Which mold is used to ferment soybeans into tempeh?

The mold used to ferment soybeans into tempeh is Rhizopus oligosporus. Its rapidly growing white mycelium binds cooked soybeans into tempeh's characteristic firm, sliceable cake.

To make tempeh, soybeans are generally soaked, dehulled, partially cooked, cooled, and inoculated with a starter culture. Rhizopus oligosporus grows through the beans and releases enzymes that break down proteins and fats, changing the beans' texture, aroma, digestibility, and flavor. Lactic-acid bacteria may also contribute to the fermentation.

Tempeh is traditionally associated with Indonesia, where the domestication of the fungus is thought to have occurred centuries ago. The mold is well suited to the warm conditions common in the Indonesian archipelago and grows quickly enough to outcompete many other microorganisms. It can also ferment some grains and legumes, although soybean tempeh is the familiar form.

A taxonomic wrinkle causes occasional confusion: many modern databases place this domesticated tempeh organism within the Rhizopus microsporus group, sometimes as Rhizopus microsporus var. oligosporus. Rhizopus oligosporus remains the standard culinary and quiz answer.