Tempeh Production — Rhizopus oligosporus Mould Incubation

Overview

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.

Flavour Context

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.

Key Principles

Remove surface moisture from cooked beans completely before inoculation — residual water drives bacterial competition and suppresses mycelial colonisation.

Maintain incubation temperature at 30-32°C ambient; probe bean mass directly and ventilate when internal temperature exceeds 38°C.

Space perforation holes at 2-3cm intervals across all bag or tray surfaces — aerobic respiration demands continuous oxygen supply through the full depth of the substrate.

Inoculate at or below 35°C; heat above this threshold kills spores and stalls germination before it starts.

Stop fermentation decisively at peak white mycelial coverage — refrigerate, freeze, or cook immediately to arrest the process before sporulation blackens the cake.

Distribute spore inoculant uniformly through the substrate; uneven mixing produces patchy colonisation with raw-bean gaps and overheated dense zones.

Pro Tips

{"After draining, spread cooked beans on sheet trays and run a fan over them for 10-15 minutes — this drives off surface moisture faster and more evenly than towel-drying, reducing contamination risk in high-volume production.","Use a digital data logger inside the incubation chamber rather than relying on ambient thermostat readings; place a second probe directly inside one bag to track internal bean-mass temperature separately from chamber air.","For high-ratio batches, interleave layers of perforated trays with a wire rack gap of at least 2cm to allow convective airflow across all surfaces — this prevents hot-spot formation between stacked trays.","Partially frozen tempeh slices cleanly on a deli slicer and develops a more pronounced Maillard crust when pan-fried than room-temperature cake — chill to -5°C for 20 minutes before portioning."}

Common Mistakes

Insufficient drying before inoculation: surface moisture creates anaerobic micro-pockets where lactic acid bacteria or Enterobacteriaceae outcompete Rhizopus, yielding a sour, slimy, poorly bound mass instead of firm cake.

Under-ventilated incubation: CO2 accumulation without adequate perforation or airflow causes the mould to produce grey-green sporangia prematurely, making the tempeh bitter and visually unacceptable.

Failing to account for metabolic heat: ambient temperature set at 30°C while the bean mass climbs to 40°C-plus cooks and kills the mycelium from the inside, leaving a dense, mealy core with only superficial white crust.

Harvesting too late: tempeh left past 48 hours at warm temperatures sporulates rapidly — grey-black pigmentation, ammoniated aroma, and a chalky, astringent mouthfeel that no preparation technique corrects.

Source

Redzepi/Zilber — The Noma Guide to Fermentation (2018)

Cross-Cuisine Parallels

• Koji incubation (Aspergillus oryzae on rice, barley, or soy) — similarly aerobic, exothermic filamentous mould colonisation requiring humidity and temperature management; produces protease and amylase rather than lipase as primary enzymatic output
• Camembert and Brie surface mould ripening (Penicillium camemberti) — another aerobic white filamentous mould that colonises a substrate surface while generating metabolic heat; similar harvest-window precision required before overripening
• Indonesian oncom (Neurospora intermedia on peanut or soy press-cake) — structural analogue to tempeh using a different mould species, producing vivid orange pigmentation and a more intense, earthy flavour profile