| Nong-flavor Baijiu is one of the typical representatives of Baijiu in China.Its main flavor substance is ethyl caproate.Caproic acid is the precursor of ethyl caproate.Traditionally,caproic acid producing microorganisms mainly come from pit mud.However,Daqu as a saccharifying and fermenting agent in the production of Nong-flavor Baijiu has a large amount of its addition and rich in many kinds of microbes.According to the principle of microbial physiological ecology,it is speculated that there are microorganisms producing hexanoate in Daqu,and these hexanoate producing microorganisms can be domesticated and proliferated and metabolized in the cellar environment.In order to prove this conjecture,this study took Daqu as the research object,yellow water as the culture medium,combined with a variety of analysis methods to explore the caproic acid production and community structure changes of Daqu microorganisms under anaerobic conditions.The main results of this paper are as follows:1.Taking yellow water and Daqu as the research object,the anaerobic culture system of Daqu microorganism was established and the composition of the system was analyzed.The 16 S r RNA amplified sequence showed that Pseudomonas,Thermoactinomyces and Capriciproducens were the dominant bacteria in Daqu.The sequencing results of its amplicon showed that seven genera including Issatchenkia and Saccharomycopsis were the dominant fungi in Daqu.The data showed that the main components of Daqu were protein and starch,while the main components of yellow water were fat and organic acids,in which lactic acid was the main organic acid.These nutrients,precursors and caproiciproducens provide raw materials for the study of caproic acid production by Daqu microorganisms.2.Under anaerobic condition,the yield of caproic acid was 4.67% ± 0.37 mg/m L.The results not only proved that Daqu microorganism had the ability to synthesize caproic acid,but also showed that caproic acid producing microorganism could adapt to the current environment.Furthermore,in order to make the caproic acid producing microorganisms in Daqu dominant in the culture system,the optimal domestication conditions of caproic acid producing microorganisms in Daqu were optimized by single factor and orthogonal test with caproic acid content as the index.The results showed that the content of hexanoic acid in fermentation broth could reach 13.62±0.08 mg/m L under the optimal culture conditions,the yield of hexanoic acid increased by 2.92 times.3.Under the optimal culture conditions,the changes of substances and microorganisms in the fermentation process were monitored.The results showed that:(1)with the fermentation,Pseudomonas,thermoactinoces and other nine dominant bacteria(relative abundance >0.05%)all declined or even died out in different degrees,while Capriciproducens and Lactobacillius gradually evolved into the dominant bacteria of the end of fermentation.In addition,the dominant fungi(relative abundance > 0.05%)in the culture system included 10 genera,such as Issatchenkia and Saccharomycopsis(2)Meanwhile,the correlation analysis showed that Capriciproducens was negatively correlated with the physical and chemical properties and the changes of acetic acid,propionic acid and butyric acid,while it was positively correlated with the change of caproic acid.Therefore,Capriciproducens was the main caproic acid producing bacteria in the anaerobic fermentation of Daqu.In conclusion,this study not only effectively confirmed the existence of caproic acid producing microorganisms in Daqu,but also found that yellow water could domesticate caproic acid producing microorganisms under anaerobic conditions.The results laid a theoretical foundation for further exploring the effect of yellow water,the changes of Daqu microorganisms in the pit environment and the source of caproic acid producing microorganisms. |
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