Effect of pH on heat stability of yak milk protein

Heat stability of yak milk protein reached a maximum at approximately pH 6.8 and was low at pH < 6.4 and 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 αS1-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.