Effect of pH on heat stability of yak milk protein

Abstract Heat stability of yak milk protein reached a maximum at approximately pH 6.8 and was low at pH   7.0. At pH 6.4, the particle size of yak milk increased by approximately 20 and 25 nm when heated at 140 °C for 2 min and 120 °C for 10 min, respectively. The particle size of heated yak milk decreased markedly with increasing pH. The content of κ-casein dissociated from the casein micelles increased from 5.7% to 9.5% at pH 6.4–76.3% and 81.5% at pH 7.2 when yak milk was heated at 140 °C for 2 min and 120 °C for 10 min, respectively. The proteins α S1 -casein and β-casein showed a much lesser extent of dissociation from the micelles than did κ-casein. The levels of α-lactalbumin and β-lactoglobulin in ultracentrifugal supernatants increased from pH 6.4 to pH 7.2 and decreased above pH 7.2.