Once san-dan-jikomi is complete and the final addition has been made we enter into moromi. Moromi starts the day after the final addition, which is tomezoe. It lasts until fermentation is almost complete. This can take anywhere from 2 weeks to about a month.
The time needed for moromi is based on both temperature and koji characteristics. In particular the characteristic diastatic power the koji can muster at the moromi temperature. The yeast work faster at the low temperatures of moromi than do the koji enzymes.
At the end of the moto the alcohol content was anywhere from about 5% to 10%. With the san-dan-jikomi additions the concentration of alcohol was also cut in the same way as the yeast and acid. However, as some yeast has been reproducing some have been producing alcohol. So by the start of moromi we have regained much of the alcohol concentration we had at the end of the moto.
With temperatures in the range of the low 60sF moromi may take between 12 to 22 days while temperatures closer to 50F 16 to 30 days. I have seen temperatures as low as about 45F used for the moromi. The lower temperatures produce more refined sake but lengthen the time needed for moromi.
On day 1 of moromi the mash is thick as the rice added over the proceeding four days has absorbed much of the liquid that had been free. Thick foam that began to rise during san-dan-jikomi continues to rise up from the ferment. Over the following days of moromi as the mash is stirred twice daily and the enzymes from the koji do their work dissolving the newly added rice, the mash becomes more fluid. After day 5 or 6 there is less need to stir the ferment because it is much more liquid and the rising foam has slowed its upward climb.
By the end of moromi, the foam no longer climbs upward. Activity in the ferment slows to be almost imperceptible. It is at this time, somewhere between days 12 to 30 when we should press the sake from the kasu (lees). This process of pressing the sake from the kasu is called joso and ends moromi.