Kimoto Chemistry: integrated graph
Nitrate-reducing bacteria survive on the small amount of nutrients provided by the koji at low temperatures. They metabolise the nitric acid in the well-water to produce nitrous acid. Their growth is hampered by the increasing level of glucose produced by the koji.
As the mash warmers slowly increase the temperature, the nutrient content in the mash also increases. This leads to a rise in the population of lactic acid bacteria. (Lactic acid bacteria thrive in nitrous acid.)
As the lactic acid bacteria begin to produce lactic acid, the synergistic effect of the lactic acid and nitrous acid destroys the nitrate-reducing bacteria and wild yeast.
Addition of yeast
After the lactic acid concentration has become so strong that other micro-organisms cannot survive, the lactic acid bacteria weaken. Once the sugar created by the koji and the acidity of the lactic acid have hit their maximum levels, the yeast is added. The alcohol produced by the yeast kills the lactic acid bacteria and the tank becomes practically sterile except for the yeast.
As yeast is the only micro-organism present that thrives in lactic acid, it swiftly begins to multiply. The carbon dioxide produced by the yeast causes the surface of the mash to swell. As lactic acid production has already reached its maximum, any further rise in acidity is caused by fermentation of the yeast. Glucose and other sugars are consumed by the yeast and decrease.
As the yeast growth and fermentation become more active, the surface of the mash begins to foam. From this point on, the heat generated by the fermentation process alone is sufficient to raise the mash temperature; mash warmers are no longer needed.
As the acidity and alcohol increase, the yeast finds it harder to survive. The temperature is quickly lowered, ending yeast activity and resting the mash. Kimoto mash has a low yeast death rate during the karashi period, so when the mash is used for moromi, yeast purity is at almost 100%.