On yeast there is a debate!

What you need to know is that unlike wine where the grapes are just pressed, in sake the rice is polished, rinsed and then steamed to make it gelatinous, so that after all these steps the rice is free of its indigenous yeasts. Traditionally, it was the yeasts in the air that transformed the sugar into alcohol. The problem in this case is that as long as there is not enough lactic acid in the yeast starter(called shubo or motto) to protect the fermenting juice, undesirable bacteria and yeasts can develop. There is a fairly high risk that they will spoil and give a bad taste to the sake. Sometimes even in the case of insufficient yeast, the fermentation cannot be completed. Sometimes, by chance, in the brewery environment, a dominant and good quality yeast would develop.

At the turn of the century, scientists succeeded in isolating sake yeasts for conducting sake fermentation. This was a revolution in sake production, as it greatly improved the quality of sake. Today, there are many types of selected yeasts. The most commonly used are yeasts No. 6, 7 and 9. Recently some breweries, very much influenced by the natural wine movement which is very fashionable in Japan, are reverting to indigenous yeasts, these sake are often very acidic, probably due to the lack of lactic acid at the beginning of the fermentation, sometimes the lactic acid will develop too much which will give a yoghurt-like taste to the sake