Koji is a Japanese term for the rice into which koji-jin has been propogated
Other definition:
Koji refers to a steamed rice into which koji-kin (which means koji mold) has been cultivated. This mold is known in English as Aspergillus Oryzae. It is used in sake brewing to break down the starch molecules into sugar molecules that can be used as food by the yeast cells. Since rice is milled, there is no husk and therefore no enzymes, so malting (as in beer brewing) is not possible. Koji provides those enzymes to create the sugars for fermentation. This cultivation of koji-kin mold onto steamed rice to create koji itself is the heart of the sake brewing process.
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