Koshuku koji (overly hardened koji)

Koshuku koji (高縮麹) refers to koji that is hard and has weak enzymatic activity, so it does not dissolve easily in the moromi (fermentation mash). This koji does not work fully during fermentation and can cause a "backing" or lining to form on the sake lees (kasu). The main causes of koshuku koji are steamed rice that is too hard and excessive loss of moisture from the steamed rice during koji making. Under these conditions, the koji's enzymatic power is not fully realized, and fermentation may not proceed well as a result. Because it greatly affects the quality of sake, avoiding koshuku koji is very important.