Making Tempeh at Home
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Tempeh is a mould ferment, and it is the one home project that most resembles running a small incubator. Cooked soybeans are inoculated with Rhizopus spores, held at a warm steady temperature for a day or two, and knitted together by white mycelium into a firm cake you can slice and fry. It is more technical than sauerkraut because you have to control temperature and airflow, but the ingredient list is short and the payoff is a food that is expensive and often disappointing when bought.
The beans need three things before inoculation. They must be dehulled, because the mould struggles to penetrate intact skins. They must be cooked until tender but not falling apart. And they must be dry on the surface, because wet beans grow bacteria instead of mould. Soaking dried soybeans overnight loosens the hulls, which you rub off by hand and float away with water changes. Cracked, pre dehulled soybeans save considerable time. Chickpeas, black beans and mixtures of beans and grains all work if soybeans are hard to find.
Acidifying the cooking water is the step beginners skip and later regret. Adding a splash of vinegar to the beans while they cook lowers the pH, which suits Rhizopus and discourages competing bacteria. After cooking, drain the beans thoroughly and spread them out, then dry them with a towel or in a warm pan until the surfaces look matte rather than glossy. They should be at body temperature or cooler before the starter goes in, since hot beans kill spores.
Mix the spore starter through the cooled beans until it is evenly distributed, following the quantity on the packet. Pack the mixture into perforated bags or a shallow container about two to three centimetres deep. The perforations matter, because Rhizopus needs some air exchange but not a draught. Holes spaced roughly two centimetres apart in a plastic bag, made with a fork or a needle, is the standard home approach. Press the beans into an even layer so the mould has a continuous surface to colonise.
Incubation is the part that needs equipment. Rhizopus wants around 30 to 32 degrees Celsius, held for 24 to 48 hours. An oven with only the light on, a proofing box, a seedling heat mat inside a closed cooler box, or a dehydrator with adjustable temperature all work. Whatever you use, put a thermometer next to the tempeh rather than trusting a dial. The important detail is that the tempeh generates its own heat once it gets going, sometimes several degrees above the ambient temperature, so around the halfway point you may need to reduce the external heat to avoid cooking your own culture.
Watch the progress rather than the clock. For the first ten hours or so nothing much appears to happen. Then a faint white fuzz spreads between the beans, and within a few more hours the cake becomes visibly knitted. Finished tempeh is firm enough to lift as one piece, densely covered in white mycelium, and smells pleasantly of mushrooms and nuts. Grey or black patches of sporulation, often around the perforations, are the mould maturing and are not harmful, though most people stop the ferment before that stage for a milder flavour.
Refrigerate the finished cake as soon as it is ready, which halts growth. It keeps for about a week refrigerated and freezes well, so a large batch is worth the trouble of setting up an incubator. Signs to reject a batch are a strong ammonia smell, slimy wet patches, or bright coloured moulds in blue, green or pink. Those usually trace back to beans that were too wet, an incubator that ran too hot, or skipping the acidification, and each of those is straightforward to correct on the next attempt.