Milling, Koji, and the Three-Stage Mash: Inside the Core Steps of Sake Brewing

What decides the flavor of a bottle before it ever reaches a glass? Much of the answer lies in three steps: rice milling, koji-making, and the three-stage mash. In The Full Sake Production Process, we traced the journey from raw rice to bottled sake as a single continuous flow. Here, we pull out just the steps that move the flavor needle the most and go down to the level of "why." Why polish the rice at all? What is koji actually doing? Why split the mash into three additions instead of just one? Once you understand the "why" behind each step, the terms on the back label start to mean something.

Where Flavor Really Gets Decided

A well-known saying among sake brewers ranks the priorities: koji-making first, yeast starter (moto) second, and mash-building third. We covered the full production sequence in the earlier article; here we zoom in on the three steps that shape flavor the most.

Before diving in, here's a quick map of what each one controls.

In between comes washing, soaking, and steaming the rice, the prep work that bridges milling and koji-making. We'll touch on it briefly before getting to the core steps, then look closely at multiple parallel fermentation later on. First, rice milling.

Rice Milling: Why Polish the Grain?

Milling (seimai) removes the outer layers of brown rice. How much has been removed is expressed as the rice polishing ratio (seimai buai): the percentage of the original grain that remains after polishing. A polishing ratio of 60% means 40% of the outer layer has been milled away. The smaller the number, the more heavily the rice has been polished. We cover how this number affects the reading on a label and the resulting flavor in Junmai, Ginjo, and Daiginjo Explained.

So why polish the rice at all? The outer layers of the grain are rich in proteins, fats, and minerals. In large amounts, these compounds are the source of off-flavors, heaviness, and rough aromas. By milling away the outer layer and keeping the starch-rich core, brewers cut off those off-flavors before fermentation even starts.

There's also craft in how the milling is done. Standard milling rounds the grain down evenly, but protein isn't only concentrated at the surface. It also collects at the two tapered ends of the elongated grain. That's where flat-polishing (henpei seimai) comes in: a technique that mills the grain thin while keeping its original oblong shape, rather than rounding it off. According to data from Hiroshima Prefecture, standard round-milling needs to remove roughly half the outer layer to bring protein down to the target level, while flat-polishing can reach the same level by removing only around 30%. Same goal, less rice lost.

Milling also generates friction heat. If that heat builds up, the grains can crack or dry out too much. That's why the polishing stones turn slowly, and the process takes time. The more heavily the rice is polished, the longer milling takes. This unglamorous, time-consuming prep step is where a sake's structural backbone is already being decided.

Washing, Soaking, and Steaming: Building a Firm Outside, Soft Inside

Once milled, the rice is washed, soaked to absorb water, and then steamed. Washing removes the rice bran and polishing dust still clinging to the surface. Soaking lets the grain absorb water, and since the amount absorbed shapes how the rice steams, brewers using highly polished rice sometimes time the soak down to the second.

Steaming aims for something different from ordinary cooking: a grain that's firm on the outside and soft inside, known as gaikō naijū. A firm exterior keeps the grain from breaking down too much, while a soft interior lets the koji mold's mycelium (its thread-like filaments) grow deep into the grain. The link between this "firm-outside, soft-inside" texture and the white starchy core (shinpaku) of the grain is discussed further in An Intermediate Guide to Sake. Once steamed, the rice is divided three ways (for koji, for the yeast starter, and for the mash), so the same batch of steamed rice ends up playing three different roles.

Koji-Making: Where Koji Creates Sugar

Koji is steamed rice inoculated with koji mold (yellow koji mold, or kōji-kin) and cultivated until the mold has spread through the grain, a process called koji-making, or seikiku. Koji mold is even named as a technical centerpiece in "Traditional Sake-Making," Japan's UNESCO-recognized Intangible Cultural Heritage entry. Koji is the linchpin of sake's flavor-building.

Koji's job is to convert starch into sugar. The koji mold produces enzymes (including amylase) that break down starch, converting it into glucose. This conversion is called saccharification. Yeast can turn sugar into alcohol, but it can't use starch directly; koji is the bridge that turns starch into something yeast can ferment.

Not all koji looks the same. The white, thread-like mycelium spreading visibly across the steamed rice is called haze; when those threads burrow deep into the core of the grain, it's called haze-komi. Depending on how the haze spreads, koji falls into two broad types. Drawing on data from Kikumasamune Sake Brewing, here's how they compare.

Starting from the same steamed rice, whether the mold spreads across the whole surface or drives straight into the core changes the strength of saccharification. Weaker saccharification tends toward a lighter sake; stronger saccharification toward a fuller-bodied one. It's often said that koji-making is what sets the direction a sake will take.

