| Chinese traditional Daqu formed by an open environment is a saccharifying and fermenting agent composed of microbes,enzymes and compounds,which plays an important role in the solid-state fermentation of vinegar.The fermentation temperature directly affects the fermentation process,quality and flavor of the final products.Increasing the maximum temperature(temperature increase)of Daqu during the making process becomes an innovation of brewing technique,however,vinegar Daqu is routinely manufactured following spontaneous low temperature fermentation and research on the metabolic pathway for the degradation of raw materials and the production of main favor compounds during vinegar Daqu is lacking.In this study,the solid-state fermentation process of vinegar Daqu was simulated in the laboratory.A high temperature(HT)batch and a medium temperature(MT)batch of vinegar Daqu were performed to analyze the effect of temperature increase on microbiota during vinegar Daqu fermentation process combined with metagenomics technology.The results indicated:(1)The fermentation temperature of the two batches vinegar Daqu reached the expected temperature.The glucoamylase activity showed a similar trend and no significant difference was exhibited between the two batches Daqu(P > 0.05).The relative content of ethanol in MT Daqu on the 9th day was significantly(P < 0.05)higher than that in HT Daqu,while the relative content of isobutanol was significant higher on the 4th day of HT Daqu fermentation(14.89±1.36 ?g/ 10 g daqu)(P < 0.05)than MT Daqu(8.03±0.61 ?g/ 10 g daqu),and the relative content of ethyl acetate in the HT batch on the 9th and 14 th days were significant higher than in MT Daqu(P < 0.05).(2)According to the temperature transition during the Daqu fermentation process,a total of 10 samples from two batches of vinegar Daqu were sequenced by metagenomics and 230,843 Unigenes sequences were obtained.Lactobacillales,Saccharomycetales and Enterobacterales were dominated orders during the whole fermentation process of the two batches of vinegar Daqu.Compared with MT batch Daqu,a higher abundance of the thermophilic Eurotiales were exhibited in the HT Daqu.The functional databases annotation results showed the main metabolic pathways of the Daqu were carbohydrate metabolism and amino acid metabolism.(3)The enzymes related to metabolic pathway of materials degradation and the main flavor in Daqu were deeply explored and encoded genes of these enzymes in gene catalogue were predicted at genus level by taxonomy annotation.The formation of lactic acid,acetic acid and ethanol were related to carbohydrate metabolism.In addition,the relative abundance of starch degrader——?-amylase and glucan 1,4-?-maltohydrolase in HT Daqu fermentation were higher than that in MT Daqu,but higher abundance sucrase-isomaltase(hydrolyze dextrin into glucose)was displayed in the MT batch Daqu than HT Daqu.Higher abundance of cellulose degrader——?-glucosidase and cellulose 1,4-?-cellobiosidase were showed in the HT batch Daqu compared with MT batch Daqu,indicating that high temperature fermentation might contribute to the degradation of the cellulose in the raw material and increase the utilization rate of the raw material.For functional microbial analysis,it was exihited that the number of functional enzymes annotated in Bacillus was the most.Besides,Eurotiales(especially Aspergillus and Thermoascus)was irreplaceable in cellulose degradation during vinegar Daqu fermentation.This paper reveals the effect of temperature increase on the microbial biomass,structure and function during vinegar Daqu fermentation process,and deeply analyzes the metabolic pathways that degrade raw materials and produce main flavor substances by microbiota.It can provide a reference for the formation mechanism of vinegar Daqu microbes and enzymes.This study is of great significance to upgrade the traditional brewing process and improve the quality of vinegar. |
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