Rhizopus oligosporus

Species of fungus

Rhizopus oligosporus is a fungus of the family Mucoraceae and is a widely used starter culture for the production of tempeh at home and industrially. As the mold grows it produces fluffy, white mycelia, binding the beans together to create an edible "cake" of partly catabolized soybeans. The domestication of the microbe is thought to have occurred in Indonesia several centuries ago. R. oligosporus is the preferred starter culture for tempeh production for several reasons. It grows effectively in the warm temperatures which are typical of the Indonesian islands; it exhibits strong lipolytic and proteolytic activity, creating desirable properties in tempeh; and it produces metabolites that allow it to inhibit and thus outcompete other molds and gram positive bacteria, including the potentially harmful Aspergillus flavus and Staphylococcus aureus. R. oligosporus is at present considered to be a domesticated form of Rhizopus microsporus, resulting in a synonym of Rhizopus microsporus var. oligosporus. R. microsporus produces several potentially toxic metabolites, rhizoxin and rhizonins A and B, but it appears the domestication and mutation of the R. oligosporus genome has led to the loss of genetic material responsible for toxin production. The synonym is not currently recognized in fungal taxonomy, so its current taxonomic position is best described as a member of the R. microsporus species group. Rhizopus oligosporus belongs to the Rhizopus microsporus group. This group is made of taxa with similar morphology that are associated with undesired metabolite production, pathogenesis and food fermentation. Although other varieties in Rhizopus microsporus may be harmful, Rhizopus oligosporus is not associated with production of potentially harmful metabolites. It is not found in nature and is frequently used by humans. Rhizopus oligosporus strains have a large diameter (up to 43 μm) and irregular spores with widely varying volume, (typically in the range 96–223 mm3). Rhizopus oligosporus has large, subglobose to globose spores, and high proportion irregular spores (>10 %). Rhizopus oligosporus also has spores with nonparallel valleys and ridges, and plateaus that sometimes are granular. It also produces an antifungal in the form of a chitinase protein.

Tempeh, a popular Indonesian food, is created by fermenting soybeans with Rhizopus oligosporus. To create tempeh, soybeans first must be soaked in water (usually overnight) at a temperature similar to the environment in which it is placed. The soybean's outer covering is then removed, and the beans are partially cooked. Lactic acid bacteria, like Lactococcus and Lb. casei species, play a major role in the fermentation of tempeh. For the tempeh to ferment, there needs to be a suitable, pure inoculum. Also needed are spores that germinate quickly.