Study On The Quality And Metabolomics Of Mixed Fermented Milk By Lactobacillus Brevis And Kluyveromyces Marxianus

Lactic acid bacteria and yeast are widely present in various traditional fermented foods and play important roles.The western Sichuan region has unique geographical characteristics and climatic conditions,nurturing a large number of microbial resources.This study uses lactic acid bacteria and yeast selected from traditional fermented dairy products in western Sichuan as fermentation agents to prepare fermented milk through mixed fermentation,with the aim of developing fermented milk with regional characteristics and discussing its quality formation mechanism,in order to enrich the types of dairy products and fermentation agent bacterial resources in China.The main research findings are as follows:(1)Using sensory scores as the main evaluation indicator,a group of lactic acid bacteria and yeast combinations with good fermentation performance,Lactobacillus brevis NZ4 and Kluyveromyces marxianus SY11,were selected.The optimal fermentation process was determined to be a 1:1 ratio of lactic acid bacteria and yeast inoculation,fermentation temperature of 38℃,sucrose addition of 6%,bacterial inoculation of 5%,and liquid loading of 70%.The sensory score of lactic acid bacteria-yeast mixed fermented milk was 88.5 points.(2)The quality analysis of lactic acid bacteria-yeast mixed fermented milk showed that the acid content(76.93±0.48 °T)of lactic acid bacteria-yeast mixed fermentation was significantly higher than that of yeast fermented milk,and the ethanol content(3.22±0.03g/L)was significantly higher than that of lactic acid bacteria fermented milk,but lower than that of yeast fermented milk;the difference in the number of lactic acid bacteria(10.35±0.27 lg CFU/m L)in the lactic acid bacteria-yeast mixed fermented milk is relatively small compared to the lactic acid bacteria fermented milk,but the number of yeast(9.86±0.33 lg CFU/m L)is significantly higher than that of yeast fermented milk;The total amount of organic acids(422.46±3.33 mg/100g)was significantly higher than that of two types of single fermented milk;The total amount of free amino acids(123.19±0.62mg/100 m L)was significantly higher than that of lactic acid bacteria fermented milk and yeast fermented milk,which were similar;Isopentyl acetate,acetone,and toluene in volatile flavor compounds contribute significantly to the formation of flavor in lactic acid bacteria-yeast mixed fermented milk.(3)Non targeted metabolomics analysis was conducted on fermented milk using different fermentation agents and different storage times.The results showed that the amino acids and their derivatives in the lactic acid bacteria-yeast mixed fermented milk were generally increased compared to the lactic acid bacteria fermented milk;Compared to yeast fermented milk,amino acids and their derivatives generally decrease.Compared to two types of single fermented milk,mixed fermented milk with lactic acid bacteria and yeast has 171 differential metabolic pathways,among which the Glycerophospholipid metabolism has the highest enrichment.In this pathway,Phosphatidylserine,CDP-diacylglycerol,and Phosphatidylglycerophosphate all increase compared to fermented milk with yeast.During the storage process of fermented milk,with the increase of storage time,there is a trend of decreasing amino acids and sugars,while increasing organic acids;Compared to 5 days and 10 days of storage,there were 151 differential metabolic pathways at 0 day storage,with higher enrichment in TCA cycle and Glycerophospholipid metabolism.Among them,TCA cycle is related to organic acid accumulation.(4)A storage experiment was conducted on the mixed fermented milk of lactic acid bacteria and yeast at 4℃.The sensory indicators,microorganisms,acidity,and ethanol content during storage were measured.The results showed that the sensory scores of fermented milk showed a downward trend during storage,and significantly decreased after10 d;The acidity and ethanol content show an upward trend;The number of yeast shows a decreasing trend,while the number of lactic acid bacteria first increases and then decreases,reaching 10.75 lg CFU/m L in 10 d.Based on comprehensive evaluation,the optimal storage time for lactic acid bacteria yeast mixed fermented milk is within 10 d.