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1
Cook the rice until soft and slightly wetter than standard steamed rice. Spread it in the fermentation vessel — a rice cooker set to 'keep warm' is well-suited — and allow it to cool to 60°C before adding the koji. Temperatures above 65°C will denature the amylase and kill the koji; verify with a thermometer.
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2
Crumble the dried or fresh rice koji thoroughly into the warm cooked rice. Mix until the koji is evenly distributed with no dry pockets. The mixture should resemble a loosely combined grain porridge.
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3
Maintain the mixture at 55–60°C for 8–12 hours. This is where the dish lives or dies: temperature control is the single critical variable — below 55°C, saccharification is sluggish and incomplete; above 65°C, the amylase is deactivated and the koji is destroyed, halting conversion entirely. If using a rice cooker, drape a folded towel over the lid to retain humidity. Stir thoroughly every two hours to prevent hot spots and ensure even saccharification.
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4
At 8 hours, taste the mixture. A pronounced, clean, honeyed sweetness with a barely perceptible sour undertone indicates sufficient amylase activity. The rice grains will be semi-dissolved into a thick, porridge-smooth consistency. Continue fermentation up to 12 hours for deeper sweetness. When the sweetness level is satisfactory, the amazake is complete.
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5
To serve warm, dilute the thick amazake with water at a 1:1 to 1:2 ratio depending on desired consistency. Heat gently to no more than 60°C — do not boil, as this destroys the active enzymes and probiotic content. The drink is ready when it is steaming, uniformly fluid, and fragrant with a clean rice-grain sweetness.
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6
Ladle into cups and finish, if desired, with a small pinch of finely grated fresh ginger per portion. The drink should be pale ivory, smooth, and gently aromatic.