What is a critical function of koji in sake brewing?

In sake brewing, koji plays a crucial role by converting starches found in rice into fermentable sugars. This process is essential because yeast, which is responsible for fermentation, cannot directly utilize starches. The action of koji, which is a specific mold (Aspergillus oryzae), produces enzymes that break down these starches into sugars. This enzymatic conversion is vital in creating the alcohol content and sweetness of the sake. Without koji, the fermentation process would not proceed effectively, as there would be insufficient sugars available for the yeast to consume and ferment, which would ultimately affect the overall quality of the sake.

The other options, while related to the brewing process, do not capture the primary function of koji. Enhancing aroma, aiding in fermentation, and clarifying sake are also important aspects of sake production, but they do not directly address the fundamental enzymatic transformation that koji provides in converting starch to sugar.