| Strong-flavor Baijiu is favored by the majority of consumers,which puts forward objective requirements for the quality of strong-flavor Baijiu to a certain extent.The flavor substances directly determine the quality of strong-flavor Baijiu,which mainly comes from the metabolic activities of various brewing microorganisms in the cellar.In recent years,there have been more and more researches on the microbial community structure and flavor substances in the fermented grains of strong-flavor Baijiu,but there are few researches on microbial community succession and dynamic changes of flavor substances in the fermented grains of strong-flavor Baijiu.In this study,samples were taken from the upper and lower levels of fermented grains every 10 days with a fermentation period of 50 days.Firstly,the physicochemical properties of fermented grains were analyzed,and the optimal extraction conditions of volatile flavor substances in fermented grains were explored.On this basis,volatile flavor substances were tracked and determined.Secondly,the structure and succession of bacterial communities in fermented grains were studied.Finally,the relationship between bacterial community structure of fermented grains and volatile flavor substances was explained from the perspective of causality.This study is helpful to further clarify the formation mechanism of important volatile flavor substances in strong-flavor Baijiu,which is helpful to guide the production of strong-flavor Baijiu.The main research results of this topic are as follows:1.The analysis results of physicochemical properties of fermented grains showed that the moisture in the fermented grains of upper and lower grains rose continuously with the fermentation process.The temperature rose continuously at first,and decreased slightly after the 40 th day of fermentation.The p H values decreased at first and increased slightly after the 40 th day of fermentation.Compared with the upper fermented grains,the moisture and temperature in the lower fermented grains were higher,but the p H was lower.The experimental results of optimizing the extraction conditions of headspace solid-phase microextraction(HS-SPME)by response surface method showed that the optimal extraction conditions of volatile flavor substances in fermented grains were as follows: the extraction temperature was 62℃,extraction time was 33 min and desorption time was 4 min.Gas chromatography-mass spectrometry(GC-MS)was used to detect volatile flavor substances in the fermented grains,and the results showed that 184 volatile flavor substances were detected in all fermented grains,including 64 esters,9 acids,26 alcohols,17 aldehydes,35 hydrocarbons,4heterocycles,10 aromatic compounds and 19 other compounds.The accumulation of different volatile flavor substances showed obvious spatio-temporal differences.The main flavor substances of strong-flavor Baijiu,such as esters,acids and alcohols,were mainly accumulated in the upper fermented grains during the 0-20 th day of fermentation,while other trace flavor substances mainly accumulated in the lower fermented grains during the 30th-50 th day of fermentation.2.Operational taxonomic unit(OTU)analysis results showed that OTU 1(Lactobacillus),OTU 6(Weissella),OTU 9(Pediococcus),and OTU 4(Roseimarinus)were the 4 bacterial OTUs with the largest abundance in the samples of fermented grains,and the abundance of OTU 1 increased continuously during the fermentation process.The abundance of other OTUs showed the opposite trend.A total of 113 bacterial OTUs were found in all samples of fermented grains,of which 92 were common OTUs.The number of unique OTUs in the fermented grains on the 10 th day was the largest,and the number of unique OTUs in the upper fermented grains was significantly higher than that in the lower fermented grains.The results of Alpha diversity analysis showed that the Coverage index of all samples of fermented grains was very close to 1,and the variation trends of Chao 1 index and Shannon index of bacterial communities in the upper and lower fermented grains were obviously different.The Chao 1 index,Shannon index and their change degree of the bacterial communities in the upper fermented grains were significantly higher than those in the lower fermented grains,and the abundance and evenness of the bacterial communities in the upper fermented grains during the first 20 days of fermentation were significantly higher than those in other fermented grains.The results of Beta diversity analysis showed that 17 phyla and 279 bacterial genera were detected in all samples of fermented grains,among which Firmicutes were the absolute dominant phyla in the fermentation process of strong-flavor Baijiu.The relative abundance of Firmicutes increased from 72.87% and 76.54% in the upper and lower fermented grains on the 0th day to 95.55% and 97.19% on the 50 th day,respectively.Lactobacillus was the main dominant genus(22.36%-98.8%).The bacterial community structure of the fermented grains on the 0th day,the upper fermented grains on the 10 th day and the lower fermented grains on the 40 th day were significantly different from other samples.3.The physicochemical properties of fermented grains(moisture,temperature and p H)and volatile acids(acetic acid,propionic acid,butanoic acid,pentanoic acid,hexanoic acid,heptanoic acid and octanoic acid)were selected as environmental factors to analyze their association with bacterial communities.Canonical correspondence analysis(CCA)showed that moisture,temperature and p H were the main environmental factors which caused the change of bacterial community structure in fermented grains during fermentation.Among the volatile acids,only hexanoic acid and acetic acid had significant effect on the bacterial community in fermented grains.In general,p H,hexanoic acid and pentanoic acid were significantly positively correlated with the samples of fermented grains on the 0th day of fermentation,but were contrary to the correlation with temperature,moisture,propionic acid,butanoic acid,heptanoic acid and octanoic acid.Although acetic acid was not significantly correlated with other environmental factors,However,there was a strong correlation with the samples of upper fermented grains on the 10 th and 20 th days of fermentation and the samples of lower fermented grains on the 40 th day of fermentation.Spearman correlation analysis showed that more than 50% of OTUs in the top 30 abundant OTUs are significantly negatively correlated with moisture and temperature,and positively correlated with p H.However,the correlation between the highest abundance OTU 1(Lactobacillus)and the 3 physicochemical properties was opposite.Most bacterial OTUs were positively correlated with volatile acids,and very few were negatively correlated.The results of Network analysis showed that during the fermentation process of strong-flavor fermented grains,there were many bacterial genera interacting with each other to varying degrees.The bacterial genera with high degree of interaction mainly belonged to Proteobacteria,Actinobacteria and Bacteroidetes.Some bacterial genera in Firmicutes had relatively low degree of interaction with other bacterial genera,but their abundance was large.Therefore,the effect on the fermentation process of strong-flavor Baijiu cannot be ignored. |
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