Tofu shows up on nearly every list of fermented foods, but the label is misleading. Standard tofu is not fermented at all. It is made by curdling soy milk with a mineral coagulant, a process much closer to cheesemaking than to fermentation. The confusion comes from the fact that many other soy-based foods are fermented, and people group them all together. This guide separates the facts, covers each major soy ferment in detail, and includes instructions for making tempeh at home.
Regular tofu is not fermented
Tofu production uses coagulation, not fermentation. You start with dried soybeans, soak them overnight, grind them with water, and strain the mixture to produce soy milk. The milk is heated, then a coagulant is added to curdle the proteins.
After the soy milk curdles, the curds are scooped into a mold lined with cloth and pressed to remove whey. More pressing produces firmer tofu. The entire process takes a few hours and involves no microorganisms, no aging, and no fermentation of any kind.
This is why standard tofu does not contain probiotics or the flavor complexity that fermented soy products develop. It is a fresh product, similar in concept to paneer or ricotta.
Fermented tofu (fu ru and sufu)
Fermented tofu is a completely different product from the blocks you find in the refrigerator section. Known as fu ru, sufu, or tofu cheese, it is a pungent, creamy condiment used across China, Taiwan, and Southeast Asia.
- 1Cut and dry fresh tofu Firm tofu is cut into small cubes (roughly 3 cm) and air-dried for several hours to reduce surface moisture.
- 2Inoculate with mold The cubes are exposed to mold spores, typically Actinomucor elegans or Mucor species. They sit at room temperature for 3-5 days until a white, fuzzy mycelium covers each cube.
- 3Salt and brine The mold-covered cubes are packed into jars with salt and a brine made from rice wine, sometimes with added chili, sesame oil, or red yeast rice for color.
- 4Age The jars are sealed and aged for 1-6 months. During this time, enzymes from the mold break down the soy proteins into amino acids, creating an intense umami flavor.
The result is soft, spreadable, and powerfully savory. White fermented tofu has a mild, slightly alcoholic taste. Red fermented tofu gets its color from red yeast rice and tastes bolder. Both are used as condiments, not eaten in large quantities.
If someone tells you that tofu is fermented, they may be thinking of fu ru. It exists, but it is a specialty condiment, not the tofu you stir-fry or add to soup.
Tempeh
Tempeh is the most widely eaten fermented soy food outside of East Asia. It originated in Java, Indonesia, likely several centuries ago. Unlike tofu, tempeh uses whole soybeans and relies entirely on mold fermentation.
The process starts with cooked, dehulled soybeans. After cooling, you mix in a small amount of Rhizopus oligosporus spore powder (the starter culture). The inoculated beans are packed into a perforated container or wrapped in banana leaves and incubated at 30-32°C (86-90°F) for 24-48 hours.
During incubation, the Rhizopus mold grows white mycelium that threads through and around every bean. This mycelium binds the loose beans into a firm, sliceable cake. The mold also produces enzymes that partially digest the proteins and reduce anti-nutritional compounds like phytic acid.
To make it yourself, see how to make tempeh.
Miso
Miso is a thick, salty paste made by fermenting soybeans with koji (Aspergillus oryzae mold), salt, and often a grain like rice or barley. It is one of the oldest fermented foods in Japan, with records dating back over 1,000 years.
The production starts with making koji. Steamed rice or barley is inoculated with Aspergillus oryzae spores and incubated for about 48 hours until the mold fully colonizes the grain. This koji is then mixed with cooked soybeans and salt, packed tightly into a vessel, and left to ferment.
The long fermentation time is what gives miso its depth. Enzymes from the koji break soy proteins into amino acids (creating umami), starches into sugars, and fats into aromatic compounds. The salt controls the pace and prevents spoilage. Darker misos have fermented longer and taste more complex.
For a closer look at each style, see types of miso; to make your own, see how to make miso and the miso salt calculator.
Natto
Natto is polarizing. The sticky, stringy texture and strong smell put off many first-time tasters, but in Japan it is an everyday breakfast food prized for its nutrition and distinctive flavor.
Production is simple compared to miso or soy sauce. Whole soybeans are soaked, steamed until very soft, and then inoculated with Bacillus subtilis var. natto bacteria. The beans are packed into small containers and incubated at about 40°C (104°F) for 22-24 hours.
Natto is typically served over rice with soy sauce, mustard, and chopped green onions. You stir it vigorously before eating to develop the stringy texture. The flavor is earthy, slightly bitter, and deeply savory.
To make it at home, see how to make natto.
Soy sauce
Traditionally brewed soy sauce is one of the most complex fermented products in existence. The process combines two distinct fermentation stages and takes 6-12 months or longer to complete.
- 1Make the koji Roasted wheat and steamed soybeans are mixed and inoculated with Aspergillus oryzae or Aspergillus sojae mold. The mixture incubates for about 3 days until the mold fully colonizes it.
- 2Prepare the moromi The koji is mixed with a strong salt brine (about 17-19% salinity) to create a mash called moromi. The salt halts the mold growth and creates an environment for the next phase.
- 3Ferment for months The moromi ferments in tanks or crocks for 6-12 months (sometimes 2-3 years for premium sauces). During this time, lactic acid bacteria lower the pH, and yeasts produce alcohol and hundreds of aromatic compounds.
- 4Press and pasteurize The finished moromi is pressed through cloth to extract the liquid soy sauce. It is then pasteurized, filtered, and bottled.
Watch for
- Chemically hydrolyzed soy sauce (made by breaking soybeans apart with hydrochloric acid) is not fermented and lacks the flavor depth of brewed soy sauce
- Check the label Traditionally brewed soy sauce lists soybeans, wheat, salt, and water with no added caramel color or corn syrup
- Light soy sauce (thin) is saltier and used for seasoning. Dark soy sauce (thick) is less salty but adds color and body to dishes. Both are fermented.
The flavor complexity of good soy sauce comes from hundreds of compounds formed during the long fermentation: alcohols, esters, organic acids, and melanoidins. No shortcut process replicates this depth.
The home version of this process is in how to make soy sauce.
Nutritional differences
The soy foods covered in this guide vary widely in their nutritional profiles. Processing method, fermentation duration, and added ingredients all play a role.
None of these foods are inherently better or worse. Tofu is a clean, versatile protein. Tempeh and natto offer more fiber because they use whole beans. Miso and soy sauce are condiments used in small amounts, so their protein contribution per serving is lower.
Making tempeh at home
Tempeh is the easiest fermented soy product to make at home. You soak and dehull soybeans, cook them, dry them thoroughly, mix in Rhizopus starter, pack them into perforated bags and keep them at 30–32°C for 24–48 hours until a firm white cake forms. Dry beans and steady warmth are what make it work.
The full method, quantities, timings and how to tell good tempeh from bad are in how to make tempeh. For the other soy and koji ferments, see the Koji, Miso & Soy guides.
