To study the effect of co-culture with Saccharomyces cerevisiae on the metabolism of caproic acid-producing bacteria and its mechanism, a strain of Clostridium celerecrescens JSJ-1 that can produce caproic acid and S. cerevisiae C-1 were cultured separately or co-cultured under different nutrient conditions. We compared the growth and metabolism (number of microbial colonies, glucose, ethanol, butyric acid, caproic acid) of the two strains in monoculture and co-culture. The results showed that S. cerevisiae C-1 grew better than C. celerecrescens JSJ-1 under anaerobic static culture conditions at 34 ℃, and could preferentially utilize glucose in the medium. When glucose was used as the sole carbon source, S. cerevisiae C-1 could utilize glucose to produce ethanol as a substrate for the synthesis of caproic acid by C. celerecrescens JSJ-1. When the medium contained 0.5% glucose and 2% ethanol, the co-culture produced caproic acid four days earlier than C. celerecrescens JSJ-1 alone. Glucose had a strong inhibitory effect on the production of caproic acid by C. celerecrescens JSJ-1. S. cerevisiae C-1 could utilize glucose to relieve its inhibitory effect on the production of caproic acid upon co-culture. During the fermentation of nongxiangxing baijiu, S. cerevisiae can not only provide substrates for caproic acid-producing bacteria to synthesize caproic acid, but also alleviate the inhibitory effect of glucose on caproic acid production.

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.

Physicochemical analysis, headspace-solid phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS) and high-throughput sequencing were used to investigate the potential relationship between the microecology of different flavor types of Daqu and their physicochemical indexes and characteristic flavor compounds. The results showed that the highest saccharification power and fermentation power were found in Fenflavor Daqu, the highest esterification power and starch content in Luzhou-flavor Daqu, and the highest acidity were found in Maotai-flavor Daqu. In the three types of Daqu, the dominant bacterial genera were Saccharopolyspora, Weissella and Bacillus, and the dominant fungal genera were Saccharomycopsis, Aspergillus and Thermomyces. Correlation analysis showed that Bacillus was positively correlated with saccharification power, liquefaction power, fermentation power, ethyl caprate and phenyl ethanol. Thermomyces was positively correlated with ethyl caproate, isoamyl alcohol, ethyl lactate, acetic acid and ligustrazine.