There are three stages of standard moromi production, but some sake breweries reduce this to only two, or increase it to 4, 5 or even 10 for cost effectiveness or marketing reasons. The last stage is called tome. Several days after “tome”, the alcohol content reaches around 10%, which is the highest concentration that will allow yeast to propagate. At this time, yeast reaches the ideal concentration of 2～3×10８/ml.
Normally when yeast produces alcohol, it also produces heat. Approximately 30% of the heat is consumed for their living activity, but the rest of the heat is released. The source of Ginjo aromas (such as isoamyl acetate) decreases when temperature increases and another major substance of the Ginjo aroma (ethyl caproate) is only produced at lower temperatures.
Additionally a higher temperature activates enzymes which leads to a fast dissolving of starch and protein. The protein is converted into amino acid which provides both umami flavor and nutrition for yeast, however, too much amino acid ruins the taste.
The mix is therefore kept at a low temperature to prevent the overheating that will destroy the balance of flavors.
For premium sake, the moromi temperature is kept at 10℃ or less for three weeks after tome, In the first week, the temperature climbs to 10℃. During the second week, it is maintained at a stable 10℃. In the third week, the temperature drops to above 5℃.
Even at this low temperature, the alcohol by volume (ABV) continues to increase. (By way of comparison, white wine is fermented at 15 to 20℃ and red wine can be over 25℃. Ginjo sake is sophisticated and aromatic, so too much umami or too little aroma is unacceptable to brewers and consumers alike.
Since meticulous control of this lower temperature creates very severe environment for yeast to produce alcohol, it takes approximately 28 to 35 days to achieve the desired ABV of 18-20%. (Normally white wine takes 14 to 21 days and red takes 7 to 10 days.)