The Physico-chemical Properties Of Yak Casein Micelles

Casein micelles are at the core of dairy industry. However, little information wasavailable on the influence of acidification/heat treatment on the physico-chemical propertiesof yak casein micelles. Hence, the quality of yak dry caseins was poor. The followingproblems are researched:Firstly, the information of composition, size and hydration of yak casein micelles wasinvestigated. The yak casein micelles were isolated by ultracentrifugation, and the relativepercentage of micellar caseins were analysis by high performance liquid chromatography(HPLC). The micellar minerals were determined by on Association of Official AnalyticalChemists Methods (AOAC). The size characteristics of yak casein micelles were determinedby dynamics light scattering (DLS). The concentrations of micellar calcium, magnesium andinorganic phosphorus in yak milk obtained during the cold period were higher than thoseobtained during the warm period, while reversed result was observed for micellar citrate. Inyak casein micelles, the relative proportion of β-casein and αs1-casein showed seasonalsignificant. Casein micelles in yak milk obtained during the cold period showed smalleraverage diameter, narrower size distribution and higher hydration. On the other hand, theseparameters of casein micelles in yak milk differed significantly compared with those in cows’milk, with different contents of the components, larger average size and higher hydration.Secondly, during acidification, the physicochemical properties (the composition, volumemean diameter and hydration) of yak casein micelles were evaluated. Meanwhile, all indexeswere made a comparison with that of cow casein micelles. The precipitation/aggregation ofcaseins at their isoelectric pH, solubilization of calcium and inorganic phosphate and decreasein hydration of casein as a function of decrease in pH were significant for both milks. Forboth species, these molecular changes were qualitatively similar but quantitatively different.These quantitative differences during acidification were related to the differences ofcomposition between the milks. All the difference between the two kinds of casein micellesshowed, the well established acidification process of cow milk, should not be directly appliedto the acidification of yak milk, some adjustment is necessary.Thirdly, the effects of heat treatment (heating temperature and pH) on the structures ofyak casein micelles were systematically studied by spectroscopy. Heat treatment from resultedin an increase in their fluorescence intensity, hydrodynamic diameter and turbidity, but decreased the size polydispersity of casein micelles. The fluorescence intensity,hydrodynamic diameter and turbidity decreased with increased heating pH, but the sizepolydispersity of caseins increased with increased pH. These results suggested that heattreatment could be used to modify the structures and emulsifying properties of caseins byappropriately selected heating conditions. The study suggested that the influence of heattreatment on the structural properties of caseins was closely dependent on the temperature andpH. The degree of denatured whey proteins and the complex of casein micelles-whey proteinswere positively corrected with the increasing heating pH. However, the percentage of thewhey protein which was combined with casein micelles was negatively corrected with theincreasing pH. Compared with the micro-environment without lactose, the micro-environmentin the presence of lactose will reduce the degree of denatured whey proteins. However, thesize of the complex of casein micelles-whey proteins changed little.In general, there exist significant difference between yak casein micelles and cow caseinmicelles. All the difference between the two kinds of casein micelles showed, the wellestablished acidification process of cow milk, should not be directly applied to theacidification of yak milk, some adjustment is necessary. Meanwhile, heat treatment was auseful tool to regulate the structural properties of yak casein micelles.