Suberi koji (slippery koji)

Suberi koji (スベリ麹, literally "slippery koji") is a condition in which specific microorganisms, mainly Bacillus subtilis (枯草菌), grow abnormally on the surface of the koji. As a result, the surface becomes smooth or takes on a sticky texture. The koji may also clump together like dumplings when touched, giving it a distinctive look and feel. Suberi koji often results from insufficient growth of the koji mold or an unsuitable fermentation environment, which causes quality problems, so it is considered difficult to brew good-quality sake from koji in this state. It is also called "nebari koji" (粘り麹, sticky koji) or "nururi koji" (ヌルリ麹, slimy koji), and it is regarded as a condition to be avoided in sake brewing. Because such koji negatively affects the flavor and aroma of the sake, proper management is required.