Fermenting soybeans is one of the oldest food preservation techniques in the world, and the process is surprisingly approachable at home. The method you choose depends on what you want to make: natto, tempeh, and miso each use different microorganisms, temperatures, and timelines, but they all start with cooked soybeans and end with a more nutritious, more flavorful food. Here’s how each one works.
Why Fermentation Improves Soybeans
Raw soybeans contain compounds called antinutrients that block your body from absorbing minerals. Fermentation breaks these down dramatically. In one study published in Scientific Reports, fermenting soybeans at 38°C for 72 hours reduced phytic acid (the main mineral-blocking compound) by roughly 90% and tannins by 53 to 65%. That means the iron, zinc, and calcium already present in soybeans become far more available to your body after fermentation.
Fermentation also generates new nutrients. Natto, for example, is one of the richest dietary sources of vitamin K2 in the MK-7 form, which plays a role in bone health and calcium metabolism. And across all fermented soy products, the process creates amino acids responsible for deep umami flavor that plain soybeans simply don’t have.
Preparing the Soybeans
Regardless of which fermented product you’re making, the first steps are the same. Soak dried soybeans overnight (about 12 hours) in plenty of water. They’ll roughly double in size. Then cook them until they’re soft enough to crush easily between your fingers. Boiling takes about two hours; a pressure cooker cuts that to 45 minutes or so. You want them tender but not mushy, since they’ll soften further during fermentation.
Sterilization matters at every step. Wash your hands, sterilize your containers and utensils with boiling water, and work on clean surfaces. Fermented soy products like tempeh and miso typically finish at a pH around 5, which is higher than the 4.5 threshold that reliably blocks harmful bacteria. That means you’re relying on your starter culture to outcompete any contaminants, so giving it a clean environment is essential.
Making Tempeh
Tempeh uses a mold called Rhizopus oligosporus, which you can buy online as a dry powder labeled “tempeh starter.” The mold binds the soybeans into a firm, sliceable cake with a nutty flavor.
After cooking and draining your soybeans, let them cool to about body temperature and pat them as dry as possible. Moisture on the surface encourages the wrong kinds of bacteria. Sprinkle the starter powder over the beans (roughly one teaspoon per pound of dry soybeans, though follow your starter’s instructions) and toss until evenly coated.
Pack the inoculated beans into perforated ziplock bags or banana leaves. The perforations are critical: the mold needs airflow to grow. Poke holes about an inch apart using a toothpick or skewer. Then incubate at 30 to 32°C (86 to 90°F) for 24 to 40 hours. By 24 hours you should see white mycelium threading between the beans. By 36 to 40 hours, the entire mass should be bound together in a dense white cake. If you see any black or green patches, that’s sporulation or contamination, and you should discard the batch.
A simple incubation setup: place the bags in your oven with just the light on, or use a cooler with a low-wattage heating pad and a basic thermometer. The mold generates its own heat as it grows, so check temperatures periodically. If it climbs above 35°C, crack the oven door or add ventilation.
Making Natto
Natto is fermented with Bacillus subtilis, a bacterium that produces the sticky, stringy texture natto is known for. You can buy natto spores (sold as natto starter powder) or use a spoonful of store-bought natto to inoculate a batch.
Cook your soybeans until very soft. While they’re still hot (the heat helps kill competing organisms), drain them thoroughly and mix in the starter. If using powdered spores, dissolve a tiny amount (typically 1/10 of a teaspoon for 2 cups of dry beans) in a tablespoon of warm, sterilized water first. If using store-bought natto, mix about a tablespoon into the hot beans.
Spread the beans in a thin layer (no more than an inch deep) in shallow containers. Cover loosely with foil, poking a few holes for airflow. Incubate at 38 to 40°C (100 to 104°F) for 22 to 24 hours. Research on natto quality found that 39°C produced the best flavor and highest amino acid content. After the initial fermentation, transfer the natto to the refrigerator for 24 hours of “after-ripening,” which mellows the ammonia smell and lets the flavor develop fully.
