yak wolfberry yoghurt, fermentation process optimization, freeze and thaw treatment times, rheological property, microstructure, Biotechnology, TP248.13-248.65, Food processing and manufacture, TP368-456

The yak milk has frozen and thawed repeatedly during processing, which will reduce the quality of yoghurt. Yak wolfberry yoghurt was prepared using frozen yak milk and red wolfberry as raw materials. The fermentation process was optimized through single-factor and orthogonal experiments, and the effects of freeze-thaw cycles of 2, 4, 6, 8, and 10 on the particle size of yak milk, the secondary structure of milk proteins, and the physicochemical indexes of yoghurt were explored. The results showed that the optimal fermentation process conditions of yak wolfberry yoghurt were as follows: red wolfberry pulp 7%, sugar 8%, starter 0.3%, fermentation temperature 41 ℃, and fermentation time 9 h. Under the optimal conditions, the sensory score of yogurt was (85.14±1.86) points, the apparent viscosity was (5.14±0.17) Pa·s, and the lactic acid bacteria count was (5.13±0.32)×109 CFU/g. With the increase of freeze-thaw treatment times, yak milk particle size decreased, secondary structure changed, elastic modulus and viscous modulus of yoghurt decreased, internal three-dimensional space structure collapsed, the lactic acid bacteria count decreased, and the water holding capacity and acidity of yoghurt decreased after 6 freeze-thaw times. Therefore, in the actual production process, the freeze-thaw treatment times of yak milk should be controlled not more than 6 times.