Rhizopus oligosporus

Rhizopus oligosporus is the domesticated mold behind tempeh, the Indonesian solid-cake soybean ferment. Unlike Aspergillus oryzae (which produces enzymes that prepare a substrate for downstream organisms), Rhizopus itself is the final fermenter — its mycelium grows through and between split soybeans, physically binding them into the firm white cake that tempeh is known for. The mold's structural role (mycelium as cement) is what makes tempeh sliceable, fryable, and meat-like in a way that paste-based soybean ferments cannot be.

The fermentation is fast and forgiving compared to koji. R. oligosporus spores inoculated onto cooked, partially-dried, split soybeans at 30-32°C in a perforated bag or banana leaf wrap colonize the substrate visibly within 12-18 hours and fully bind the cake by 24-48 hours. The mold tolerates wider conditions than A. oryzae — warmer temperatures (up to 35°C without harm), more variable humidity, and less precise sanitation requirements. This forgiveness makes tempeh substantially more home-accessible than koji.

The biochemical effects on the substrate are substantial. R. oligosporus secretes proteases that break down soybean protein into more digestible peptides; lipases that work on soybean oil; and phytase that reduces phytic acid (an antinutrient that impedes mineral absorption) by 30-60% during fermentation. The mold also produces antibacterial compounds that suppress competing organisms during the warm aerobic incubation. The vitamin B12 claim sometimes made for tempeh is disputed — R. oligosporus itself doesn't produce B12, but Klebsiella and Citrobacter bacteria that often co-colonize tempeh in traditional fermentations can produce small amounts. Reliable dietary B12 from tempeh should not be assumed.

Beyond pure tempeh, Rhizopus species contribute to several other Asian ferments. In Korean *meju (the dried-soybean precursor to doenjang and gochujang), wild Rhizopus oligosporus is one of the surface molds that colonize the cooked-soybean bricks alongside Aspergillus and Bacillus. In Chinese furu (fermented tofu), Rhizopus oryzae or Actinomucor (a related zygomycete) drive the surface inoculation that precedes the brining stage. These cross-tradition appearances are documented in the encyclopedia's reverse-linking — R. oligosporus*'s page surfaces tempeh + doenjang + furu as the three connected uses.

The other Rhizopus species — R. oryzae and R. arrhizus — are used variously in tempeh production and in Chinese fermented-tofu traditions; the differences between them are subtle but matter for specific traditional outcomes. Commercial tempeh starter from suppliers like GEM Cultures or Cultures for Health is typically labeled as R. oligosporus but in practice may include strain mixtures.