Presentation Information
[2Brew-16]Sake Yeast: Characteristic Features and Breeding
○Nami Goto1, Hitoshi Shimoi1 (1. Brewing Society of Japan (Japan))
Keywords:
Yeast,Sake,Breeding,Alcoholic beverage
[Purpose] Sake is a traditional Japanese alcoholic beverage produced by fermenting steamed rice, rice koji, and water, and then filtering the mash. Undiluted sake (genshu) contains nearly 20% (v/v) ethanol, which is unusually high for a fermented beverage. Although rice itself is relatively low in aroma, sake develops a rich and characteristic flavor largely due to the metabolic activities of sake yeast, which has led to extensive studies on sake yeast. Sake yeast forms a distinct clade within Saccharomyces cerevisiae (Azumi and Goto-Yamamoto, 2001) and exhibits unique physiological and biochemical traits. In addition to screening and isolation of desirable strains, strain improvement through mutagenesis has long been practiced; several representative examples are summarized here.
[Results and Consideration] A characteristic feature of sake yeast is the formation of a thick foam layer during fermentation. This phenomenon results from the adhesion of yeast cells to CO2 bubbles and is associated with the hydrophobic cell-surface protein Awa1p (Shimoi et al., 2002). Because the foam disappears as ethanol concentration increases, it has traditionally served as an indicator of fermentation progress. However, excessive foaming requires additional tank headspace and increases cleaning labor. To address this, a method for isolating non-foaming mutants was developed by Ouchi and Akiyama (1971), and such strains are now widely used. The AWA1 gene is located near the telomere of chromosome IX, and a non-foaming mutant K701 was shown to arise from chromosomal translocations that disrupt the AWA1 gene.
Yeast-derived esters, e.g., isoamyl acetate and ethyl caproate, play a major role in sake aroma, distinguishing it from wine. Consequently, ester production has been intensively studied. In particular, breeding of ethyl caproate–high-producing strains carrying a FAS2 mutation selected by cerulenin resistance (Ichikawa et al., 1991) has markedly contributed to the production of fruity ginjo sake. Many strains have been bred using this technique.
Another important feature of sake yeast is its high ethanol fermentation capacity. This was previously attributed to strong ethanol tolerance; however, Watanabe et al. (2012) demonstrated that sake yeast exhibits impaired stress response systems, allowing continued fermentation under high ethanol conditions but increasing susceptibility to cell death in the late stages. Yeast cell death adversely affects flavor quality and promotes the formation of dimethyl trisulfide (DMTS), a key compound responsible for aged off-flavors during storage. Isogai et al. (2008) showed that the DMTS precursor is linked to yeast methionine metabolism. In collaboration with Inoue and colleagues, mutant strains that do not produce this precursor compound were developed and have been commercially applied since 2021.
[Results and Consideration] A characteristic feature of sake yeast is the formation of a thick foam layer during fermentation. This phenomenon results from the adhesion of yeast cells to CO2 bubbles and is associated with the hydrophobic cell-surface protein Awa1p (Shimoi et al., 2002). Because the foam disappears as ethanol concentration increases, it has traditionally served as an indicator of fermentation progress. However, excessive foaming requires additional tank headspace and increases cleaning labor. To address this, a method for isolating non-foaming mutants was developed by Ouchi and Akiyama (1971), and such strains are now widely used. The AWA1 gene is located near the telomere of chromosome IX, and a non-foaming mutant K701 was shown to arise from chromosomal translocations that disrupt the AWA1 gene.
Yeast-derived esters, e.g., isoamyl acetate and ethyl caproate, play a major role in sake aroma, distinguishing it from wine. Consequently, ester production has been intensively studied. In particular, breeding of ethyl caproate–high-producing strains carrying a FAS2 mutation selected by cerulenin resistance (Ichikawa et al., 1991) has markedly contributed to the production of fruity ginjo sake. Many strains have been bred using this technique.
Another important feature of sake yeast is its high ethanol fermentation capacity. This was previously attributed to strong ethanol tolerance; however, Watanabe et al. (2012) demonstrated that sake yeast exhibits impaired stress response systems, allowing continued fermentation under high ethanol conditions but increasing susceptibility to cell death in the late stages. Yeast cell death adversely affects flavor quality and promotes the formation of dimethyl trisulfide (DMTS), a key compound responsible for aged off-flavors during storage. Isogai et al. (2008) showed that the DMTS precursor is linked to yeast methionine metabolism. In collaboration with Inoue and colleagues, mutant strains that do not produce this precursor compound were developed and have been commercially applied since 2021.
Comment
To browse or post comments, you must log in.Log in