In a clean, food-safe container, combine the dried koji rice, distilled water, and salt. Stir thoroughly until the salt is completely dissolved and the rice is evenly hydrated.
Cover the container loosely with a layer of cheesecloth or a vented lid to allow limited airflow while protecting the contents from contaminants.
Leave the mixture at ambient temperature (68–75°F / 20–24°C) for 7 days. Stir gently once per day with a clean utensil to support consistent enzyme activity and even distribution.
After 7 days, transfer the container to the refrigerator. The shio koji is now ready to use.
You may blend the mixture into a smooth paste for marinades and dressings, or leave the rice grains intact for greater flexibility in use. Strain if desired based on your recipe.
Notes
Storage Tips: Store in the refrigerator in a sealed container. At a salt concentration of approximately 6.5%, shio koji will remain stable and usable for 6 months or longer.