How does the brewer stop the growth of koji?

The correct answer is moving the rice to a cooler part of the room. This method is utilized in the sake brewing process, particularly during the koji making stage. Koji mold, specifically Aspergillus oryzae, is cultivated on steamed rice and thrives in warm, humid conditions, promoting the conversion of starches in the rice into sugars, which are essential for fermentation.

By relocating the rice to a cooler environment, the brewer effectively slows down or halts the growth of the koji mold once it has achieved the desired level of sugar conversion. This precise control is crucial to ensure that the koji does not over-ferment the rice, which could result in undesirable flavors or too high a sugar content that complicates the subsequent fermentation process.

Moving the rice to a warmer area would actually encourage further mold growth, which is not the objective at this point. Adding salt could inhibit microbial activity but is not a standard practice in koji production, as it would adversely affect the fermentation process. Reducing moisture content would also be counterproductive since koji requires a certain level of humidity to grow effectively. Hence, cooling the environment is the most effective and precise method for controlling koji growth.