Sake · Production · Study guide
Fermentation
A study guide to sake fermentation — the shubo starter, kimoto, yamahai and sokujo, the three-stage main mash, sake yeasts, and how temperature sets the style.
Fermentation is where sake's style is truly decided. The rice, the polish and the kōji have set the stage, but it is in the fermenting tank — how the yeast is grown, which strain it is, and above all how warm it is allowed to work — that a batch becomes a fragrant, delicate ginjō or a rich, savoury junmai. The old ranking of brewing skill put it second only to kōji: ni moto, "second the starter".
Two ideas carry the whole subject: the starter (shubo, or moto) — a small, dense seed mash whose only job is to grow a strong yeast population, defended from spoilage by lactic acid — and the three-stage build (sandan shikomi) that scales it up into the main mash without ever letting the yeast be overwhelmed. Fix how the acid gets there and how the mash is built, and the rest is temperature.
The one thing to fix first: the starter is about clean, strong yeast
A brewer cannot simply pitch yeast into a huge tank of sweet mash — wild bacteria and stray yeasts would spoil it first. So sake begins with a starter: a concentrated mash of steamed rice, kōji, water and yeast, kept small so the yeast can multiply into a dense, healthy, dominant population. The thing that keeps it clean is lactic acid, which lowers the pH until only the sake yeast thrives. How that acid arrives splits the starters into two families:
- Added lactic acid — the acid is simply added at the start. Fast, safe, and overwhelmingly the norm (sokujō-moto, ready in about two weeks).
- Natural lactic acid — wild lactic-acid bacteria are allowed to generate the acid over weeks before the yeast takes over (kimoto and yamahai). Slower, riskier, and prized for the depth it brings.
The starter methods
| Method | How the acid arrives | Time | Character |
|---|---|---|---|
| Sokujō-moto | Lactic acid added directly | ~2 weeks | Clean, precise, the modern default |
| Kimoto | Wild bacteria; rice mashed to paste by hand with poles (yama-oroshi) | ~4 weeks | Rich, complex, higher acidity, savoury |
| Yamahai | Wild bacteria, but the pole-mashing is omitted | ~4 weeks | Earthy, gamey, full-bodied |
Kimoto is the old, laborious way: brewers once rammed the mash to a paste with poles in the cold (the yama-oroshi step) to help the enzymes work. In 1909 researchers showed this pounding was unnecessary — the mash breaks down on its own — and the labour-saving yamahai ("yama-oroshi haishi", abolished pole-ramming) was born. Both let wild bacteria build acidity slowly, which is why they taste deeper and more savoury than the squeaky-clean sokujō. When you see kimoto or yamahai on a label (reading a sake label), expect body and tang.
The three-stage main mash (sandan shikomi)
The starter — a few percent of the final volume — is scaled up into the main mash (moromi) by adding rice, kōji and water in three stages over four days, roughly doubling the volume each time. The staged build keeps the yeast concentration and acidity high enough to stay clean while the tank grows:
| Day | Name | What happens |
|---|---|---|
| 1 | Hatsuzoe | First addition of rice, kōji and water |
| 2 | Odori ("dance") | A rest day — no addition; the yeast multiplies |
| 3 | Nakazoe | Second, larger addition |
| 4 | Tomezoe | Third and largest addition; the mash is complete |
Then the parallel fermentation runs — kōji releasing sugar, yeast making alcohol — for about two to four weeks, cool and slow for ginjō, warmer and shorter for rich styles.
Key facts
| Starter (shubo/moto) | Small seed mash — rice, kōji, water, yeast — protected by lactic acid |
| Fast starter | Sokujō-moto (added lactic acid, ~2 weeks) |
| Traditional starters | Kimoto and yamahai (natural lactic acid, ~4 weeks) |
| Main mash build | Sandan shikomi — three additions over four days |
| Fermentation | Multiple parallel fermentation, ~2–4 weeks |
| Style lever | Temperature — cool & slow = fragrant; warm & fast = rich |
Temperature sets the style, briefly
Because temperature governs the pace of both conversions at once, it is the brewer's main dial. A cool, slow fermentation (down around 10 °C, sometimes for a month) stresses the yeast just enough to throw off fruity, floral esters — the hallmark of ginjō. A warmer, faster fermentation builds body, savour and a fuller texture — the junmai end. Everything else in a ginjō recipe (high polish, tsuki-haze kōji, an aromatic yeast) exists to support that long, cool ferment.
In this guide
The full guide below goes deeper into the yeast and the choices:
- The sake yeasts — Kyōkai #6, #7, #9 and #1801 — and what each gives
- How brewers deliberately build "ginjō aromas"
- Ending the fermentation, and adjusting the final strength
- Why kimoto and yamahai taste the way they do
- Classic exam questions
The sake yeasts
Yeast supplies the alcohol and much of the aroma, so strain choice matters. The Brewing Society of Japan distributes the reference Kyōkai (association) yeasts by number, and a handful are worth knowing:
| Yeast | Known for |
|---|---|
| #6 (Aramasa) | Reliable, clean, steady fermenter; one of the oldest still in use |
| #7 (Masumi) | Versatile, dependable, good aroma; the most widely used |
| #9 (Kumamoto) | The first great ginjō yeast — fruity apple and banana esters |
| #1801 | Highly aromatic, ester-rich; the modern competition ginjō yeast |
Many are also sold as low-foaming variants, which don't throw the thick foam cap that traditionally forced brewers to leave headroom in the tank. Beyond the Kyōkai strains, prefectures and individual breweries breed their own house yeasts.
Building ginjō aromas
The fragrant, fruity ginjō character (ginjō-ka) is not an accident — it is engineered by pushing several choices in the same direction:
- a high polishing ratio, so there is little protein and few nutrients;
- a restrained tsuki-haze kōji, releasing enzymes slowly;
- low nutrient levels, which stress the yeast into producing esters;
- an aromatic yeast (like #9 or #1801);
- and a long, cool fermentation.
Starve and chill the yeast, in other words, and it rewards you with apple, banana, pear and melon aromatics. Feed it and warm it, and you get body and savour instead.
Ending the mash and setting the strength
When the sugar and alcohol reach the brewer's target — often 18–20% ABV, the highest of any brewed drink — the fermentation is stopped by chilling and by pressing the mash off its lees. At this point some brewers add a measured dose of distilled jōzō alcohol (in honjōzō and many ginjō/daiginjō) to lift aroma and lighten the body; junmai styles add none. The finished sake is later diluted with water to drinking strength, unless it is bottled undiluted as genshu. What happens next — pressing, filtration, pasteurisation — is covered in pressing & finishing.
Classic exam questions
- What is the shubo (moto)? — the yeast starter: a small, dense mash grown to build a strong, clean yeast population.
- What protects the starter from spoilage? — lactic acid, either added (sokujō) or made by wild bacteria (kimoto, yamahai).
- How does yamahai differ from kimoto? — yamahai omits the yama-oroshi pole-mashing step; both still rely on natural lactic acid.
- Name the three-stage addition and its four days. — sandan shikomi: hatsuzoe, odori (rest), nakazoe, tomezoe.
- Which yeast is the classic fruity ginjō strain? — Kyōkai #9 (with #1801 the modern competition choice).
- How does a brewer create ginjō aromas? — high polish, tsuki-haze kōji, low nutrients, aromatic yeast, and a long cool fermentation.
Grow clean yeast, build the mash in stages, then rule it with temperature — and the same tank can give you either a perfumed daiginjō or a savoury yamahai junmai.