The average fermentation tank holds between 1,500 and 3,000kg but for premium sake, such as (Dai) Ginjo and Junmai (Dai) Ginjo, a smaller tank (around 500kg) tends to be used.
Day 1: First addition (Soe or Soe-jikomi)
Throughout the creation of moromi, different proportions of water, koji and steamed rice are added. The first step is called soe, literally meaning “to add something”. At this stage, approximately 13% of the total mixed materials are added to the starter (which is, itself, 8% of the total). Normally water and koji is mixed in advance to create “water koji” in order to more easily extract enzymes from the koji. The temperature at this stage is quite warm, around 12-15℃, in order to propagate yeasts rapidly. This addition of water koji dilutes the lactic acid to around 40% of the original amount, but the combination of alcohol and acidity continues to provide contamination resistance.
Day 2: Rest day (odori)
It is not clear why day 2 is called “odori” (meaning “dance”). Perhaps it provided the original makers a brief respite from the rigors of regular monitoring and checking. Or perhaps it refers to the appearance of the mix as it develops through the day. Nothing is added on this day, which is reserved for the propagation of yeast. The temperature of the mix on odori is the highest of the whole production process (even more so for premium sake). Lines of bubbles appear when fermentation has started. (If bubbles are not detected, the mix is left for an additional day of odori to confirm the proper propagation and fermentation.
Day 3: Middle addition (naka or naka-jikomi)
Approximately 30% of the total volume of mixed materials (water, koji and steamed rice) are added on day 3. Rather predictably, day 3 is called naka, meaning “middle”, being the middle one of the three days of additions.
Day 4: Final addition (tome or tome-jikomi)
On the final day of additions, approximately 49% of mixed materials are added again at a temperature of 8-9℃.