Tao Zeng’s research while affiliated with Southwest University and other places

Mucor racemosus is the predominant fungal in the zhiqu stage of the fermentation of Yongchuan Douchi (Mucor-type), which plays an important role in the fermentation process of Yongchuan Douchi. However, there is a lack of information on the genetic analysis of M. racemosus. In this study, we isolated and identified M. racemosus C (accession no JAPEHQ000000000) from Yongchuan Douchi and analyzed the physiological indicators, then genomic information of the strain to perform a comprehensive analysis of its fermentation capacity and safety. M. racemosus C had neutral protease activity up to 68.051 U/mL at 30 °C and alkaline protease activity up to 57.367 U/mL at 25 °C. In addition, comparing the genomic data with the COGs database (NCBI), it was predicted that M. racemosus C undergoes extensive amino acid metabolism, making C suitable for the production of fermented foods (e.g., Douchi, Syoyu, and sufu). Finally, we performed virulence genes and resistance genes analysis, hemolysis experiment, aflatoxins assay, antibiotic resistance assay to evaluate the safety of M. racemosus C, and the results showed that M. racemosus C was safe, non-toxin-producing and non-hemolytic.

Yongchuan douchi is a traditional fermented food in China. This study investigated the microbial community of two major types (Natural and artificial) of Yongchuan douchi, and their influences on quality. The results found that natural-type douchi had significantly better quality in its physicochemical properties such as higher amino nitrogen, darker color, and higher umami amino acids after post-fermentation (p < 0.05). A total of 58 volatile components were found in natural-type douchi compared to artificial-type with 42 identified. Microbial analysis found that natural-type douchi had the richness and diversity of the bacterial (fungus) community. However, artificial-type douchi was high in Candida genera may associated with its bitter after taste compared to natural-type. Following post-fermentation, natural-type douchi had higher abundance of Bacillus sp., Enterobacter sp., and Pseudomonas sp., while Bacillus sp., Weissella sp., and Aspergillus sp. compared to artificial-type douchi. Correlation analysis showed that physicochemical properties and sensory quality could be correlated positively or negatively with the microbial composition during the last stage of post-fermentation. In conclusion, the zhiqu process significantly affected the final quality of the two types of Yongchuan douchi as evidenced by the microbial composition in the post-fermentation process.