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

<strong>Abstract</strong> Fermented products have high nutritional value and constitute an important part of the Chinese dietary profile; they are also gaining popularity throughout the world. <em>Daqu</em> 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 <em>Daqu</em>. The review provided a preliminary understanding of the link between the fermentation process and the characteristics of the final <em>Daqu</em> product. Then the occurrence, levels, and diversity of microorganisms were studied in different types of <em>Daqu</em> produced by various fermentation processes. The results showed that <em>Bacillus licheniformis</em> and <em>Saccharomycopsis fibuligera</em> were present in all the tested samples of <em>Daqu</em>. Regional comparisons showed <em>Staphylococcus gallinarum</em> and <em>St</em><em>aphylococcus</em><em> saprophyticus</em> in southern <em>Daqu</em>. The fungi <em>Sm. fibuligera</em> and<em> Lichtheimia ramosa </em>were found in low/medium-temperature <em>Daqu </em>and <em>Thermomyces lanuginosus</em> occurred in high-temperature <em>Daqu</em>. In order to study the functionality of <em>Daqu </em>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 <em>Fen</em>-<em>Daqu</em> 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 <em>Bacillus</em> spp. was associated with prevailing acidity, moisture content, and temperature during <em>Daqu</em> fermentation. The predominant species in fermentation were <em>B. licheniformis</em>, <em>Pediococcus pentosaceu</em>s, <em>Lactobacillus plantarum</em>, <em>Pichia kudriavzevii, Wickerhamomyces <em><em>anomalus</em></em></em>, <em>Sacchromyces cerevisiae</em>, and <em>Sm. fibuligera</em>. 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. <em>S. cerevisiae</em> provided superior ethanol production.<em> Sm. fibuligera</em> and <em>B. licheniformis</em> provided the amylolytic activity that converted starch and polysaccharides into fermentable sugars. Finally, <em>W. anomalus</em> 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 <em>Daqu</em> will enable commercial producers to improve and/or scale-up traditional processes and enhance product quality and safety, thus facilitating entry into international markets.