How Soy Sauce Is Made Through Fermentation

The Basics of Soy Sauce Fermentation

Soy sauce fermentation is a traditional process that transforms soybeans and wheat into a dark, savory liquid seasoning. The key steps involve mixing cooked soybeans with roasted wheat, inoculating with koji mold, then fermenting the mixture in a salt brine called moromi. Over several months, enzymes from the koji, along with yeasts and lactic acid bacteria, break down proteins and starches into amino acids, sugars, and aromatic compounds, giving soy sauce its characteristic umami flavor and deep brown color.

Thai soy sauce production follows similar principles but varies in ingredients and fermentation time, resulting in distinct varieties like si-io khao (light soy sauce) and si-io dam (dark soy sauce).

Key Ingredients and Their Roles

The primary ingredients in soy sauce fermentation are:

• Soybeans: Provide protein that is broken down into amino acids, especially glutamic acid, which creates umami.
• Wheat: Adds carbohydrates for fermentation, contributing sweetness and aroma. In some Thai soy sauces, wheat may be reduced or omitted for gluten-free varieties.
• Salt: Creates a high-salt environment in the brine that inhibits spoilage organisms while allowing halotolerant yeasts and bacteria to thrive.
• Koji: A culture of Aspergillus oryzae or Aspergillus sojae mold grown on the soybeans and wheat. It produces enzymes like proteases and amylases that break down the raw materials.
• Water: Used to make the brine that forms the moromi.

These ingredients combine to create a fermentation environment where a succession of microorganisms works together to develop flavor.

The Fermentation Process Step by Step

1. Preparing the Koji

Soybeans are soaked and cooked until soft, and wheat is roasted and cracked. The two are mixed and inoculated with koji spores. The mixture is incubated in a warm, humid room for about 2-3 days, during which the mold grows and produces enzymes.

2. Making the Moromi

The koji is transferred to a fermentation vessel and mixed with a salt brine (about 20-23% salt) to create a thick mash called moromi. This high salt concentration selects for specific microorganisms that can tolerate the salty conditions.

3. Brine Fermentation

The moromi is left to ferment for several months to years. During this time:

• Enzymatic breakdown: Koji enzymes continue to hydrolyze proteins into amino acids and starches into simple sugars.
• Lactic acid bacteria (such as Tetragenococcus halophilus) grow, producing lactic acid that lowers pH and adds tanginess.
• Yeasts (such as Zygosaccharomyces rouxii) ferment sugars into alcohol and create aromatic esters.
• The moromi gradually darkens and develops complex flavors.

Fermentation time varies: typical soy sauce ferments for 6-8 months, but some Thai soy sauces may be fermented for only 2-3 months for a lighter flavor, while premium varieties age over a year.

4. Pressing and Filtration

After fermentation, the moromi is pressed to separate the liquid soy sauce from the solid residue (soybean cake). The raw soy sauce is then filtered to remove particles, resulting in a clear liquid.

5. Pasteurization

The raw soy sauce is heated to pasteurize it, which stops fermentation, kills any remaining microorganisms, and develops additional flavor and color through Maillard reactions. The soy sauce is then bottled.

The Microorganisms Behind the Flavor

Three main types of microorganisms drive soy sauce fermentation:

These microorganisms work in sequence: the mold first prepares the substrate, then bacteria and yeasts take over as oxygen is depleted and salt selects for halotolerant species.

Thai Soy Sauce Varieties

Thai soy sauce production yields several distinct products:

• Si-io khao (light soy sauce): Thin, salty, and light in color. Used for seasoning and dipping sauces, similar to Chinese light soy sauce.
• Si-io dam (dark soy sauce): Thicker, darker, and slightly sweet, often with added molasses. Used for color and flavor in stir-fries and braises.
• Si-io wan (sweet soy sauce): Very thick and sweet, often used in noodle dishes like pad see ew.

These soy sauces differ in fermentation time and post-fermentation additions. Light soy sauce is fermented for a shorter period and may have less wheat, while dark soy sauce often gets additional molasses and longer aging.

Thai soy sauces are essential in Thai cooking: light soy sauce seasons dishes without darkening them, dark soy sauce adds color and richness to stir-fries, and sweet soy sauce balances flavors in noodle dishes.

Making Soy Sauce at Home: A Simplified Approach

While traditional soy sauce takes months, you can make a small batch at home with patience:

1. Make koji: Cook soybeans and mix with roasted wheat flour, then inoculate with koji spores (available online). Incubate in a warm place for 2-3 days until covered with white mold.
2. Prepare moromi: Mix the koji with a brine of 20% salt (20g salt per 100ml water) in a clean jar.
3. Ferment: Cover loosely and stir daily for the first week, then let it sit in a cool place for 3-6 months, stirring occasionally. The mixture will darken and develop aroma.
4. Press and pasteurize: Strain the liquid through cheesecloth, then heat to 70-80°C for 20 minutes to pasteurize.
5. Bottle: Store in a clean bottle in the refrigerator.

Note: Homemade soy sauce may differ in flavor from commercial versions, but it offers a fascinating glimpse into the fermentation process.

Conclusion

Soy sauce fermentation is a complex synergy of molds, bacteria, and yeasts transforming simple ingredients into a deeply flavorful seasoning. Thai soy sauce production adds its own nuances, yielding essential condiments for Thai cuisine. Understanding the process enhances appreciation for this everyday ingredient.