|Rhizopus oligosporus on homemade tempeh|
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 at high temperatures (30-40 °C) which are typical of the Indonesian islands, it exhibits strong lipolytic and proteolytic activity that create desirable properties in tempeh and it produces metabolites that allows 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 and its proper taxonomic position is thus 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.
Rhizopus oligosporus is a fungus that belongs to the class Zygomycetes, which is one of two classes in the phylum Zygomycota. 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 microscopus 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 um) and irregular spores with the most variable sizes. This is, for instance, reflected as high values in the spore volume (96–223 mm3/spore). 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.
Role in tempeh fermentation
A popular Indonesian food, tempeh, is created by fermenting soybeans in combination with Rhizopus oligosporus. In order to create tempeh, soybeans must first be soaked in water (usually overnight) at a temperature similar to the environment it is placed in. 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, spores with a tendency for fast germinability are needed, as well. In order for the tempeh to attain its characteristic compact 'cake' form after fermentation, the soybeans become compressed due to the mycelia of Rhizopus oligosporus. Rapidly growing mycelia helps speed up the growth of this fungus. Because mycelia are quite sensitive to dehydration and adverse temperatures, preserving tempeh for extended periods of time can be challenging. When the soybeans are bound together by the white mycelium, the fungus releases enzymes that make this heavily protein-rich product more digestible for humans. Tempeh-like foods (black oncom) can also be created from cereal grains such as wheat and rice. Many times, a good inoculum for this new fermentation actually comes from tiny pieces of old tempeh that have already been fermented.
Uses of tempeh
Tempeh has the potential to be used in many high-protein foods due to its mild flavor when fried in vegetable oil. Containing more than 40% protein, tempeh is often used as a meat-substitute. This product is used in soups or can simply be sliced and seasoned.
Even after it is consumed, Rhizopus oligosporus produces an antibiotic that limits gram-positive bacteria like Staphylococcus aureus (potentially harmful) and Bacillus subtilis (beneficial). Thus, people who eat tempeh tend to have fewer intestinal infections. Tempeh contains ergosterol (provitamin D2). Beneficial effects of tempeh include inhibiting tumor development, lowering cholesterol and decreasing diarrhea issues, iron-deficient anaemia, lipid oxidation and hypertension. This fungus can also treat waste and wastewater, produce industrial enzymes and ferment other substrates like other legumes and cereals.
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