How soy sauce is made
Soy sauce is brewed for months by a mold that digests soybeans and wheat into amino acids.
- stations
- 7
- things upstream
- 8
- raw sources
- 6
- steps from the ground
- 2
The short version
Soybeans are cooked and wheat is roasted and crushed, then mixed and seeded with koji mold. After the mold grows, the culture goes into salt brine and ferments for months, while enzymes break proteins into amino acids and starches into sugars. The mash is pressed, the liquid is pasteurized, and the sauce is bottled.
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Soak and cook the beans
Soybeans are soaked in water and then steamed or boiled until soft. Heat unfolds the proteins so mold enzymes can attack them later.
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Roast and crush the wheat
Wheat is roasted and cracked. The roasting gives the sauce its toasty aroma, and the wheat starch will later turn into sugars.
Goes in -
Seed with koji mold
Equal amounts of cooked soybeans and roasted wheat are mixed and sprinkled with spores of Aspergillus oryzae or Aspergillus sojae. The mold spreads through the grain in warm, humid rooms for a few days and makes enzymes.
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Drown it in brine
The mold-covered grain, called koji, is mixed into strong salt brine to make a thick mash called moromi. Salt keeps spoilage bacteria out while the right microbes do the work.
Goes in -
Brew for months
Mold enzymes break the grain proteins into free amino acids and the starches into simple sugars. Lactic acid bacteria and yeast then work on the sugars and add acids, alcohol and many flavor compounds. Traditional soy sauce takes months.
months
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Press out the liquid
The mash is wrapped in cloth-lined containers and squeezed. Raw soy sauce runs out, and the solid cake that is left is used for animal feed or fertilizer.
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Heat and bottle
The raw sauce is heated to kill remaining yeast and mold, then filtered. It is aged or bottled directly.
Worth repeating
- Soy sauce began as a way to stretch salt, which used to be expensive.
- The Dutch East India Company listed soy sauce as a commodity in 1737, when seventy-five large barrels shipped from Dejima, Japan, to Batavia.
- Some cheaper soy sauces are made by chemically hydrolyzing soy protein, and some blend both methods.
Back to the ground
Follow every input upstream and soy sauce starts as these 6 raw sources.
Keep walking
Sources
Cite this page: "How soy sauce is made." Assembly Line, 2026-11-03. https://assemblyline.org/make/soy-sauce