Six millilitres of yak milk of each pH was transferred to glass vials, the vials were immersed in a thermostatically controlled oil bath maintained either at 120 C for 10 min with an additional 2.5 min come-up time or at 140 C for 2 min with an additional 3.0 min come-up time using an immersion circulator heater (Suzhou Bolinger Analytical Instruments Co., Ltd.), and rocked in a horizontal plane with an oscillation of 10 cycle min1.