In the low-nutrient (glucose and other sugars) environment, the nitrate-reducing bacteria metabolise nitric acid in the well-water to produce nitrous acid. The rising concentration of glucose and other sugars hampers their growth.
As the mash warmers slowly increase the temperature, the koji produces more and more glucose and other sugars. The increase in nutrient availability leads to a rise in the population of lactic acid bacteria. They produce lactic acid, resulting in higher acidity.
Addition of yeast
The lactic acidity becomes so strong that normal micro-organisms cannot survive. Once the concentration of glucose and other sugars and acidity reach their peak, the yeast is added.
Yeast is the only micro-organism present that thrives in lactic acid, so it swiftly begins to multiply. As lactic acid production has already reached its maximum, any further rise in acidity is caused by fermentation of the yeast.
As glucose and other sugars are consumed by the yeast, the amount remaining decreases.
As the acidity and alcohol concentration increase, the yeast finds it harder to survive. The temperature is quickly lowered, ending yeast activity.