Tempeh Production — Rhizopus oligosporus Mould Incubation

Tempeh originates on the island of Java, Indonesia, with documented production going back at least to the early nineteenth century, likely longer in village practice. It spread through the Indonesian archipelago as a protein staple and entered Western food science consciousness seriously only in the latter half of the twentieth century.

Tempeh is made by encouraging Rhizopus oligosporus — a filamentous mould — to colonise cooked, dehulled soybeans, binding them into a firm, sliceable cake through dense mycelial growth. The craft sits at the intersection of substrate preparation, precise humidity control, and temperature management over a 28-to-48-hour window. Get any of those wrong and you get either a stalled ferment or an overheated, ammonia-reeking brick. Start with dried soybeans soaked 8-12 hours, hulls removed by hand-rubbing and flotation, then simmered until just tender but not falling apart — you need structural integrity for the mould to lace through. Drain aggressively and dry the surface moisture before inoculation. Wet beans are the enemy: excess surface water suffocates mycelial growth and invites bacterial contamination. Once cooled to below 35°C, mix in Rhizopus oligosporus spore powder — typically 1-2 g per kilogram of cooked beans — ensuring even distribution throughout the mass. Pack into perforated bags or trays with 2mm holes punched every 2-3cm. Hole spacing matters: the mould is aerobic and generates substantial metabolic heat as it grows. Without gas exchange, CO2 builds, oxygen depletes, and the ferment stalls or shifts toward undesirable organisms. Incubate at 30-32°C. After 16-20 hours you'll see white mycelial fuzz beginning. At 24-30 hours, metabolic heat from the mould's own respiration can push the interior temperature 5-8°C above ambient — monitor with a probe and ventilate or reduce chamber temperature accordingly. The CIA's The Professional Chef and Redzepi and Zilber's The Noma Guide to Fermentation both flag this runaway exotherm as the single most common production failure in kitchen environments. At 36-48 hours, a well-incubated cake is chalk-white, firm enough to slice cleanly, and smells of mushrooms and warm grain — faintly nutty, never ammoniated. Stop the ferment by refrigerating immediately or cooking. Overrun tempeh goes grey-black as sporulation begins and develops bitter, astringent off-notes. The window between perfect and over-fermented is narrow.

Rhizopus oligosporus secretes lipases and proteases throughout incubation. The proteases partially hydrolyse soy protein into shorter peptides and free amino acids, generating umami depth and reducing the beany, metallic flavour compounds associated with raw soy. Lipase activity on soybean oils produces fatty acids including linoleic and oleic, contributing richness. Simultaneously, the mould synthesises vitamin B12 precursors and reduces antinutritional factors such as phytic acid, improving mineral bioavailability. When cooked — pan-fried, roasted, or grilled — these free amino acids and reducing sugars undergo Maillard reaction readily, producing the deep, roasted, nutty crust that makes tempeh compelling as a standalone preparation rather than merely a protein substitute.