| Yogurt is favored by consumers because of its unique flavor and rich nutrition.At the same time,how to reduce the production cost of yogurt and further optimize the process of yogurt is also a problem that people have been paying attention to.However,traditional research often uses simple sensory,physicochemical and microbiological indicators as criteria for determining the quality of yogurt.Studies showed that fermentation conditions have a greater impact on the growth and metabolism of fermentation strains.Among them,fermentation temperature,anaerobic and non-anaerobic fermentation,and the addition of probiotics had a greater impact on yogurt production.However,the current fermentation conditions for yogurt nutrients and the specific effects of metabolites are still unclear.In this study,fresh milk tested by dairy companieswas seleted as raw material,and DANISCO’s Lactobacillus bulgaricus and Streptococcus thermophilus were used as fermentation strains,which were fermented at different temperatures(30℃,37℃,40℃,42℃,45℃),anaerobic and non-anaerobic conditions and the addition of different probiotics(xylo-oligosaccharide,oligo-isomaltose,galactooligosaccharide)three fermentation conditions to prepare test yogurt samples.The yoghurt was subjected to conventional physical and chemical analysis,color difference analysis,electronic tongue analysis,full texture analysis,rheological analysis,and observation of the microstructure of the sample by scanning electron microscopy;the contents of fatty acids and free amino acids in yogurt were determined by GC-MS and HPLC;Finally,non-targeted metabolomics technique was used to analyze the effects of different fermentation conditions on yoghurt metabolites,in order to obtain new discoveries and accurately guide the industrial production of yoghurt.The following results were obtained through research:1.Fermentation temperature had different effects on the nutritional quality and metabolites of yoghurt.Among them,as the fermentation temperature increased,the chewiness,bitterness and umami taste of the yogurt decreased significantly,while the sour taste showed an upward trend.In addition,the fermented yoghurt at 37℃,40℃ and 42℃ had better water holding capacity;the yoghurt fermented at 45℃ showed a strong viscosity,and the yoghurt had the best elasticity at 37℃ fermentation;The yoghurt fermented at 37℃ and 42℃ had a more uniform microstructure and no large agglomeration;the relative content of citric acid,lauric acid and myristic acid in the yoghurt fermented at 40℃ was increased,and the fermentation condition at 37℃ can better ensure the content of free amino acids in yoghurt.A total of 83 differential metabolites were detected in the 40℃ and 30℃ comparison groups.A total of 111 differential metabolites were detected in the 45℃ and 40℃ comparison groups.The metabolic pathways enriched to significant changes were glycerol phosphate metabolism,ornithine.Biosynthesis of lysine and niacin alkaloids,changes in metabolic pathways of tryptophan metabolism,etc.2.Anaerobic fermentation affected the nutritional quality and metabolites of yogurt to varying degrees.Anaerobic fermentation can improve the water holding capacity of yoghurt.At the same time,from the cross-section of the anaerobic fermentation yoghurt,the three-dimensional network structure of anaerobic fermentation yoghurt was more complete.The relative content of lauric acid,palmitic acid,oleic acid,octadecanoic acid and linoleic acid increased significantly under anaerobic fermentation conditions;anaerobic fermentation of yogurt increased the free amino acid content of yogurt,among which Val,Ile,Leu,His The contents of Glu,Ala,Tyr and Pro were significantly increased.For metabolomics research on anaerobic fermentation yoghurt,66 differential metabolites were detected,including the significantly up-regulated metabolite ergoflavin,pyridinium and short peptide.38 kinds of compounds,enriched into the changed KEGG metabolic pathway for glycerophospholipid metabolism,arachidonic acid metabolism,α-linolenic acid metabolism,riboflavin metabolism,in addition to terpenoid synthesis,tyrosine metabolism,refined ammonia Acid biosynthesis,lysine biosynthesis,etc.3.The addition of different probiotics has different effects on the nutritional quality and metabolites of yoghurt.The addition of probiotics improved the viscosity and elasticity of yoghurt.The yoghurt added with galacto-oligosaccharide had the highest viscosity;the addition of xylooligosaccharides and galacto-oligosaccharides increased the water holding capacity of yoghurt and improved the stability of yoghurt;the yoghurt of the xylose had a relatively good microstructure;the addition of probiotics increased the relative content of palmitic acid in the yoghurt,and the yoghurt,octanoic acid,lauric acid,(Z)-hexadecenoic acid of the xylo-oligosaccharide yoghurt was added.The relative content of 11-octadecenoic acid increased significantly;the addition of xylooligosaccharide increased the content of Val and Ile,but the free amino acid content of galacto-oligosaccharide yoghurt decreased.In summary,the fermentation conditions had a certain degree of influence on the nutritional quality and metabolites of yoghurt.Fermentation at 37℃ and 40℃,anaerobic fermentation and addition of xylooligosaccharide can better ensure the quality of yoghurt. |
