Polishing removes the hard outer layers of the rice grain, leaving mostly the starchy center of the rice grain exposed. It's a slow process—polishing to 70% can take about 10 hours, while polishing down to 50% can take almost two days—so that the heat generated prevents the grains from cracking and instead gives them a uniform, round shape.

After polishing, the rice is washed and soaked to remove any remaining rice flour (nuka) from the polishing process and to achieve just the right moisture content—usually around 30%. The more polished the rice, the faster it absorbs water.
Brewers must be very precise here, working with very small batches at a time and soaking in cold spring water to the second to maintain consistent levels.

Steaming has two main functions: it sterilizes the rice and modifies the starch so that it can be used in brewing. The goal is to cook each grain so that it is firm on the outside but soft on the inside. After steaming, part of the rice goes to the production of koji (the mold that converts starch into sugar), and the rest goes into the tank for fermentation. Rice added only for fermentation is called kakemai.

Koji (Aspergillus oryzae) is grown on steamed rice and helps break down starch into sugar so that yeast can convert it into alcohol. Approximately 20% of the rice in a sake batch is processed into koji. This 40-hour process is extremely labor-intensive and requires constant monitoring, day and night. It involves ensuring the even growth of the mold across all rice grains, which the brewers control through careful mixing and adjusting temperature and humidity.

After steaming, the rice is cooled and placed in a special warm, humid room and spread out. Koji mold spores are sprinkled over it, and after a few hours, the mold begins to grow under continuous monitoring.

After about 10–12 hours, the rice is mixed again. In traditionally handmade koji, brewers unwrap and re-wrap the rice several times, breaking up clumps by hand and spreading it evenly. While the mold is rapidly growing, the resulting heat and moisture must be able to escape to prevent the koji from overheating and spoiling during this very active phase.

The rice is then usually divided into boxes or trays, or simply left on the large bed. Dividing it into smaller batches allows brewers to better control humidity and temperature. This middle part of koji production is called "mori."

After about 36-40 hours, the mold is encouraged to spread across the surface. Finally, the process is stopped by cooling the rice.

Before the primary fermentation begins, brewers prepare a small, powerful starter called shubo or moto. This mixture of rice, water, and koji is designed to cultivate a strong yeast population. The goal is to create a healthy, acidic environment where the yeast can thrive and harmful bacteria stay away. Sokujō-moto vs. Kimoto/Yamahai
Today, most sake is produced using the Sokujō-moto method, the rapid starter method developed in the early 20th century. Lactic acid is added directly to the starter, which protects the yeast from harmful bacteria and speeds up the process. This method takes about two weeks instead of a full month. The older Kimoto method required manual labor, with brewers using poles to pound rice and koji into a paste and relying on natural bacteria to produce lactic acid.

Moromi

Moromi is the stage where all the main ingredients – rice, koji, water, and the yeast starter – come together in a large tank. This stage is crucial, as it creates many of sake's unique aromas and flavors. Brewers must maintain a balance between the conversion of starch to sugar (by koji) and the fermentation of sugar to alcohol (by yeast). The process typically takes 21–28 days at temperatures of 12–18°C, although ginjo styles require cooler temperatures (10–12°C) and take longer (30–35 days).

Moromi occurs in three stages, in which rice, koji, and water are gradually added to keep yeast activity strong and avoid bacterial contamination. This step-by-step method is called "three-stage addition," or sandan jikomi.

Filtration

The main goals of filtration are to stop fermentation at the right time to capture as many desirable flavors as possible and remove unwanted solids. Importantly, “filtered, pressed liquid” is part of the legal requirements set out by the Japanese government for brewing sake in Japan. After filtration, distilled alcohol (jozo alcohol) can be added to improve aroma, flavor balance, and texture. This practice is common in non-Junmai sake styles (aruten).

There are several different filtration methods, each producing slightly different results:

Yabuta-shibori: Uses accordion-style filters and air pressure. It's efficient and widely used, but too much pressure can release harsh flavors.

Funa-shibori: Traditional method using cloth bags. Slower, gentler, and often used for premium sake. Produces finer textures.

Fukuro-shibori: Here, the mash is suspended in cloth bags and drips out without pressure, only by gravity. A very slow process, but it produces the purest, most delicate sake. Often reserved for competition entries.

During filtration, brewers can separate different fractions: the first pass (Arabashiri, rough and lively), the middle pass (Naka-dori, the highest quality), and the last pass (Seme, stronger but coarser). These fractions can be bottled separately or blended together. The remaining solids, called sake kasu, are rich in nutrients and are often used for cooking or pickling.

There are two main methods of pasteurization. Bulk pasteurization treats large tanks, while bottle pasteurization heats the sake in the bottle, often used for premium ginjō to protect delicate flavors. Pasteurization usually occurs twice: once before storage and again before shipping. Some sakes are left unpasteurized (nama-zake), which gives them a fresh, vibrant flavor but makes them more delicate to handle.