| The ester-producing yeast,also known as fragrant yeast,is a kind of yeast that can synthesize organic compounds with aromatic odors,especially producing ethyl acetate,and is widely used in the fields of seasoning,functional beverage,brewing and the like.The aim of this study was to screen and identify a strain of yeast with high ester-producing ability and optimize ester-producing conditions.In this study,9 strains of yeast were screened by sensory evaluation as the indicators,and the yeast obtained by the initial screening was rescreened again with the total ester production as the evaluation indicators to obtain the yeast strain with the strongest ability to produce flavor;the yeast obtained by screening was identified through morphological,physiological,and biochemical characteristics;the esterproducing conditions were optimized by single factor experiment combined with response surface optimization design.The results were mainly as follows.1.Through sensory evaluation and two-stage screening of total ester production,a yeast strain(No.Y-213)with strong ability of producing flavor was obtained.The initial total ester yield concentration of Y-213 yeast was 1.25g/L by saponification titration.Through morphological and microscopic examination,Y-213 yeast was large,thick,and bulging,with neat edges,thick and opaque appearance in milky white.Y-213 yeast cells were oval or round,with multilateral sprouting and sporulation.2.Through sugar fermentation experiments,starch compound experiments,urease experiments,nitrate reduction experiments,Y-213 yeast could ferment glucose to produce acid and gas,while couldn't produce starch compounds,produce urease for decomposition of urea,or reduce nitrate.The results of carbon source assimilation experiments showed that Y-213 yeast could not use lactose to ferment,and could ferment glucose,fructose,maltose and sucrose.The optimal carbon source was sucrose.The results of nitrogen source assimilation experiments showed that Y-213 yeast couldn't use nitrate and ammonium salts,but could use beef extract,peptone,and yeast extract.The optimal nitrogen source was yeast extract.The results of tolerance test showed that the optimum initial growth pH of Y-213 yeast was 6 and the optimum growth temperature was 34 °C.The maximum tolerated concentrations of Y-213 yeast for ethanol,acetic acid and NaCl were 9% and 0.4%,10%,respectively.The 26 S rDNA of Y-213 yeast was sequenced by molecular biology method,and the Y-213 yeast phylogenetic tree was compared to confirm that Y-213 yeast was abnormal Wickham's yeast.3.The effects of carbon source concentration,nitrogen source concentration,ethanol addition,acetic acid addition,temperature and time on the ester-producing ability of Y-213 yeast were investigated by single factor experiments.The results showed that the optimal sucrose concentration was 3%,the optimal yeast extract concentration was 5%,the optimal ethanol addition amount was 2%,the optimum acetic acid addition amount was 0.2%,the optimal culture time was 96 h,and the optimal culture temperature was 32.5 °C.4.The response surface design was used to optimize the esterification conditions of Y-213 yeast by four factors: nitrogen source concentration,acetic acid addition amount,temperature and time.The results showed that the optimal culture temperature was 32.1 °C,and the optimal culture time was 158 h.The optimal acetic acid addition amount was 0.23%,and the optimum addition amount of yeast extract was 4.30%.After optimization,the concentration of total ester produced by Y-213 yeast could reach 2.32g/L,which was 1.86 times before optimization.GC-MS detection of Y-213 yeast fermentation product showed that ethyl acetate was the main volatile substance,accounting for 54.60%. |
PDF Full Text Request