Daqu : a traditional fermentation starter in China: microbial ecology and functionality

Fermented products have high nutritional value and constitute an important part of the Chinese dietary profile; they are also gaining popularity throughout the world. Daqu is a traditional natural fermentation starter culture that has a significant impact on the quality and flavour of Chinese liquor and vinegar. A review of the literature was conducted focusing on the classification, composition, and manufacture of Daqu. The review provided a preliminary understanding of the link between the fermentation process and the characteristics of the final Daqu product. Then the occurrence, levels, and diversity of microorganisms were studied in different types of Daqu produced by various fermentation processes. The results showed that Bacillus licheniformis and Saccharomycopsis fibuligera were present in all the tested samples of Daqu. Regional comparisons showed Staphylococcus gallinarum and Staphylococcus saprophyticus in southern Daqu. The fungi Sm. fibuligera and Lichtheimia ramosa were found in low/medium-temperature Daqu and Thermomyces lanuginosus occurred in high-temperature Daqu. In order to study the functionality of Daqu and the contribution of the predominant microorganisms to alcoholic fermentation, the mesophilic and thermophilic bacteria and spores, Enterobacteriaceae, lactic acid bacteria, yeasts, and moulds present in the core and outer portions of Fen-Daqu were isolated. The isolates were identified by culture-dependent sequencing of rRNA genes (16S rRNA for bacteria; 18S rRNA, 26S rRNA, and ITS rRNA for fungi). A succession of fungi, lactic acid, and Bacillus spp. was associated with prevailing acidity, moisture content, and temperature during Daqu fermentation. The predominant species in fermentation were B. licheniformis, Pediococcus pentosaceus, Lactobacillus plantarum, Pichia kudriavzevii, Wickerhamomyces anomalus, Sacchromyces cerevisiae, and Sm. fibuligera. One strain of the each of the above-mentioned predominant species, with the highest starch degrading ability and alcohol tolerance, was selected and used in different combinations to perform alcoholic fermentation. Metabolite composition differed significantly between various fermentation trials. S. cerevisiae provided superior ethanol production. Sm. fibuligera and B. licheniformis provided the amylolytic activity that converted starch and polysaccharides into fermentable sugars. Finally, W. anomalus was found to be an important contributor to formation of the liquor aroma. Understanding the microbial diversity and functional activity, as well as the production dynamics and safety of Daqu will enable commercial producers to improve and/or scale-up traditional processes and enhance product quality and safety, thus facilitating entry into international markets.