Never eat a wild mushroom identified only from a website, a book or an app. Have every find checked in person by an expert, such as a mycological society.
Identification
Rhizopus oligosporus is a mould of the order Mucorales. Its hyphae are broad and largely without cross-walls, and they grow extremely fast, covering a substrate within a day. Root-like rhizoids anchor the mycelium, and short sporangiophores carry small, round, grey to black sporangia containing many sporangiospores. On tempeh, the mycelium should be dense and white; scattered grey or black spots are sporangia forming at the edges or where air reaches, and are normal in small amounts.
Taxonomically, many authors treat it as Rhizopus microsporus var. oligosporus, a domesticated variety of the widespread species R. microsporus. Strains used for tempeh have been selected for good growth on soybeans and low sporulation.
Habitat and ecology
The tempeh mould is known mainly from food fermentation. Traditional starters in Indonesia were made by growing the mould on leaves of hibiscus or teak or on rice, and today pure spore cultures are produced commercially. Its wild relative Rhizopus microsporus is found in soil, on plant material and in decaying foods in warm regions worldwide. Rhizopus species tolerate high temperatures, grow rapidly on starchy and protein-rich materials and are common contaminants of bread and fruit.
Uses and importance
Tempeh originated on Java, where it has been made for centuries. During fermentation the mould’s enzymes break down part of the protein and fat of the beans, the mycelium knits them into a sliceable cake, and the flavour becomes mild, nutty and mushroom-like. Tempeh is now produced worldwide and is valued as a protein-rich plant food.
Beyond soybeans, tempeh-style fermentations are made with chickpeas, lentils, fava beans, lupins, grains and food by-products such as okara, which is an active area of food research. Rhizopus enzymes are also studied in food biotechnology.
Cultivation
To make tempeh, soybeans are soaked, dehulled, cooked and often slightly acidified with vinegar, which helps to suppress unwanted bacteria. After drying and cooling to below about 35 °C, the beans are mixed with tempeh starter and packed into perforated bags or trays. Incubation at about 30–32 °C lasts roughly 24–48 hours. The mould generates heat as it grows, so ventilation matters. Finished tempeh is firm, white and smells fresh and slightly yeasty; it is refrigerated or frozen promptly.
Safety and edibility
Tempeh made with commercial food-grade starters and good hygiene has a long record of safe consumption. It is usually cooked (fried, steamed or baked) before eating. Discard tempeh that is slimy, smells of ammonia or rot, or shows pink, yellow, green or other unusual colours, which indicate contamination by bacteria or other moulds. Never use spontaneous mould growth as a starter.
Wild strains of Rhizopus microsporus can cause mucormycosis in severely immunocompromised people, so laboratory work with Rhizopus requires appropriate biosafety practice. Serious poisonings have been associated with a different coconut-based product, tempe bongkrek, when it became contaminated with toxin-producing bacteria; this is not linked to normal soybean tempeh made under hygienic conditions. People with mould allergy or soy allergy should take care.
If you suspect mushroom poisoning, call your local emergency number or poison centre straight away, even before any symptoms appear, and keep the remains of the mushrooms.
Look-alikes
In the kitchen, the main confusion is with other fast-growing moulds. Rhizopus stolonifer, the black bread mould, grows on bread and fruit and forms larger black sporangia; it is not a tempeh culture. Green, blue or yellow moulds such as Penicillium or Aspergillus species on tempeh are contaminants. Some traditional starters also contain Rhizopus oryzae (R. arrhizus).
Around the world
Worldwide, Indonesia remains the centre of tempeh production and consumption, with many regional variants. Outside Indonesia, tempeh is made in the Netherlands, other European countries, North America and Australia, often by small producers using imported or locally produced starters.