You can make tempeh without commercial starter by cultivating wild Rhizopus mold from plant leaves or by propagating spores from a previous batch of tempeh. Both methods have been used for centuries in Indonesia, where tempeh originated. They require more patience and carry more risk than using packaged starter, but they work.
The Traditional Leaf Method
Long before commercial tempeh starter existed, Indonesian producers made their own inoculum called “usar” using leaves from the waru tree (Hibiscus tiliaceus). Researchers at Wageningen University confirmed that Rhizopus oryzae, one of the key tempeh molds, naturally lives on fresh Hibiscus leaves with considerable frequency. The theory is that tempeh was likely discovered by accident when cooked soybeans were wrapped in these leaves for storage and the resident mold took over.
The traditional process from East Java works like this: soak and cook soybeans as you normally would for tempeh, then place 10 to 20 cooked soybean halves on the underside of a fresh, rinsed Hibiscus leaf trimmed to roughly 10 centimeters across. Cover with a second leaf, underside facing down, creating a sandwich. Stack the sandwiches under banana leaves or a loosely draped plastic sheet and incubate for two to three days in a warm, humid, well-aerated spot. You’re looking for dense white mold growth with visible dark spore patches. Once fully sporulated, open the sandwiches and sun-dry them until crisp. The dried usar keeps at room temperature for four to six weeks, though traditional makers consider fresh usar superior to the dried version.
Research found that Hibiscus leaves don’t contain any special growth-promoting compounds. They simply provide a convenient surface that retains moisture while still allowing enough airflow for the mold to develop. This means other broad, flexible leaves could work in theory, though Hibiscus species have the advantage of naturally harboring Rhizopus spores on their surface.
Growing Starter From Store-Bought Tempeh
The most accessible method if you don’t have Hibiscus trees nearby is to propagate spores from a piece of existing tempeh. You need unpasteurized tempeh with live culture, which rules out most grocery store brands that have been frozen or heat-treated. Look for fresh tempeh from a local producer, or use a single batch made with purchased starter to generate enough homemade culture for dozens of future batches.
To do this, let a small portion of finished tempeh continue fermenting past the point where you’d normally eat it. As the mold matures, the white mycelium will darken and produce black or grey spores. This is what you want. The over-ripened tempeh needs to be thoroughly dried to create a shelf-stable starter. You can do this by breaking it into small pieces and placing it in a warm, dry, well-ventilated area. A dehydrator on low heat or a warm cupboard works. If it still feels paste-like after grinding, let it dry for several more hours and grind again. The goal is a fine, dry powder you can store in an airtight container.
To use this homemade starter, sprinkle it over your prepared beans at a rate roughly seven to ten times higher than you would with commercial starter. The spore count in homemade culture is less concentrated, so you need more of it. Mix thoroughly, then ferment as usual.
Keeping Fermentation Safe
The biggest risk with starter-free tempeh is that you’re working with wild microbes. Rhizopus mold is what you want, but your beans are also exposed to bacteria and potentially harmful molds. Two things tilt the odds in your favor: acidity and temperature.
Adding vinegar to your cooked beans before fermentation lowers the pH to around 3.5, which selectively suppresses unwanted bacteria while Rhizopus tolerates it well. Use roughly one tablespoon of white vinegar per cup of dry beans, mixed in after cooking and draining. The beans must also be thoroughly dried on the surface before you begin. Excess moisture encourages bacterial growth instead of mold growth.
Temperature matters enormously. Rhizopus thrives between 30°C and 35°C (86°F to 95°F). Below that range, fermentation slows and competing organisms gain ground. Above 40°C (104°F), you risk killing the mold entirely. A consistent, warm environment is more important than any other single factor. Many home fermenters use a cooler with a low-wattage light bulb, a seedling heat mat, or an oven with just the light on.
Watch your tempeh carefully during the first 24 hours. Healthy Rhizopus growth appears as white, cotton-like mycelium spreading evenly across the beans. By 36 to 48 hours, the beans should be knit into a firm, sliceable cake. If you see green, blue, orange, or bright yellow patches, or if the tempeh smells strongly of ammonia rather than having a mild mushroomy scent, discard it. These are signs of contamination by other mold species or bacterial overgrowth.
Practical Tips for Wild Fermentation
Your beans need to be cooked but not mushy. Overcooked beans hold too much moisture and collapse into a paste that doesn’t allow airflow between the pieces. Split the beans (or dehull soybeans) so the mold can access the interior. Spread them in a thin, even layer no more than about 2.5 centimeters (one inch) deep in your fermentation container.
Airflow is critical. Rhizopus is an aerobic mold, meaning it needs oxygen to grow. If you’re using plastic bags, punch small holes every couple of centimeters. If you’re using a container with a lid, leave it slightly cracked or covered with breathable cloth. The traditional leaf-sandwich method works partly because the leaves allow gas exchange while holding in moisture.
Expect wild fermentation to be less predictable than using commercial starter. Your first batch may take longer, produce patchier growth, or fail entirely. This is normal. Each successful batch gives you material to make stronger starter for the next round. Indonesian usar makers traditionally use their freshest, most vigorous usar to inoculate the next generation, gradually selecting for the most productive strains over time.
Storing Homemade Starter
Dried homemade starter keeps for four to six weeks at room temperature in a sealed container stored away from light and moisture. For longer storage, refrigeration or freezing extends viability significantly. The key is making sure the starter is completely dry before sealing it. Any residual moisture will cause it to clump, degrade, or grow unwanted mold during storage.
Grind dried sporulated tempeh into a fine powder using a spice grinder or mortar and pestle. A finer powder distributes more evenly across your beans and produces more consistent results. Label it with the date, since potency decreases over time. If a batch of tempeh made with older starter takes noticeably longer to ferment or produces weak, patchy growth, it’s time to make fresh starter from your most recent successful batch.