Evolution of bacterial flora in raw yak milk and its effect on quality characteristics during frozen storage

Due to the scarcity of yak milk in Tibetan areas, local herders primarily use traditional freezing methods for preservation. However, the extent of microbial contamination in raw yak milk during frozen storage and its impact on milk quality remains largely unexplored. This study analyzed the changes in bacterial populations and quality of raw yak milk stored at -18 °C for 6 months. The results indicated that, compared to fresh milk, the freshness of yak milk showed no significant change after 2 months of frozen storage (P > 0.05). After 4 months, the bacterial diversity of flora in yak milk increased significantly, with Pseudomonas and Acinetobacter emerging as the dominant psychrophilic bacteria. Correspondingly, the expression of metabolic pathways related to quality deterioration increased, with a significant increase in physicochemical indicators such as acidity, proteolysis degree, acid value and fat oxidation degree (P < 0.05). Extending the frozen storage to 6 months resulted in a slight but no significant decrease in bacterial diversity, with no significant difference in quality deterioration compared to the 4-month group (P > 0.05). Correlation analysis further revealed that Acinetobacter and Psychrobacter were significantly positively correlated with quality deterioration of frozen milk (P < 0.05). In summary, when frozen for more than 4 months, raw yak milk exhibited the highest bacterial diversity and predicted the most abundant metabolic pathways, with dominant psychrophilic bacteria massive proliferation leading to a decline in quality and safety.