Effects of Freezing Raw Yak Milk on the Fermentation Performance and Storage Quality of Yogurt

In this study, the effects of freezing yak milk at -20 °C and -40 °C for 30, 90 and 180 days on the fermentation characteristics and storage quality of the corresponding yogurt were discussed. The results showed that, compared with that of yogurt made from fresh yak milk, the lactic acid bacteria (LAB) growth and acid production rate of the yogurt in the -20 °C group decreased at 90 d. The water-holding capacity, viscosity and hardness decreased during storage, and a sour taste was prominent, while no significant changes were observed in the -40 °C group. At 180 d of freezing, the post-acidification of the yogurt in the -20 °C and -40 °C groups increased after 21 d of storage. Compared with the -40 °C group, the -20 °C group showed a significant decrease in LAB counts, a decrease in pH value to 3.63-3.80 and poor texture and sensory quality.