The Kankyo Brewery (whose name means “sweet and strong”) has made its mirin the same way since 1862.
Producers apply koji, a special Japanese rice mold, to steamed sweet rice and let it ferment for 24 hours, just long enough to activate sugar production. Distilled sake is then added to the mixture, which is left to develop the sugars and amino acids that give mirin its characteristic golden hue and silky mouthfeel. After 60 to 90 days, a raw version of the mirin is pressed from the rice mixture and aged for another nine months, finally producing a liquid that is subtly sweet, mellow and full of umami.