How Soy Sauce Is Made: The Complete Science and Process Guide

Soy sauce is one of the most complex fermented liquids in food production. The premium version — naturally brewed from soybeans, wheat, water, and salt — undergoes a sequence of microbial transformations involving multiple species of mold, yeast, and bacteria over 6 months to 3 years. The result contains over 300 identified chemical compounds contributing to flavor.

Understanding how soy sauce is made changes how you use it — knowing what's been lost in speed-produced commercial versions and what's present in traditionally brewed soy sauce informs when quality matters.

The Two Production Methods

Before looking at the process, the fundamental division in soy sauce production:

Naturally brewed (本醸造, hon-jōzō): The traditional method using actual koji fermentation, moromi (mash) fermentation, and pressing. Minimum 6 months; premium versions 18-36 months. Most Japanese soy sauce sold globally.

Chemically produced (アミノ酸液混合, amino acid liquid mixed): Soybeans are hydrolyzed with hydrochloric acid, producing free amino acids (including glutamic acid) in a matter of days. Caramel coloring and salt are added. The result mimics the appearance and salt-savory profile of soy sauce but lacks the complex flavor development of fermentation. Used in cheaper products. Sometimes blended with naturally brewed soy sauce.

This guide covers naturally brewed soy sauce.

The Ingredients

Soybeans: The protein source. Defatted soybean meal (the byproduct of soybean oil extraction) is common in commercial production because the fat fraction slows fermentation; premium producers use whole soybeans. More fat in the soybean produces a slightly richer, fuller-flavored sauce.

Wheat: Contributes carbohydrates for fermentation, adds flavor complexity (wheat fermentation produces different compounds than soybean fermentation alone), and affects color development. Koikuchi (standard dark soy sauce) uses roughly equal parts soybean and wheat. Tamari uses little or no wheat — it developed as the liquid drained from miso barrels, which are wheat-free.

Salt: 17-20% by weight in the finished product. Salt determines the safety of fermentation (inhibiting pathogenic organisms while allowing beneficial ones) and is the primary preservative in the finished sauce.

Water: Traditionally local — the specific water chemistry of a region's water source affects the fermentation and flavor. This is similar to the water terroir of Japanese whisky production.

Koji starter (Aspergillus sojae or A. oryzae): The critical inoculation that drives the initial mold fermentation.

Stage 1: Koji Making (3-5 Days)

What happens: Soybeans are soaked, cooked (steamed or pressure-cooked to soften and denature proteins). Wheat is roasted and crushed. The two are combined and inoculated with Aspergillus koji spores.

The mixture (koji substrate) is spread in a temperature- and humidity-controlled room. Over 3-5 days, the Aspergillus mold colonizes the grain-bean mixture, growing white to yellow-green mycelium throughout.

What the mold does: Aspergillus produces enzymes — most critically protease, amylase, and glutaminase:

• Protease: Breaks down soy proteins into peptides and free amino acids, including glutamic acid (the primary umami compound)
• Amylase: Converts wheat starches into sugars that will feed later fermentation
• Glutaminase: Converts glutamine into free glutamic acid, increasing umami potential dramatically

This stage is called the koji stage, and the result is a dense, enzyme-rich grain-bean solid called shoyu koji.

Why this matters: The koji stage determines how much enzyme activity will be available for the next stage. More koji time, more enzymes, more complete protein breakdown, more free amino acids, more flavor. This is why longer fermentation produces more complex soy sauce.

Stage 2: Moromi Fermentation (6-36 Months)

What happens: The shoyu koji is mixed with brine (heavily salted water — approximately 24-25% salt solution) in large fermentation vessels. This mixture is called moromi (醪, the same word used for sake mash).

The salt inhibits pathogenic organisms while allowing salt-tolerant microorganisms to work. The fermentation sequence:

Phase 1 (Early moromi — months 1-3): Lactic acid bacteria (Tetragenococcus halophilus, a highly salt-tolerant LAB species) dominates. They produce lactic acid, which lowers the pH and acidifies the moromi. This creates the right conditions for yeast fermentation that follows.

