This research aimed to study the bacterial community, free amino acid and fatty acid composition in featured dry fermented dairy products from some major pastoral areas of China and to analyze the correlation between bacterial community and free amino acid and fatty acid composition. The V4–V5 region of the bacterial 16S rDNA gene from Suannai Geda from Xinjiang, Qula from Tibet, Naizhazi from Inner Mongolia and milk fan from Yunnan was sequenced by high-throughput sequencing. The contents of fatty acids and free amino acids in these dairy products were determined by gas chromatography-mass spectrometry (GC-MS) and liquid chromatography (LC), respectively. Finally, the correlation of bacterial community with α diversity, fatty acids and free amino acids was evaluated by redundancy analysis (RDA). High-throughput sequencing results showed that a total of 2421338 reads were obtained from these dry fermented dairy samples, of which 1871524 were clean reads. The bacterial Chao1 and Shannon indexes in milk fan were significantly higher than those in the other three dairy products. The bacterial community compositions in the four dairy products were significantly different, but Firmicutes was dominant in all these products, among which the relative abundance of Firmicutes in Suannai Geda was the highest. The relative abundance of Proteobacteria in Qula was the highest. At the genus level, the bacterial community compositions in the four dairy products were dominated by Lactococcus or Lactobacillus. The contents of 16fatty acids in milk fan were higher than those in the other three products. A total of 24 free amino acids were detected, among which Suannai Geda contained the largest number (24) of free amino acids, and the contents of most free amino acids in Suannai Geda were higher than those in the other three products. Redundancy analysis showed that alanine was correlated with Acetobacter, Lactobacillus and Streptococcus, and acetic acid and butyric acid were correlated with Acinetobacter. The results of this study may provide a theoretical basis for the industrial production of traditional dry fermented dairy products in different provinces.

In order to investigate the effect of long-term consumption of yak milk on the intestinal flora and metabolism and to explore the correlation between acclimation to monodiet and yak milk intake in high-altitude pastoral areas, this study used metagenomics and metabolomics to evaluate the gut microbial community structure and metabolites of mice fed yak milk or Holstein cow milk. Results showed that compared with the control group (normal saline solution), at the phylum level, ingestion of yak milk improved the relative abundance of Actinobacteria (33.27%), Bacteroidetes (24.31%) and Verrucomicrobia (11.08%), while ingestion of Holstein cow milk increased the relative abundance of Firmicutes (43.74%) and Bacteroidetes (24.75%). At the genus level, yak milk consumption increased the relative abundance of Ackermania (11.80%), Bacteroides (6.09%) and Limosilactobacillus (4.77%), while Holstein cow milk consumption increased the relative abundance of Lactobacillus (28.62%), Duncaniella (6.15%) and Limosilactobacillus (6.49%). Both yak and Holstein cow milk up-regulated carbohydrate and nucleotide metabolism. Furthermore, yak milk significantly up-regulated amino acid metabolism, cofactor and vitamin metabolism, energy metabolism, the biosynthesis of secondary metabolites, and the biosynthesis and metabolism of peptidoglycan. Metabolomics revealed that yak milk increased the contents of metabolites such as VB₁, (+/-) 12 (13)-DiHOME, coenzyme Q2 and sulfonymethazine, as well as steroid hormone biosynthesis, cofactor biosynthesis, 2-oxyarboxylate metabolism, VB₆ metabolism and phenylalanine metabolism in mice. Investigation of intestinal tissue morphology showed that the intestinal wall became thicker with increasing diversity and richness of the gut microbiota and the intestinal villi became longer and denser without deformity or breakage. In conclusion, yak milk has a positive effect on the intestinal flora and metabolism in mice, and this study provides a scientific basis for further development and utilization of yak milk.

Yak milk is a dietary source of high-quality protein in the plateau region of China but as yet uncharacterized oxidative changes occur during heat treatment. Therefore, oxidation of and proteomic changes to milk proteins from plateau pasture-fed yaks after at different temperatures were investigated. Content of carbonyl groups, surface hydrophobicity increased, and total sulfhydryl, disulfide bond content decreased. Endogenous fluorescence intensity decreased after at increasing temperatures, indicating increased particle size, and absolute values of the zeta potential decreased. Analysis by Fourier transform infrared spectroscopy showed changes of the secondary structure, with relative content of α-helices increasing and then decreasing, β-sheet showed a trend of decreasing and then increasing while the relative content of random curl did not change. The close range of the β-turn gradually decreased, breaking the protein microstructure, and folding stacking occurred. Proteomics analyses showed a temperature dependent effect. Sixty-two proteins were suppressed and 49 elevated with 4 pathways up-regulated and 7 down-regulated at 65 °C. Thirty-one proteins were suppressed and 37 elevated with 5 pathways up-regulated and 4 down-regulated at 90 °C. The most extensive changes were observed at 120 °C, when 327 proteins were suppressed and 308 elevated with 11 pathways up-regulated and 33 down-regulated.