What role does koji play in sake production?

Koji plays a crucial role in sake production by converting starches in rice into sugars. This process is essential because the fermentation of sake relies on sugars as the primary substrate for yeast, which produces alcohol and carbon dioxide during fermentation. The mold used to create koji, typically Aspergillus oryzae, enzymatically breaks down the rice starches into fermentable sugars. This conversion is a key step that allows the yeast to thrive and effectively produce alcohol, making it foundational to the sake-making process.

The other options do not accurately represent the function of koji. While acidity is a characteristic of sake, it is primarily influenced by other factors in the fermentation process, not directly by koji. Koji does not enhance the alcohol content itself; rather, it provides the sugars that the yeast converts to increase alcohol levels. Additionally, while preservation is important for sake, this is achieved through proper brewing techniques and storage rather than through the role of koji directly.