Pour a well-made junmai ginjo into a glass and hold it to the light. The liquid is almost perfectly clear — or faintly golden if it has spent time maturing. The aroma is delicate: fruit, perhaps, or fresh rice, or something floral that is hard to name. What you are holding is the product of one of the most technically sophisticated fermentation processes ever developed. Calling it rice wine does not begin to cover it.

Why Sake Is Not Wine

Wine fermentation is, in biochemical terms, relatively direct. Grapes contain glucose and fructose — sugars that yeast can ferment immediately into ethanol and carbon dioxide. The winemaker’s primary job is to manage that conversion: temperature, yeast selection, timing.

Sake starts from rice, and rice contains no fermentable sugar. Its carbohydrate is starch — long, complex chains of glucose molecules that yeast cannot access. Before fermentation can begin, those starches must be broken down into simple sugars. This saccharification step is what separates sake from wine, and it is where the extraordinary complexity of sake brewing originates.

The organism that performs this saccharification is not yeast. It is Aspergillus oryzae — the koji mold — secreting amylase enzymes that systematically cleave starch chains into glucose. And the central technical achievement of sake brewing is that this saccharification and the subsequent yeast fermentation happen simultaneously, in the same vessel.

What Is Koji? The Mold Behind Japanese Fermentation

Step One: The Rice

Sake begins with rice, but not the short-grain table rice you eat with dinner. Sake-specific cultivars — Yamada Nishiki, Omachi, Gohyakumangoku, and others — are bred for a distinctive characteristic: a large, opaque starchy core called the shinpaku (白心). This pure starch centre is surrounded by layers of protein and fat — components that, during fermentation, produce undesirable flavour compounds including fusel alcohols and certain fatty acid esters.

Polishing: Seimaibuai

To remove these outer layers, sake rice is milled — a process measured by seimaibuai (精米歩合), the percentage of the original grain remaining after milling. A rice polished to 60% seimaibuai has had 40% of its mass removed. The lower the number, the more has been milled away, and the purer the remaining starch.

After milling, the rice is washed, steeped in water to a precise moisture content, and steamed — not boiled. Steaming gelatinises the surface starch while keeping the grain firm enough for koji hyphae to penetrate without the grain dissolving into mush.

Step Two: Making Koji

Approximately 20–25% of the steamed rice is set aside for koji production. Aspergillus oryzae spores are dusted over the cooled rice and the inoculated grain is incubated in a temperature- and humidity-controlled koji room (kojimuro) for 40–50 hours.

During this time, the mold’s hyphae penetrate each grain and begin secreting amylase and protease enzymes. The amylases will later saccharify starch in the main fermentation mash. The proteases break down rice proteins into free amino acids — contributing to the body and amino acid profile of the finished sake.

The quality of the koji is arguably the single most important variable in sake production. An experienced toji (杜氏, master brewer) manages the kojimake process by touch, smell, and visual inspection — adjusting temperature and timing to achieve the precise enzyme balance required for the style of sake being produced.

Step Three: The Yeast Starter (Shubo)

Before the main fermentation mash can be assembled, a concentrated yeast culture must be established. This starter — called shubo (酒母) or moto (酛) — is a small, highly acidic environment designed to cultivate a massive population of sake yeast while suppressing unwanted bacteria.

The acidity is critical. Sake yeast (Saccharomyces cerevisiae strains selected for sake production) are relatively acid-tolerant; most spoilage bacteria are not. By establishing a low-pH environment before scaling up fermentation, brewers create a selective advantage for the yeast they want.

Two Approaches to Acidification

Step Four: The Main Mash (Moromi)

The moromi is where the biochemical complexity of sake brewing reaches its peak. Into a large fermentation vessel, the brewer combines the yeast starter, koji rice, additional steamed rice, and water — and allows simultaneous saccharification and fermentation to proceed over the next 20–35 days.

The Sandan-Jikomi System

The ingredients are not added all at once. Instead, they are introduced in three stages over four days — a system called sandan-jikomi (三段仕込み):

The logic of this staged addition is biochemical. Adding all the rice at once would flood the mash with starch, crashing the sugar concentration to a level that would stress the yeast and risk stalling fermentation. The staged approach keeps glucose concentration in a narrow range — high enough to sustain active fermentation, low enough to avoid osmotic inhibition of the yeast. It is a self-regulating system that Japanese brewers developed empirically over centuries before the underlying biochemistry was understood.

The Biochemistry of Simultaneous Saccharification and Fermentation

At its peak, the moromi is a remarkable biochemical environment. Koji amylases are continuously converting starch to glucose. Sake yeast are continuously converting that glucose to ethanol and carbon dioxide. Neither process waits for the other — they run in parallel, in the same liquid, simultaneously.

This simultaneous saccharification and fermentation (SSF) system is what allows sake to achieve ethanol concentrations of 18–22% ABV by natural yeast fermentation — among the highest in any fermented beverage worldwide. Wine yeast typically die at 14–16% ABV; sake yeast tolerate higher alcohol concentrations, and the SSF system keeps glucose supply gradual enough that the yeast are never overwhelmed.

Temperature and Aroma Development

Fermentation temperature profoundly shapes the aroma of the finished sake. Ginjo-style sake is fermented cold — typically 8–12°C — over a longer period of 60–90 days. Low temperatures slow the yeast’s metabolic rate, favouring the production of aromatic esters over rapid alcohol yield. The key compounds are ethyl caproate (apple-like, fruity) and isoamyl acetate (banana-like), produced by yeast ester synthase enzymes that are more active at low temperatures.

Warmer fermentation produces sake with less aromatic lift but more body and structural complexity — characteristics valued in junmai and kimoto styles.

Step Five: Pressing, Filtration, and Pasteurisation

When fermentation is complete, the moromi — now a mixture of sake, lees (rice solids and spent yeast), and residual koji — is pressed to separate the liquid from the solids. The pressing method affects the character of the finished sake: gentle pressing by inflatable bladder produces cleaner, more delicate sake; traditional fune (槽) pressing by gravity and weighted boards produces richer, more textured sake with more lees contact.

Most sake is then filtered, both through activated charcoal (to remove colour and certain harsh flavour compounds) and through fine filters for clarity. Nigori sake (にごり) is an exception — it is roughly filtered, leaving rice solids in suspension and producing the characteristic cloudy appearance.

Finally, most sake is pasteurised — heated briefly to around 65°C to deactivate enzymes and kill remaining microorganisms, stabilising the sake for storage. Nama sake (生酒, “raw sake”) skips this step, retaining active enzymes and a more vibrant, volatile flavour profile — but requiring refrigeration throughout the supply chain.

Reading a Sake Label

Understanding the brewing process makes sake labels legible in a new way. Each term on a label describes a specific decision made somewhere in the process above:

A Biochemist’s Guide to Sake Classification(coming soon)

Two Thousand Years of Refinement

The core principles of sake brewing — koji saccharification, simultaneous fermentation, staged mash addition — were established in Japan over a millennium of empirical refinement, long before the microbiology and biochemistry were understood. The toji tradition, in which master brewers passed their knowledge through apprenticeship rather than textbooks, produced a fermentation system of remarkable sophistication.

Modern sake production layers scientific understanding on top of that tradition. Yeast strains are now maintained and distributed by the Brewing Society of Japan, with documented aroma profiles and fermentation characteristics. Temperature is controlled to the degree. Enzyme activity can be measured rather than inferred from smell alone.

But the best sake still comes from breweries where the toji’s judgment — built from years of reading a mash by its colour, its viscosity, its CO₂ production, its smell — remains the final arbiter of quality. The biochemistry explains what they are doing. It does not replace the doing.