Rinse the rice under cold water three times until the water runs mostly clear.

In a large pot, combine rinsed rice with 4 cups of water and bring to a boil over high heat.

Reduce heat to low, cover, and simmer for 15 minutes until the rice is tender.

Remove the pot from heat and let the rice cool uncovered for 10 minutes to reach 140°F.

Stir in 3 cups of water to the cooled rice to create a thin porridge mixture.

When the mixture reaches 140°F, crumble in 1 cup of rice koji spores and stir thoroughly for one minute.

Transfer the pot to an insulated container and maintain the temperature between 130°F and 145°F for 8 to 12 hours, checking every 2 hours.

Heat the fermented mixture to 170°F and hold for 5 minutes to stop fermentation.

Strain the mixture through a cheesecloth-lined strainer or blend until smooth, depending on desired consistency.

Cool to room temperature, transfer to glass jars, and refrigerate for at least 2 hours before serving.