Rhizopus mycelium colonizes soybean surfaces over one to two days
Tempeh is created by inoculating cooked soybeans with Rhizopus oligosporus (or related Rhizopus species), a mold that rapidly grows through the beans. Over 24-48 hours at warm temperature (29-32C), the mold's thread-like mycelium spreads across and between beans, binding them into a solid cake. The growth is aggressive: the mold colonizes the surface first, then gradually penetrates deeper until the entire bean mass is held together by white mycelium.
The transformation is visible: pale, separate beans gradually fuse into a cohesive block as mycelium density increases. A fully matured tempeh has mycelium so thick that individual beans are barely distinguishable. The cake is firm enough to slice, fry, or crumble.
Temperature and moisture control final texture
Warmer incubation accelerates colonization: 32C achieves full coverage in 24 hours, while 26C may take 36-48 hours. Too cool (below 24C) and coverage stalls; too warm (above 35C) and the mold struggles to grow. Humidity matters: the mold needs moisture but not standing water. Excess moisture promotes bacterial growth and slime; inadequate moisture slows mycelium spread. Properly made tempeh has a nutty aroma and a firm, sliceable texture. Immature tempeh (undericolonized) falls apart when cooked. Overgrown tempeh develops darker pigmentation (sporulation) and a stronger, sometimes bitter flavor.