Nurihaze (koji mold growing only on the grain surface)

Nurihaze refers to koji in which the koji mold (Aspergillus oryzae) has grown only on the surface of the rice grains, without penetrating well into the interior. Such koji can look normal from the outside, but enzyme activity inside the grain is low and its saccharifying and fermenting capacity is insufficient, so it is not well suited to sake brewing. Properly developed koji breaks the rice down evenly and helps bring out sweetness and umami, but these effects are weakened when the koji is in a nurihaze state. In sake brewing, making good koji is extremely important for producing high-quality sake.