| Qu which is used as a sacchariferous starter is a core material in traditional Chinese liquormaking process.And it is produced by multispecies solid-state fermentation.Microbial community,enzyme system and flavor substances formed from Daqu fermentation are very important to the fermentation of Baijiu.However,studies of microbial diversity,succession mechanism and its contribution in the fermentation of Baijiu are still limited,so midtemperature Daqu(Lu-type)was used as a study target in this paper.The structure of microbial community and succession of microbe were analyzed,and the main environmental variables which influenced the succession of microbial community were illustrated during Daqu fermentation process;the research on microbial community of Daqu participating in the fermentation of Baijiu was studied,and the influence of Daqu volatile compounds on fermented grains and base wine were analyzed.We investigated the contribution of Daqu to the fermentation of Luzhou-Baijiu from two aspects: microbial community and volatile compounds.The major results of this paper are as follows:1.The high-throughput sequencing technology was used to analyze the microbial community structure during the Daqu fermentation process.A total of 45 orders,154 genera bacteria were detected.Among them,Weissella,Leuconostoc,Bacillus and other 6 genera were dominant bacteria(sum of relative abundances was more than 83.3% on average);at the same time,A total of 34 orders,87 genera fungi were detected,Thermoascus,Candida,Thermomyces and other 7 genera were dominant fungi(sum of relative abundance was more than 84.7% on average).According to the succession of microbial community,the Daqu fermentation process could be divided into two stages: stage I(0-12 d)and stage II(12-32 d).2.Significance test of the spearman's correlation between the dynamics of environmental variables and the succession of the microbial community was performed.In stage I of fermentation(0-12 d),temperature correlated significantly with bacterial community succession(r = 0.71,P = 0.02),while no significant correlation was shown between moisture and acidity(P > 0.05).All the tested environmental variables(temperature,moisture,and acidity)were closely associated with the dynamics of fungal cell number(P < 0.05).In stage II(12-32 d),no significant correlation between environmental variablesand succession of bacterial and fungal communities was observed(P > 0.05).Thus,temperature was determined to be environmental variables playing a major role in stage I of MT-Daqu fermentation.The temperature data of 10 batches of MT-Daqu were collected and fitted by the weighted least square method.the first 12 d of MT-Daqu fermentation could be divided into two substages: stage I-1(0-4 d)and stage I-2(4-12 d).In substage I-1,the temperature of MT-Daqu increased rapidly to 55°C,the relative abundance of most bacterial taxa and most genera from Saccharomycetales in MT-Daqu showed a positive relation with the increase in temperature.At a relatively high temperature(53 to 56°C)in substage I-2 of MT-Daqu fermentation,heatresistant microtia(Bacillus and Thermoascus et al.)could survive during this substage.3.Microbial community structure,physicochemical variables and enzyme activity of Daqu under different fermentation temperature were analyzed.Microbial community structure was significant different between T37 group and T50 group.The relative abundance of heat-resistant microorganisms including Bacillus,Thermoactinomyces and Thermoascus in T50 group was significantly higher than that of T37 group.the relative abundance of Staphylococcus,Weissella,Aspergillus and Gibberella in the T50 group was significantly lower than that of the T37 group.It showed that maintaining a high fermentation temperature would selected the microbial community to adapt to the heat-resistant microbial communities.4.The microbial community succession of fermented grains was analyzed during the fermentation of Baijiu.A total of 397 genera bacteria were detected.Among them,Lactobacillus,Bacillus,Weissella and other 2 genera were dominant bacteria(relative abundance > 1.0%);meanwhile,155 genera of fungi were detected,mainly composed of Kazachstania,Aspergillus and Thermoascus.According to the succession rules of bacterial community structure,the fermentation process can be divided into three stages: stage ?(0-5 d),stage ?(5-17 d)and stage III(17-40d),meanwhile the fermentation process could also be divided in to three stages by succession rules of fungal community: stage ?'(0-4 d),stage ?'(4-17 d)and stage III'(17-40d).Through analysis of different stages of microorganisms,there was a significant correlation between community succession and environmental variables(temperature,moisture,acidity and ethanol concentration)dynamics,and the succession of bacterial community in stage I(stage I')was significantly correlated with the changes in temperature,moisture,and ethanol concentration(P < 0.05).The succession of fungal community was significantly correlated with the changes of moisture content of fermented grains.There was no significant correlation(P > 0.05)between the microbial community succession environmental variables(temperature,moisture,acidity and ethanol concentration)dynamics in stage II and stage III(stage II' and stage III').5.Structures of microbial community between Daqu and fermented grains were quite different during the fermentation of Luzhou-flavor Baijiu.Bacteria and fungi,source from Daqu,and its relative abundance were 81.5% and 79.2% in fermented grains(d 0),respectively.The dominant microbes(Bacillus,Leuconostoc,Aspergillus and Thermoascus et al.)from Daqu hold a high relative abundance in fermentation grains(0-5 d),but some of these microbes were disappeared or others were kept a low relative abundance in later fermentation stage of fermented grains.Lactobacillus(OTU2667 and OTU174)and identified as Kazachstania(OTU001)which were low relative abundance in Daqu,became dominant microbes in later fermentation stage of fermented grains.Additionally,bacteria(OTU539 and OTU737 et al.),and fungi(OTU010 and OTU020 et al.)which were from environment also became dominant microbes in later fermentation stage of fermented grains.It suggested that Daqu indeed was considered as a starter in Luzhou-flavor Baijiu,and the microbe from environment also played an important role in the fermentation of Baijiu.6.The volatile compounds during the fermentation of Luzhou-flavor Baijiu was analyzed by GC-MS.A total of 63 substances were detected in Daqu samples,and the average concentration of volatile compounds was 12047.41 ± 578.59 ng/g dry Daqu;a total of 75 substances were detected in the fermented grains,and the average concentration of volatile compounds was 20.51 ± 3.63-148.73 ± 18.21 ?g/g dry fermented grains;a total of 49 substances were detected in the base wine samples,and the average concentration of 1002.75 ± 78.14 ?g/mL.Volatile compounds in samples(Daqu,fermented grains and base wine)were significant different,and fermented grains contained most of the volatile compounds in the liquor-making process.Most volatile components(esters,alcohols and acids)of Daqu also found in fermented grains,at the end of fermentation,the concentration of these compounds in fermented grains(d 40)and base wine samples had significantly increased.However,pyrazines in Daqu samples were not detected in fermented grains. |
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