Making tempeh at home requires soybeans (or other legumes), a starter culture containing Rhizopus oligosporus mold, and about 48 hours of patience. The process is straightforward: soak, cook, inoculate with spores, and let the mold knit everything into a firm, sliceable cake. Once you understand the basics, it’s one of the simplest fermentation projects you can do in a home kitchen.
What You Need
The ingredient list is short. You need dried soybeans (about 2 cups makes one block), tempeh starter (sold online as a dry powder containing Rhizopus oligosporus spores), and white vinegar. For equipment, you’ll need a pot, a perforated bag or container for incubation, and a way to hold a steady warm temperature for two days.
Perforated zip-lock bags work well for beginners. Poke small holes every inch or so with a toothpick or skewer. The mold needs oxygen to grow, and without those holes, fermentation will stall or produce off flavors. Banana leaves are the traditional Indonesian wrapper and work beautifully if you can find them.
Step-by-Step Process
Soak and Dehull
Start by soaking your soybeans in plenty of water for 6 to 18 hours at room temperature (around 25°C / 77°F). The beans will roughly double in size. After soaking, rub the beans between your hands or in a towel to split them and loosen the hulls. The hulls float, so fill the pot with water and skim them off the surface. You don’t need to remove every last one, but getting most of them off helps the mold penetrate the beans evenly.
Cook
Boil the dehulled beans for 30 to 45 minutes. You want them tender but not mushy. They should hold their shape and feel slightly firm when you bite one. Overcooking creates a wet, pasty mass that’s harder for the mold to colonize.
Dry and Acidify
Drain the beans thoroughly and spread them on a clean towel or baking sheet. Pat them dry and let surface moisture evaporate for 10 to 15 minutes. This step matters: excess water encourages bacterial contamination rather than mold growth.
Once the beans have cooled to below body temperature (around 35°C / 95°F or lower), add 1 to 2 tablespoons of white vinegar and toss to coat. The vinegar lowers the pH into the range of 4.1 to 4.5, which suppresses harmful bacteria while still allowing the mold to thrive. In traditional Indonesian production, natural lactic acid bacteria handle this acidification during the soak, but vinegar is the reliable shortcut for home production.
Inoculate
Sprinkle the starter evenly over the beans (typically about 1 teaspoon per 2 cups of dry beans, but follow your starter’s instructions). Toss gently with clean hands or a spoon until the powder is distributed throughout. Pack the mixture into your perforated bags or containers in a layer about 2 to 3 centimeters (roughly 1 inch) thick. Thinner layers ferment more evenly because the mold generates its own heat as it grows.
Incubate
Place the bags in a warm spot and hold the temperature between 30 and 35°C (86 to 95°F). Rhizopus oligosporus grows well across this range and can tolerate temperatures up to 40°C, but above 35°C the fermentation speeds up and becomes harder to control. Many Indonesian tempeh producers simply ferment at room temperature (around 25°C) for about two days. At 30°C, expect the process to take roughly 36 to 48 hours.
For the first 12 hours, not much will happen visually. Around 18 to 24 hours, you’ll start seeing white fuzz forming between the beans. By 36 to 48 hours, the entire block should be covered in dense white mycelium that binds the beans into a solid cake you can slice.
Setting Up a Simple Incubator
If your kitchen stays around 30°C, you can ferment tempeh on the counter. Most people in cooler climates need a little help. The simplest option is an oven with just the light on. The bulb generates enough warmth to keep the interior in the right range, though you should verify with a thermometer first.
A cooler or insulated box with a low-wattage incandescent bulb or a seedling heat mat also works well. The key is steady warmth and some airflow. Drill or prop open a small gap for ventilation. The mold is aerobic and needs fresh air, and the fermentation itself produces heat and carbon dioxide that need to escape. A completely sealed chamber will overheat once the mold gets active, potentially killing it.
How to Tell If It Worked
Good tempeh is bright white with dense mycelium that completely covers and binds the beans. It should smell pleasantly nutty or mushroom-like. A few gray or black spots are normal, especially near the air holes in your bag. Those dark patches are areas where the mold has begun to sporulate (produce spores), which happens when it’s exposed to more oxygen. This is safe to eat but indicates the tempeh is fully mature or slightly overfermented in those spots.
Tempeh that has turned brownish-yellow throughout has spoiled. If you see bright green or pink spots, or if the block smells strongly of ammonia or alcohol, discard it. These are signs of bacterial contamination rather than healthy mold growth. The most common causes of failure are excess moisture on the beans, insufficient air holes, or temperatures outside the ideal range.
Beyond Soybeans
Tempeh works with a surprising range of legumes and grains. Black beans, navy beans, black-eyed peas, and mung beans all make excellent tempeh. The general rule is to use small beans. Large ones like kidney beans don’t culture well because the mold can’t penetrate deeply enough between the pieces.
Chickpeas work if you sort for the smaller ones in the bag or crack them in half before inoculating. Lentils are another good option, but stick with whole brown, green, or French lentils. Split lentils fall apart during cooking. Lentils also hold onto a lot of moisture, so drain them well and stir them as they cool to let extra water evaporate. Boil them for only about 15 minutes, keeping them slightly firm.
You can also mix legumes with grains like rice or barley, or add seeds like sunflower or sesame for flavor and texture. A quarter cup of seeds mixed into a batch of bean tempeh adds a pleasant nutty quality. The fermentation process and timing stay the same regardless of the substrate.
What Fermentation Does to the Beans
The mold doesn’t just glue the beans together. As it grows, its enzymes break down compounds in the beans that normally block mineral absorption, particularly phytic acid. This means the iron, zinc, and calcium in tempeh are significantly more available to your body than in plain cooked soybeans. The fermentation also converts certain plant compounds into more biologically active forms, loosening the cell structure of the beans and making nutrients easier to digest overall.
Storage
Once your tempeh reaches full white coverage, stop the fermentation by refrigerating or cooking it. Fresh homemade tempeh keeps in the refrigerator for about 5 to 7 days. Wrapping it tightly helps, but the mold will slowly continue to mature in the fridge, so the window is limited.
For longer storage, freezing is the best option. Tempeh freezes well without significant changes to taste or texture and will keep for about 3 months. Slice it into portions before freezing so you can thaw only what you need. If your stored tempeh develops bright green or colored spots, or a strong unpleasant smell, it has gone bad. A slight yellowing of the surface is normal and still safe.