The acidity of a sake or its sando is a measure of how much base liquid is needed to neutralize 10ml of sake. Acidity in sake balances its sweetness. The sweeter the sake the higher its acidity can be without being sour or annoying. In general the higher acidity the thinner the sake will seem. However, as with all the characteristic parameters of sake, we cannot say that a sake with a high acidity level will seem thin, only thinner than if it had lower acidity.
Acidity levels tend to range from 0.8 to 1.7. As we saw previously, the nihonshudo values (SMV) tend to be between -5 and +10. Using these two metrics together is more useful than individually. Recall that the more negative the nihonshudo value the sweeter it is and the more positive the dryer it is. So producing a sake with SMV -4 and acidity of 1.7 (two extremes) could create a heavy dry sake; that’s right, dry. This is because the acidity balances out all of the sweetness. We must keep in mind that these are only trends and not absolutes. In the same way, if we produce a sake with high SMV of +7 and a acidity of 0.8 may be sweet and thin or watery.
The acidity comes naturally from several contributors: oenococcus, pediococcus and lactobacillus bacteria (lactic acid), acetic acid bacteria including acetobacters (acetic acid) and yeast (succinic acid, malic acid, citric acid, D-lactic acid, inosinic acid). However, the most common method for producing the moto is sokujo moto where the majority of the non-yeast produced lactic acid (L-lactic acid) is added directly rather than through bacteria cultivation as is done for kimoto and yamahai moto. D-lactic acid produced by yeast during fermentation generally falls in the range of 140mg/L to 270mg/L. L-lactic acid levels are more easily controlled by the brewer, generally fall in a broader range of 60mg/L to 450mg/L.
These acids provide a flavor component as well as their balancing effects. Lactic acid has a smooth acidity but can approach astringency. Acetic acid is the acid in vinegar. Succinic acid adds a slight heaviness and earthiness. Malic acid is light with some bitterness. Finally, citric acid brings a strong astringency. The relative ratio of these acids as well as amino acids (not talked about here) offer dials for the brewer to adjust.
A major goal for sake brewers is to strike a good balance in the acid levels with each other, with the sweet / dry level and other components so they come together for a nicely unified experience. Kimoto and yamahai moto procedures are largely about balancing these acid levels while with sokujo moto is more direct – measure and add.