Study On The Process And Characteristic Nutrients Of Fermented Yak Milk

Yak milk is a unique milk source in Tibetan areas of China.The yield of yak milk is increasing year by year.At present,yak milk products are mainly milk powder and ghee.The type is relatively single,and the degree of industrial production and utilization is low.Yak milk fermented by traditional strains has a single taste,insufficient aroma,insufficient fat degradation,and weak market competitiveness.In order to enrich Yunnan yak milk products,find lactic acid bacteria suitable for yak milk fermentation,and explore its characteristic nutrients,Shangri-La yak milk was used as the research object in this study.Lactobacillus plantarum L3 with good fermentation performance（acid production,aroma production,strong ability to degrade protein and fat）and traditional starter（Lactobacillus bulgaricus,Streptococcus thermophilus）were selected in the early stage of the laboratory to optimize the fermentation process conditions,and the quality index and storage performance of the product were studied.Metabolomics was used to analyze its characteristic small molecule metabolites and clarify the main metabolic pathways;potential bioactive peptides were explored by peptidomics combined with bioinformatics analysis.The results of the study are as follows:1.The processing technology and product quality of yak milk fermented milk（1）Processing technology of yak milk fermented milkThrough single factor experiment screening and response surface optimization,the best fermentation agent was determined as Lactobacillus plantarum L3 and Lactobacillus bulgaricus,Streptococcus thermophilus compound（strain addition amount 2%,compound ratio:3:1:1,sucrose addition amount 8%）,fermentation temperature 28°C,fermentation time30 h.（2）Product quality of yak milk fermented milkThe color of yak milk fermented milk is milky white,the smell is fragrant,the sour flavor is rich,the taste is soft,the tissue state is delicate,and there is no whey precipitation;protein 4.46±0.16 g/100 g,fat 4.18±0.14 g/100 g,acidity 98.07±1.48°T,viscosity9564±23.14 m Pa.s,water holding capacity 96.5±1.25%,viable count of lactic acid bacteria1.68×10⁹ CFU/g;mold,yeast and coliform bacteria were not detected.The number of lactic acid bacteria in fermented yak milk fermented milk stored at 4°C for 28 days was greater than 10⁶ CFU/g stipulated by the national standard,and the acidity was within the acceptable range of consumers.2.Study on the characteristic nutrients of yak milk fermented milk（1）Characteristic small molecule metabolites of yak milk fermented milk.Through non-targeted metabolomics technology,ultra-high performance liquid chromatography-quadrupole-flight mass spectrometry was used to collect the metabolite information of yak milk fermented milk in positive and negative ion modes.A total of 705 small molecule metabolites were identified.Combined with multivariate statistical analysis and screening,a total of 100 main metabolites were obtained,including acids,lipids,carbohydrates,amino acids,dipeptides and other small molecule metabolites.Among them,the characteristic small molecule metabolites are L-lactic acid,gentiobiose,L-carnitine,hexanoic acid,acetyl L-carnitine,and docosapentaenoic acid.These characteristic small molecule metabolites have a positive effect on improving the taste and functionality of the product.（2）Metabolic pathway enrichment analysis of small molecule metabolites.Through the correlation analysis of the main metabolites and pathways,9 key metabolic pathways were screened out,mainly including aminoacyl t RNA biosynthesis,glyoxylic acid and dicarboxylic acid metabolism,arginine and proline metabolism,citric acid cycle（TCA cycle）,D-glutamine and D-glutamate metabolism.The 16 main metabolites that were significantly mapped were aspartic acid,hydroxypyruvate,D-glycerol ester,ethyl acetate,glycine,L-phenylalanine,L-proline,L-glutamic acid,citric acid,D-glutamic acid,etc.（3）Bioactive peptides in fermented yak milk.The water-soluble peptides of yak milk fermented milk were extracted and separated by ultrafiltration,and the ultrafiltration components with MW>3 k Da and MW<3 k Da were obtained.The activity determination showed that the water-soluble protein peptide component with MW<3 k Da had higherα-glucosidase inhibitory activity(IC₅₀ value was 4.879 mg/m L),antibacterial activity IC₅₀value(Staphylococcus aureus IC₅₀value was 11.148 mg/m L,Salmonella IC₅₀ value was17.774 mg/m L,Listeria IC₅₀ value was 15.878 mg/m L);antioxidant activity(DPPH·clearance rate IC₅₀=19.209 mg/m L,ABTS·+clearance rate IC₅₀=10.446 mg/m L).A total of 469 water-soluble peptides were identified by liquid chromatography-tandem mass spectrometry,and the molecular weight was concentrated between 500～4500 Da.The primary structure amino acid sequence was analyzed by the active peptide database,and 24potential antioxidant peptides,3 hypoglycemic peptides,and 3 antibacterial peptides were retrieved,mainly fromαs1,αs2-casein,β-casein and k-casein.A new antimicrobial peptide APFKEVFGK(IC₅₀=0.318 mg/m L)and a newα-glucosidase inhibitory peptide RHPHPHLSFM(IC₅₀=0.621 mg/m L)were screened by using the structure-activity relationship of active peptides,molecular docking technology and solid-phase synthesis activity verification.It was clarified that yak milk fermented milk had antibacterial activity and hypoglycemic activity,which improved the biological value of yak milk fermented milk.3.Market acceptance test of yak milk fermented milk productsYak milk fermented milk was pre-packaged according to GB 28050-2011《General Principles for Nutrition Labeling of Pre-packaged Foods》.In the campus of Yunnan Agricultural University,the market acceptance test of yak milk fermented milk products was carried out in the form of questionnaires with the help of“Yunnong Double Innovation”activities.It was evaluated that the product had a delicate taste,rich milky aroma and pure texture.Yak milk fermented milk products have high public acceptance and good market potential.