fermenting grain, grain which has sprouted, yeast
糵 centers on grain that has been transformed by sprouting or fermentation, used as a starter for brewing. The yeast sense is the active agent in that process.
糵 combines 薛 (a plant-related component) with 米 (rice/grain), suggesting grain that has undergone a plant-like transformation such as sprouting or fermentation. The exact historical development is uncertain.
The top part 薛 looks like a plant growing, and the bottom 米 is rice. Picture rice grains sprouting and fermenting into yeast, ready to start brewing.
For ゲツ, imagine a brewer getting yeast ready: 'get' the yeast -> ゲツ, and the grain starts to ferment.
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