Screening Of Fat And Fatty Acid Degrading Bacteria And Its Application In Fermented Soymilk

During the fermentation,the fat,protein,and starch in the raw materials are degraded by microorganisms into fatty acids,amino acids and sugars,which determine the final flavor,texture and organoleptic characteristics of the fermented products.Degradation products of fat and fatty acids are one of the main sources of flavor substances in fermented foods.In this study,the strains with the ability to degrade fat and fatty acids were screened,then were applied in fermented soybean milk to improve the flavor.This study lays a foundation for the production of fermented soy milk.The main results were as follows:(1)Ten strains of fat-degrading bacteria and eight strains of fatty acid-degrading bacteria were isolated from fermented foods which were collected from different regions of Inner Mongolia,using fat and fatty acids(oleic and linoleic acids)as the sole carbon sources,respectively.Among them,the strain F-YC2 showed the highest fat degradation rate of 50.6±2.3%.The strain A-BEF514 had the highest oleic acid degradation rate of6.6±2.4% and linoleic acid degradation rate of 12.7±1.1%.By 16 S r DNA sequencing,the strain F-YC2 and A-BEF514 were identified as Bacillus alpinus and Bacillus licheniformis,respectively.(2)The effects of fermentation temperature,initial p H,inoculum amount,and shaking speed on fat degradation of strain F-YC2 were investigated by single-factor experiments.According to response surface analysis,the optimal degradation conditions were as follows:fermentation temperature 37 ℃,initial p H 7.0,inoculation amount 1.5%.The fat degradation rate could reach 65.5±1.9% under this condition.Meanwhile the fatty acid degradation conditions for A-BEF514 were optimized by single factor and orthogonal test.The results showed that the degradation rate of oleic acid was 7.3± 0.6% at p H 7.5,inoculum amount of 2.5% and 175 r/min at 31 ℃,and the degradation rate of linoleic acid was 13.8±1.9% at p H 6.5,2.0% inoculum,225 r/min at28 ℃.The degradation rate of linoleic acid was 13.8±2.0% at p H 6.5,inoculum amount of 2.0% at 225 r/min and 28℃.In addition,it was found that the degradation products of oleic and linoleic acids for strain A-BEF514 were some esters,acids,and ketones by GCMS analysis.(3)the fermented soybean milk was produced using Streptococcus thermophilus and Lactobacillus bulgaricuse as starer,which combined with strain F-YC2 and A-BEF514 respectively.The optimal fermentation conditions for the addition of F-YC2 were determined as follows: the inoculum level of 3.0%,the ratio of 1:1:1(Streptococcus thermophilus: Lactobacillus bulgaricus: F-YC2),the fermentation time of 12 h at 40 ℃,and the sensory score of fermented soymilk was 8.8 ±0.2.For A-BEF514,the optimal conditions were the inoculum level of 3.0%,the ratio of 1:1:0.5(Streptococcus thermophilus: Lactobacillus bulgaricus: A-BEF514),the fermentation time of 12 h at37 ℃,and the sensory evaluation score of the fermented soymilk was 9.0±0.2.