Yak milk whey protein denaturation and casein micelle disaggregation/aggregation at different pH and temperature
At the natural pH of yak milk (pH 6.6), a low level (<30%) of k-casein (k-CN) was found in the serum phase after heating at 95 oC for 30 min, indicating that as much as 70% of the b-lactoglobulin (b-Lg) and k-CN complexes is associated with the micelle colloidal particles. The b-Lg and k-CN levels increased from 13.2% and 2.6% at pH 6.0 to 35.2% and 60.1% at pH 7.0, respectively, when yak milk was heated at 95 oC for 30 min. At pH 6.0-6.4, the denatured whey proteins were associated with the caseins in the colloidal phase, resulting in milk gelation upon heating. The distribution of b-Lg and k-CN complexes increased in the serum phase, demonstrated by the increasing levels of both b-Lg and k-CN with increasing pH; at high pH (6.6-7.0), large proportions of b-Lg and a-lactalbumin were lost, presumably forming complexes in the colloidal phase.
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