Effect Of Mixed Fermentation Of Lactic Acid Bacteria And Yeast On Cheese Flavor

Lactic acid bacteria and yeast are the main flora in traditional handmade cheese,which is closely related to the development of flavor.The existing research mainly focuses on the effect of lactic acid bacteria fermentation on cheese flavor.Using only lactic acid bacteria as starter is easy to control fermentation,but it may lead to insufficient flavor and product homogenization,and there are few studies on the effect of yeast on cheese flavor.Based on the previous research,the mixed fermentation of Lacticaseibacillus paracasei SMN-LBK and Kluyveromyces marxianus SMN-S7-LBK was used to improve the flavor quality of cheese and explore its effect on the flavor of cheese.The following conclusions are obtained:(1)To evaluate application potential of mixed fermentation in dairy fermentation,the fermentation characteristics of mixed fermentation and single-strain fermentation were compared,and the growth curve under different carbon sources,reducing sugar content,free amino nitrogen content,volatile flavor compound content,texture characteristics were measured,and sensory evaluation was carried out.Lacticaseibacillus paracasei SMN-LBK and Kluyveromyces marxianus SMN-S7-LBK can ferment lactose and galactose,and the growth of the two strains was significantly increased by mixed fermentation with glucose and galactose as carbon sources.The two strains have the ability to metabolize lactose and galactose,and the mixed fermentation enters the logarithmic growth phase quickly and has a large growth amount.Mixed fermentation quickly consumed glucose and galactose,and the consumption of galactose reached 3.71 g/L.The free amino nitrogen content of mixed fermentation can be as high as 1.35 g/L,and the ability of mixed fermentation to hydrolyze protein was stronger than that of single-strain fermentation.Mixed fermentation and single-strain fermentation samples have different composition of volatile flavor compounds,mixed fermentation has higher concentrations of 3-methyl-1-butanol,2-methyl-1-propanol,2-heptanol,1-butanol,1-nonanal,furfural,isobutyric acid,furfuryl acetate,ethyl caprylate and phenethyl acetate.Mixed fermentation reduced the texture characteristics of fermented milk,and the hardness and viscosity index were lower than those of single-strain fermented milk.Although the mixed fermented milk has a lower tissue state score,the aroma score is higher than that of the single-strain fermented milk,and the overall sensory perception is acceptable.(2)To evaluate the effect of yeast-assisted fermentation on the flavor quality of cheese,mixed and single-strain fermentation cheeses were prepared,and the basic physical and chemical indexes and changes in the contents of organic acids,amino acids and volatile flavor compounds during the ripening were determined.The moisture,protein and salt contents of mixed fermentation cheese were not significantly different from those of single-strain fermentation cheese.The mixed fermentation exhibited stronger protein hydrolysis,with higher free amino nitrogen content in the middle and late stages of ripening than in single-strain fermentation cheeses.Although the mixed fermentation cheese in the late ripening stage had lower contents of organic acids and amino acids,the contents of ethyl lactate,3-methyl-1-butanol and2-methyl-1-propanol related to the metabolic transformation of organic acids and amino acids were increased,reaching 963.83,86.49,and 4778.10 μg/kg,respectively.Mixed fermentation cheese increases the content of acids,alcohols and esters,with higher concentrations of isopentyl isobutyrate,ethyl caprate,isoamyl acetate,ethyl caprylate,phenethyl acetate,phenethyl alcohol,ethyl lactate,2-methyl-1-propanol,ethyl hexanoate,2-butanol,isobutyric acid.Changes in the aroma style of mixed fermentation cheeses are sensory acceptable and can increase the variety of cheese flavors.(3)In order to explore the flavor formation mechanism and interaction of mixed fermentation of lactic acid bacteria and yeast,the differential metabolites and metabolic pathways between mixed fermentation and single-strain fermentation cheese were measured by untargeted metabolomics.Protein and lipid derived metabolites(amino acids,glycerophospholipids,fatty acids and fatty acyls)are the main metabolites in cheese.Amino acids and fatty acids are the different nonvolatile flavor metabolites between mixed fermentation and single-strain fermentation cheese.Amino acid metabolism was the main differential metabolic pathway between mixed and single-strain fermentation cheeses.The metabolism of glutamate and glutamine is a key pathway with high correlation with the difference of metabolites,which has the potential of flavor regulation and production of functional compounds.Overall,this study evaluated the potential of mixed fermentation of Lacticaseibacillus paracasei SMN-LBK and Kluyveromyces marxianus SMN-S7-LBK for cheese production and clearly demonstrated the differences in flavor substances and changes in metabolite activity between mixed and single-strain fermentation cheeses.This study provides an interesting perspective for the production of mixed fermentation cheeses with flavor enhancement and will contribute to the progressive identification of flavor formation mechanisms and microbial interactions.