Phase 2 (Mid moromi — months 3-6): Salt-tolerant yeasts (Zygosaccharomyces rouxii and others) produce ethanol and hundreds of flavor compounds including:

• Furanones (caramel-like character)
• Various esters (fruity, floral notes)
• Pyrazines (roasted, nutty aromas — from amino acid-sugar Maillard reactions that occur slowly at fermentation temperatures)
• HEMF (4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone) — the compound most characteristic of soy sauce aroma

Phase 3 (Continued fermentation): Further enzyme activity, flavor development, and maturation. The color deepens from enzymatic browning reactions. Organic acids accumulate. The flavor becomes more complex.

Temperature management: Traditional kioke (wooden barrel) fermentation allows natural temperature fluctuation with the seasons. Cold winters slow fermentation (and allow flavor development without over-fermentation); warm summers accelerate the yeast and enzyme activity. This seasonal rhythm is considered by traditional producers to produce more complex flavor than temperature-controlled fermentation.

Moromi management: Large producers stir the moromi regularly to prevent stratification and promote even fermentation. Traditional producers stir less, allowing some natural stratification.

Stage 3: Pressing

What happens: The moromi is pressed to separate the liquid from the solid residue. Traditional pressing uses a mechanical press — layers of moromi-filled cloth alternating with pressing boards. Hydraulic or screw presses are used at scale.

The pressed liquid is raw soy sauce (nama-shoyu) — full of active enzymes and microorganisms.

The solid residue (shoyu kasu) — the pressed-out grain and soybean solids — can be used as animal feed, composted, or occasionally used as a seasoning.

Stage 4: Heat Treatment and Pasteurization

What happens: The raw soy sauce is heated to approximately 70-80°C (158-176°F) for a specific time. This process:

1. Pasteurizes the sauce, killing residual microorganisms and halting fermentation
2. Develops color through heat-induced Maillard reactions between free amino acids and sugars — producing the characteristic dark brown color and additional roasted flavor compounds
3. Develops aroma through the formation of additional volatile compounds during heating
4. Clarifies the liquid by precipitating proteins that were soluble at fermentation temperature

The heating step is the final major flavor development stage. Under-heating leaves raw, harsh notes; over-heating destroys volatile aromatics. The specific temperature and time is a key producer variable.

Stage 5: Blending, Filtering, and Bottling

The heated soy sauce is blended (most commercial producers blend batches for consistency), filtered to remove precipitates, and bottled. Some premium producers bottle unfiltered.

Preservatives: Most commercially sold soy sauce contains sodium benzoate or alcohol as preservatives. Traditional soy sauce relies on its salt content (17-20%) for preservation. Premium naturally brewed soy sauce often contains added ethanol (sake lees or brewery alcohol) rather than sodium benzoate.

What Distinguishes Premium from Commodity Soy Sauce

Time: The primary differentiator. Standard commercial soy sauce in Japan is often produced in 6 months. Premium soy sauce is 18 months or longer. The additional time allows more complete enzyme action, more secondary fermentation products, and more complex Maillard development.

Whole soybeans vs. defatted meal: Whole soybeans produce more complex flavor. Most premium producers use whole soybeans.

Wooden kioke vs. stainless steel: Traditional cedar barrels contribute trace aromatic compounds from the wood and allow natural temperature variation. Fewer than 1% of Japanese soy sauce is produced in wooden barrels today — it's considered an artisanal premium.

Specific koji strains: Different Aspergillus strains produce different enzyme profiles and flavor characters. Some premium producers use proprietary koji strains maintained over generations.

Free glutamic acid content: A direct quality proxy. Premium naturally brewed soy sauce contains more free glutamic acid (more umami) than commodity versions, because longer fermentation allows more complete protein breakdown.

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The 300+ chemical compounds in naturally brewed soy sauce are not incidental complexity — they're the product of one of the most sophisticated microbial transformations in food production. When that process is shortened or bypassed, the sauce tastes flat, salty, and one-dimensional by comparison. The difference is directly tasted in the glass.

Related reading: Japanese vs Korean Soy Sauce | What Is Umami? | Korean Fermentation Science