. The aim of this research is to provide theoretic and technique supports for process of yak milk yoghurt.In this study, protein compositions of yak milk were analyzed by RP-HPLC, the results of experiments showed that the content of κ-CN, αs2-CN, β-CN and β-Lg in yak milk was significant higher than in bovine milk. When yak milk was heated at p H<6.8, the heat-induced aggregation between κ-CN and β-Lg occurred on the surface of casein micelle; when yak milk was heated at p H>6.8, the heat-induced aggregation between κ-CN and β-Lg served as soluble heat-induced aggregation of κ-CN/β-Lg exists in serum. The protein compositions of soluble heat-induced aggregation of κ-CN/β-Lg were analyzed by RP-HPLC, it is found that the level of κ-CN in soluble heat-induced aggregation of κ-CN/β-Lg decreased with increase of p H, the level of β-Lg increased with the increase of p H. The mass ratio of κ-CN to β-Lg is 1:2.The heat-induced aggregation was simulated by mix κ-CN, casein micelle with whey protein of yak milk, respectively. After heat treatment, soluble heat-induced aggregation of κ-CN/β-Lg in serum was separate by SEC; the formation pathway of soluble heat-induced aggregation of κ-CN/β-Lg was discussed. It is found that the denatured whey protein heat-induced aggregated with κ-CN and then dissociation from casein micelle. Finally, heat-induced aggregation of κ-CN/β-Lg enters yak milk serum.Adding yak milk serum with soluble heat-induced aggregation of κ-CN/β-Lg in unheated casein micelle, on the same condition, adding casein micelle with heat-induced aggregation of β-Lg /casein micelle in unheated yak milk serum. The properties of yak milk acid gel from two different heat-induced aggregations and heat treatment yak milk at different p H was analyzed, including rheological properties, gel time, p H, firmness, water-holding capacity and syneresis, the microstructure of yak milk acid gel was observed by scanning electron microscope(SEM). The results of experiment showed that two different heat-induced aggregations can increase elastic modulus, firmness and water-holding capacity, decrease the gel time and syneresis. However, soluble heat-induced aggregation of κ-CN/β-Lg has an advantage in increasing elastic modulus, firmness and water-holding capacity, decrease the gel time and syneresis when compared to heat-induced aggregation of β-Lg /casein micelle. The elastic modulus, firmness and water-holding capacity after heat-treatment of yak milk at p H 7.0 is higher than p H 6.6 and p H 7.4 significantly. The results of SEM showed that the acid gel of yak milk with higher content of soluble heat-induced aggregation of κ-CN/β-Lg has smaller and more homogeneous porous structure. |
