Sake Brewing and Bacteria Inhabiting Sake Breweries.

Abstract

Sake is a Japanese traditional fermented alcoholic drink. It is brewed using koji mold to convert the starch in rice into sugar, which is then converted into ethanol by sake yeast. Two eukaryotic microorganisms, Aspergillus oryzae and Saccharomyces cerevisiae, are used for sake brewing, leading to highly efficient ethanol fermentation. The final ethanol concentration is ~20%, which is higher than that of beer and wine. The use of technology in the sake brewing process is remarkably high and involves parallel double fermentation. In contrast, beer is brewed using malt to convert the starch in barley into sugar, which is then converted into ethanol by beer yeast. In winemaking, wine yeast converts sugar from grapes into ethanol. Beer and wine processes can include also serial yeast-based double fermentation. It is well-known that bacteria, mainly lactic acid bacteria, can have a role in the production process of beer (sour beer) and wine (malolactic fermentation) (Berbegal et al., 2019; De Roos et al., 2020; Dysvik et al., 2020; Virdis et al., 2020). On the opposite, several alcoholic beverages, such as sake, are considered fermented only by eukaryotic microorganisms. Beer, sake, and wine originated long before the discovery of microorganisms. Thus, these alcoholic drinks were produced without understanding the mechanism of ethanol fermentation. Surprisingly, sake undergoes pasteurization, called hiire, in the process of sake storage, and it has been recorded as performed 300 years before Pasteur reported the pasteurization method. The high level of biotechnology involved in sake brewing has been maintained for a long time. Sake breweries are widely distributed in Japan, and there may be much more to learn from the sake brewing process. Sake has different grades depending on the degree of polishing of rice. The highest grade (Daiginjo) is produced using polished rice with 50% or more removal of the outer layer of the grain. In addition, sake is distinguished by the addition of distilled alcohol. Sake without distilled alcohol is called Junmaishu. This opinion paper proposes Sake as a model matrix to highlight bacterial role in traditional beverages that are considered exclusively mold/yeast-based. In addition, the opinion underlines the importance of considering all microbial determinants for a complete safety assessment.