Influence of Feeding Systems on the Microbial Community and Flavor Characteristics of Raw Milk: A Comparative Analysis.

This study analyzed the composition of the microbial community in raw milk and its effects on flavor compounds by comparing samples from cows raised on pasture (PXN) and in housing (HXN). In August 2023, 9 raw milk samples from Holstein cows were collected from a pasture in the Xilin Gol League, Inner Mongolia, China, and labeled as the PXN group. Simultaneously, 9 raw milk samples were collected from Holstein cows raised by individual farmers in the region and labeled as the HXN group. Metagenomic sequencing technology was used to detect and analyze the dynamics of microbial community in milk samples. At the same time, headspace solid phase microextraction, electronic nose and electronic tongue technology were used to monitor the flavor characteristics of raw milk under the 2 feeding conditions and to analyze the correlations between them. The results showed that raw milk protein (PXN 3.43%; HXN 3.36%) and fat (PXN 4.01%; HXN 3.79%) content in pasture group were higher than that in the housed group, while lactose content (PXN 4.98%; HXN 5.12%) and total solid content (PXN 4.98%; HXN 5.12%) were lower than that under the housed system. However, there was no significant difference between non-milk fat solid contents of the 2 groups. W1C, W5C, W5S, W2S sensors of electronic nose played an important role in distinguishing the 2 groups of samples. The sour taste of raw milk in pasture group was different from that in house feeding group. There were significant differences in the microbial communities of the 2 groups, with Macrococcus caseolyticus (39.85%) in PXN group and Lactococcus lactis (49.59%) in HXN group being the dominant bacteria. The volatile compounds of raw milk in PXN and HXN groups were detected by headspace solid phase microextraction method. A total of 62 and 53 flavor compounds were detected in PXN and HXN groups, respectively. Terpenoids, esters and aldehydes were dominant in PXN group, while acids were dominant in HXN group. The saturated fatty acid (SFA) content in HXN group (76.13%) was higher than that in PXN group (72.47%). The gamma-linolenic acid (GLA) content in PXN group (3.56 g/100g) was higher than that in HXN group (1.32 g/100g). The dominant strains, i.e., Macrococcus caseolyticus, Lactococcus lactis and Lactococcus cremoris, in both PXN and HXN groups exhibited positive effects on the formation of alcohols, esters, phenols and unsaturated fatty acids. In addition, Macrococcus caseolyticus in PXN group showed significant positive correlation with calcium lactate (CAL). This study comprehensively elaborates the microbial community structure and flavor quality of raw milk under different feeding methods, which may serve as a reference to control the quality of raw milk.