The microbial community diversity during the fermentation process of digital high-temperature Daqu and traditional high-temperature Daqu was analyzed by high-throughput sequencing on the Illumina MiSeq platform. Meanwhile, correlation analysis and bioinformatics analysis were conducted based on microbial species information and physicochemical properties. The results showed that the microbial community structure of high-temperature Daqu significantly changed during the fermentation process. At the end of fermentation, the major bacterial genera in high-temperature Daqu were Kroppenstedtia, Saccharopolyspora, Bacillus, Thermomyces, and Lactobacillus, and the major fungal genera were Thermoascus, Thermomyces, and Aspergillus. The results of non-metric multidimensional scale (NMDS) analysis and hierarchical clustering analysis (HCA) showed that the microbial community composition of the two types of Daqu was roughly similar at the same time points of fermentation.

This study compared the differences in the growth and metabolism of Clostridium celerecrescens JSJ-1, a strain able to produce caproic acid, and Clostridium tyrobutyricum D-1, a stain able to produce butyric acid, when cultured separately or co-cultured in different systems (solid medium, and liquid medium with or without use of dialysis bag). Furthermore, this study investigated the effect and mechanism of co-culture with C. tyrobutyricum on caproic acid metabolism in caproic acid-producing bacteria by culturing JSJ-1 alone in the presence of butyric acid or sodium butyrate at different concentrations. The results showed that when co-cultured on a composite solid medium containing glucose in anaerobic conditions at 34 ℃, D-1 grew better than JSJ-1, and could preferentially utilize glucose for growth. The generation time of caproic acid was three days earlier when D-1 and JSJ-1 were co-cultured in a liquid medium than when JSJ-1 was cultured alone under the same conditions, and the concentration of caproic acid was 9.93 and 9.79 g/L on the 12^th day in the two cases, respectively. Glucose had a strong inhibitory effect on the production of caproic acid by JSJ-1, which could be relieved by preferential utilization of by D-1 upon co-culture. The co-culture in the liquid medium with and without use of dialysis bag were compared, revealing that whether there was direct contact between JSJ-1 and D-1 had no effect on the yield of caproic acid. By purely culturing JSJ-1 in the presence of butyric acid or sodium butyrate, it was found that butyric acid generated by D-1 at pH 6–6.5 could promote the synthesis of caproic acid by JSJ-1. During co-culture, the preferential utilization of glucose by D-1 could relieve the inhibitory effect of glucose on the production of caproic acid, and butyric acid produced by D-1 could promote the synthesis of caproic acid.