Maintaining a steady temperature for 24 hours is the main challenge. An oven with the light on often runs too cool for natto. Better options include a food dehydrator with a temperature dial, an Instant Pot set to its yogurt function, or a small cooler rigged with a reptile heating pad and a plug-in temperature controller. These controllers cost about $30 to $40 and let you set a precise target temperature using a probe.
Making Miso
Miso is the longest fermentation on this list, but the hands-on time is short. It requires koji (rice or barley that has been inoculated with Aspergillus oryzae mold) and salt. You can buy ready-made koji from specialty food stores or online.
The process: mash your cooked, cooled soybeans into a rough paste. Mix the koji and salt together first, then combine everything. The ratio of salt, koji, and soybeans determines both flavor and fermentation time. A good starting ratio for a medium-bodied miso is equal parts koji and cooked soybeans by weight, with salt making up about 10 to 12% of the total mixture weight. More salt and less koji produces darker, more complex miso but requires a longer ferment. Less salt and more koji gives you a sweeter, lighter miso that finishes faster.
For a white (shiro) miso, aim for roughly 7% salt in the final mixture with a higher proportion of koji. This can be ready in about three months. A darker, red-style miso with 11 to 13% salt needs six months to a year at room temperature. Some traditional misos age for two years or more.
Pack the mixture tightly into a clean jar or crock, pressing out all air pockets. Smooth the surface flat, press plastic wrap directly onto the paste, and weight it down with a plate or a sealed bag of salt. This prevents air exposure, which causes mold on the surface. Store it at room temperature in a spot out of direct sunlight. If a thin layer of white mold appears on top, scrape it off. A layer of liquid (called tamari) may pool on top, and that’s normal and desirable.
Keeping Temperatures Steady
Temperature control is the single biggest factor in a successful ferment. For tempeh and natto, where you need 30 to 40°C for one to two days, your options from simplest to most reliable are:
- Oven with the light on: Many ovens hold around 27 to 35°C with just the incandescent bulb. Test yours with a thermometer before committing a batch.
- Cooler with a heat source: A small insulated cooler with a heating pad, seedling mat, or even a jar of hot water can maintain a narrow temperature range. Add a digital thermometer to monitor.
- Plug-in temperature controller: These devices connect to a heat source (like a heating pad) and cycle it on and off to hold a set temperature. They turn any enclosed space into a reliable incubator.
- Instant Pot or yogurt maker: The yogurt setting on many pressure cookers holds roughly 38 to 40°C, which is nearly perfect for natto.
For miso, temperature control isn’t an issue since it ferments at whatever room temperature you have. Warmer rooms (around 25°C) simply speed the process along.
Troubleshooting Common Problems
The most common failure with tempeh is beans that are too wet when inoculated. Excess surface moisture encourages bacterial growth instead of mold growth, producing a slimy or sour-smelling result. After cooking, drain the beans in a colander and spread them on a clean towel to air-dry for 15 to 20 minutes before adding the starter.
With natto, an overpowering ammonia smell usually means fermentation went too long or too hot. Aim for the 22 to 24 hour window and don’t let temperatures creep above 42°C. The after-ripening period in the fridge also helps reduce ammonia. If the beans aren’t sticky or stringy, the starter may not have been viable, or the incubation temperature was too low.
Miso is forgiving because the salt content inhibits most harmful organisms. The main enemy is surface mold from air exposure. Keep the surface sealed tightly and weighted. If your miso develops pink or orange mold, scrape it away generously. The paste underneath is fine.
Storing Fermented Soybeans
Fresh tempeh keeps in the refrigerator for about a week and freezes well for several months. Slice it before freezing so you can pull out portions as needed. Natto also stores well in the fridge for a week or in the freezer for two to three months. Miso is the most shelf-stable of the three. Once it reaches a flavor you like, move it to the refrigerator to dramatically slow fermentation. Refrigerated miso keeps for a year or longer.