How Acids in Moromi Change the Way Yeast Ferments

Sake is brewed by carrying out two processes at the same time in a single tank: koji (rice on which the koji mould has been grown) turns the rice starch into sugar, and yeast turns that sugar into alcohol. The thick liquid that forms during this fermentation is called “moromi”. Besides yeast, moromi also contains living lactic acid bacteria and other microbes, and a variety of organic acids are produced. A research group from Ryukoku University in Kyoto and Fukushima University has shown that these organic acids may be more than mere by-products and may change the way yeast works. The findings were published in June 2026 in the journal Yeast (Wiley).

Yeast cells that stop prioritising glucose

When enough glucose is available, yeast puts off using other sugars. This is known as “glucose repression”. However, within a yeast population, cells sometimes appear in which this repression has been lifted and which can use several sugars at once, a state known as [GAR+]. These cells have a weaker capacity for alcoholic fermentation, which affects how fermentation proceeds. In 2023, the research group reported that, in sake brewing, lactic acid may influence the fermentation characteristics of yeast. It was not known, however, whether other organic acids have the same effect.

A mechanism shared by three acids

Professor Koichi Tanabe of the Faculty of Agriculture at Ryukoku University and Professor Jun Shima of the Faculty of Food and Agricultural Sciences at Fukushima University, together with colleagues, compared three acids abundant in moromi, lactic acid, acetic acid and pyruvic acid, under the same conditions. They confirmed that all three increased the number of [GAR+] cells. According to the team, this is the first time that the main organic acids have been compared under identical conditions and shown a common effect. When they then varied the pH of the culture medium, they found that the effect appeared only when the acid was in its undissociated form. Because undissociated acids pass easily through the cell membrane, they are thought to be taken up into the yeast cell, where they act.

Possible relevance for wine and beer

The result suggests that microbes in moromi may influence one another through organic acids. The research group says the same phenomenon may occur in beer, wine, miso, soy sauce and other fermented products. The next step is to investigate in detail which molecules respond and what happens inside the cell, and the work is expected to be applied to the control of fermentation and the development of new brewing yeasts.

For European readers familiar with wine and beer, lactic acid and acetic acid will be well-known terms. This is still basic research, and it will not change the sake on the shelves any time soon. Even so, the acidity of sake is an important element that forms the backbone of its flavour. If you look at the “acidity” figure printed on the label next time, you may feel a little closer to the work of the microbes in the moromi.