Research On The Relationship Between Microorganisms And Milk Metabolites During The Quality Change Of Refrigerated Raw Milk

The cold storage process of raw milk is a dynamic and complex process.Microorganisms change the milk environment and make metabolites change.At the same time,metabolites drive the directional succession of microorganisms,causing the quality of raw milk to gradually deteriorate over time.This experiment is aimed at 4? refrigerated raw milk,through analysis of quality changes,metagenomics,milk metabolites and their relationship,to explore the effect of microorganisms on milk metabolites,so as to provide a theoretical basis for improving the quality control technology of raw milk cold storage.With the extension of cold storage time,the fat,protein,lactose,and pH in raw milk changed significantly(p<0.05),and the number of colonies,psychrophilic bacteria,lactic acid bacteria,coliform bacteria also observably increased(p<0.05),aerobic spores were detected in cold storage 1d?2d,but thermophilic aerobic spores were not detected during cold storage.At the same time,lipase,protease,plasmin,lactoperoxidase,catalase,xanthine oxidase and alkaline phosphatase changed significantly,while the activity of lipoprteinlipase did not change significantly.The increase and decrease of fat and protein content were related to protease and microorganism,respectively.The research of metagenomics was carried out by IlluminaHiseq Xten sequencing platform.With the extension of cold storage time,the composition of microbial community in raw milk changed significantly.The taxonomy annotation showed that the genus mainly successively from Acinetobacter,Streptococcus,Anaplasma to Flavobacterium,Pseudomonas and Lactococcus.Functional annotation showed that the top three relative abundances were amino acid biosynthesis,carbohydrate metabolism and ABC transport.In addition,the metabolic pathways related to lipids,amino acids and carbohydrates were significantly enriched in 1 day,3 days and 6 days of cold storage,respectively.The correlation analysis between dominant microorganisms and metabolic pathway showed that Acinetobacter was positively correlated with lipid metabolism,Pseudomonas was positively correlated with amino acid metabolism,and Lactococcus was positively correlated with carbohydrate metabolism.The differential metabolites of raw milk were identified and separated by UHPLC-Q-TOFMS.With the extension of cold storage time,the differential metabolites in raw milk can be divided into six types,lipids and lipid molecular metabolites were the most diverse,followed by amino acids and their analogs,organic acids,carbohydrates and their conjugates,nucleotides and vitamins.In the early stage of cold storage(2 h?1d),there were few kinds of significant difference metabolites and weak correlation with bacteria,while in the middle stage of cold storage(3 d?4 d),the types of significant difference metabolites were the most and the correlation between metabolites and microorganisms was complex,which can be used as the key stage of raw milk quality control.Lipid-related differential metabolites was significantly positively correlated with Acinetobacter,vitamin C was positively correlated with Flavobacterium,and amino acid and carbohydrate differential metabolites were positively correlated with small abundance bacteria,such as Enterococcus,Brochothrix,Serratia.And the correlation between lactic acid and Lactococcus was less than that of Pseudomonas,Streptococcus,Enterococcus,Carnobacterium and Brochothrix.