Nitric acid; nitrous acid

The presence in minute quantities of nitrate ions and nitrite ions is important in the production of seishu. Nitric acid exists in the form of salts in water, and the quantity is expressed as ppm (parts per million) of NO3- .

When making kimoto-style yeast starter, it is first necessary to propagate nitrate-reducing bacteria in the mash in the presence of nitrate ions. These bacteria reduce nitrates, producing nitrite ions (NO). The resulting nitrous acid inhibits yeast growth and lactic acid bacteria begin to grow in the meantime. Lactic acid is produced as these bacteria multiply, then the nitrate-reducing bacteria die off because of the lactic acid, and the nitrous acid also soon disappears (decomposing into N2, NO2, NH3 and so on). Only at this point are the conditions created in which it is possible for suitable  sake yeast (seishu kobo)  to grow preferentially. Consequently, where sake yeast is to be added to the starter mash, this is done after the disappearance of the nitrous acid reaction. The presence or otherwise of nitrous acid can be simply established by using commercially-available test papers.

Miyamizu contains 20-30ppm of nitric acid. However, where large quantities of nitric acid are present, they are (along with nitrous acid and ammonia) frequently derived from nitrogenous compounds of animal origin, and, as indicating possible contamination by sewage and such like, are undesirable.

Nitric acid can be quantified with colorimetry using the brucine and aniline methods. However, it is also common to simply carry out qualitative analysis, expressing the result as shown by the colour change in grades as “small quantity”, “trace”, “not detected”, and so on.