koji

Koji is steamed rice that has had koji-kin, or koji mold spores, cultivated onto it. This magical mold, for which the official scientific name is Aspergillus Oryzae, creates several enzymes as it propagates, and these are what break the starches in rice into sugars that can be fermented by the yeast cells, which then give off carbon dioxide and alcohol. Without koji, there is no sake.