What happens during the "Final work" step of koji making?

During the "Final work" step of koji making, maintaining the temperature above 38°C but below 43°C is crucial for the optimal growth of the koji mold, Aspergillus oryzae. This temperature range allows the mold to propagate effectively while also facilitating the enzymatic activities that break down starches in the rice into sugars. The regulated warmth is essential to maximize the koji's potential to convert starches into fermentable sugars, crucial for subsequent fermentation processes when sake is produced.

While halting mold growth is essential at certain stages, this specific step is focused on allowing sufficient development for flavor and aroma compounds, which requires maintaining this precise temperature range. Cooling the rice significantly would inhibit the enzymatic activity necessary for the koji to develop properly, and fermentation occurs later in the sake production process when yeast is added. Thus, maintaining the temperature within this critical range is the foundation for successful koji production.