Koji-making takes place over roughly two days in a temperature- and humidity-controlled room called the kōji-muro (koji chamber). Temperature is typically kept around 32–38°C (90–100°F) and humidity around 60–70%, though exact figures vary by brewery and equipment. Because the growing mold generates its own heat, brewers regularly turn and loosen the rice (a process called kirikaeshi) to keep the temperature in check. This careful management of temperature and humidity is exactly what determines whether the koji develops as tsuki-haze or so-haze. It's a hands-on step where a brewery's individual character tends to show through.

Multiple Parallel Fermentation: Saccharification and Fermentation at Once

Koji's saccharification and yeast's fermentation aren't separate, sequential steps. As soon as sugar is produced, the yeast starts converting it into alcohol. Saccharification and alcoholic fermentation proceed simultaneously, inside the same mash. This is called multiple parallel fermentation (heikō fukuhakkō). The mash itself (a blend of steamed rice, koji, water, and yeast, all fermenting together) is called moromi. Multiple parallel fermentation is, arguably, the single most important concept for understanding sake.

Compared with other alcoholic beverages, sake's approach stands out. Wine grapes already contain their own sugar, so yeast only needs to ferment it, a single-step process called simple fermentation. Beer first converts malt starch into sugar, then ferments it in a separate stage, a two-step process known as sequential fermentation. Sake does both jobs in a single mash, at the same time.

That simultaneity has real advantages: sugar never builds up to excess, and yeast keeps a steady supply to work with. Multiple parallel fermentation is why sake can reach an alcohol content of around 20% (per Gekkeikan's explanation), well above wine (around 11%) or beer (around 5%), over a fermentation period of roughly 20 to 30 days. You can see exactly where this mechanism fits into the overall brewing sequence in The Full Sake Production Process.

The Three-Stage Mash: Why Split It Into Three Additions?

Once the yeast starter (moto) is ready, brewing moves to the main mash. The moto is a concentrated mass of yeast cultivated under the protection of lactic acid, which suppresses unwanted bacteria. Rather than adding all the rice, koji, and water at once, brewers build up the mash in several additions. This staged approach is called dan-jikomi (staged brewing), and doing it in three additions specifically is called san-dan jikomi, the three-stage mash.

Why split it up? Adding a large volume of rice and water all at once would dilute the yeast and lactic acid that have been carefully cultivated, opening the door for unwanted bacteria to take hold. Adding ingredients gradually lets the yeast population keep growing without losing concentration, and keeps the acid level (which holds contaminants in check) from dropping too fast. Each addition roughly doubles the previous one, by design, so the yeast is never suddenly diluted. The whole process unfolds over four days.

The key moment is day two, odori (literally "the dance"), a deliberate pause the day after the first addition, giving the yeast time to build up its numbers. This lets the yeast establish a strong, dominant population before the volume is ramped up. By taking it step by step, the mash can grow safely in size while yeast stays firmly in control of the fermentation.

How These Three Decisions Show Up in the Glass

Each of these three core steps leaves its own signature on the final flavor. Here's a quick reference for what to notice when you taste.

The more heavily the rice is milled, the fewer off-flavors remain and the more aromatic the sake tends to be, though heavier polishing also mutes the rice's own character. Neither direction is objectively "better"; it depends on the flavor the brewer is aiming for. The relationship between naming conventions and milling levels is laid out in Junmai, Ginjo, and Daiginjo Explained.

The type of koji haze shapes the balance of sweetness, body, and finish: tsuki-haze tends toward a light, aromatic sake, while so-haze tends toward a fuller, umami-rich one. The temperature and pace of the three-stage mash shape how fermentation develops from there. Layer on the choice of rice variety, yeast strain, and water, and you arrive at the final glass. How individual ingredients play into this is covered in An Intermediate Guide to Sake, and the vocabulary of taste and aroma in Taste and Aroma.

Summary

Here are the essentials from the three core steps:

• Rice milling removes surface protein and fat, reducing off-flavors, measured as the rice polishing ratio.
• In koji-making, enzymes from the koji mold convert starch into sugar (saccharification). Tsuki-haze koji suits light, ginjo-style sake; so-haze koji suits rich, full-bodied sake.
• Multiple parallel fermentation is the mechanism where saccharification and fermentation happen together in the same mash, the core of what makes sake unique.
• The three-stage mash adds ingredients gradually to avoid diluting yeast and acid, with day two's rest period (odori) letting the yeast population build first.

Once the "why" behind each step clicks, the next step is connecting it to the vocabulary of taste. See An Intermediate Guide to Sake for how ingredients shape flavor, and Taste and Aroma for the language of aroma and taste.

• This article is intended for readers of legal drinking age (20 in Japan). Please drink responsibly. Avoid alcohol during pregnancy or breastfeeding, and never drink and drive.