Study On The Influencing Factors Of P-resol And Its Precursor In Strong Aroma Type Chinese Liquor

The liquor-making process of Chinese liquor is a solid-state fermentation process in which multiple strains work together under natural conditions.Different microorganisms not only produce flavoring substances but also off-odor chemicals which affect the taste of Chinese liquor.p-Cresol(PC),a kind of compound with off-odor,has been detected in liquors with different flavors,affecting the taste and quality of Chinese liquor directly.Currently,people know little about p-cresol-associated microorganisms in Chinese liquor due to the complexity of the brewing process,thus the control measures for p-cresol content remain to be studied.In this study,changes of p-cresol content and microbial community structure at different positions of fermented grains during the brewing process of Chinese liquor were investigated.Factors affecting the p-cresol content were explored using culture-dependent methods.Finally,we clarified the main source and generation stages of tyrosine which are precursor of p-cresol.The main results obtained are as follows:(1)The increasing of p-cresol content in fermented grains are mainly occurred in the bottom layer at early stages(0-7 d)and later stages(40-60 d)of fermentation process.The pcresol content in fermented grains at the same layer are similar,and the bottom layer of fermented grains has a higher p-cresol content than the upper layer.Fermented grains which have fermented for the same time from different geographic sites of the cellar has similar prokaryotic and eukaryotic microbial communities,while microbial communities from fermented grains which have fermented for different time are quite different.Among them,Clostridium was positively correlated with p-cresol content,while Bacillus,Lactobacillus and Pichia were negatively correlated with p-cresol content.(2)The p-cresol-producing capacity of single strains is weaker than that of pit mud microorganisms under the same culture conditions,and this is mainly caused by the action of other microorganisms in pit mud,while the complex chemical compositions in pit mud is generally restraining for p-cresol generation.There are no synergistic effect between strains of different species on the production of p-cresol,while the presence of lactic acid bacteria can significantly inhibit the growth of p-cresol bacteria.Different environmental conditions have a great influence on the production of p-cresol.The pH,lactate concentration and ethanol concentration in fermented grains are all within the range of inhibiting for p-cresol production.(3)The saccharification stage of the raw material and the early stages of fermentation are the main stages for tyrosine production.Bacteria and fungi play a leading role in the process of proteolysis of proteins into amino acids.Both strains and the raw materials have influences on the total amount of amino acids and tyrosine content in the fermentation process,and strains are main factors influencing the amino acid composition.Compared with other strains,Bacillus subtilis JQ11 and Bacillus amyloliquefaciens JQ8 tend to generate more tyrosine when they hydrolyze grain proteins.Glutinous rice and rice tend to be hydrolyzed to generate the relatively higher contents of tyrosine,while sorghum produces less tyrosine after hydrolysis.