Rhizopus oryzae

Rhizopus oryzae (also called R. oryzae) is a group of filamentous fungi in the family Rhizopodaceae in the order Mucorales. Fungi in this order are commonly called “pin molds” because of their appearance, which resembles a long pin with a round head. Like other filamentous fungi, it secretes enzymes.

R. oryzae, which was first identified in 1895, can cause infections in humans and animals. It is responsible for about 70 percent of cases of mucormycosis, a rare fungal infection that can be fatal to individuals with reduced immunity. It is commonly found in the environment, and its spores can be inhaled, leading to sinus or lung problems. It can also infect injured skin.

R. oryzae is commonly used in industry to produce cortisone and lactic acid, biosorption, and alcoholic fermentation; to treat wastewater; and to create biofuel. R. oryzae is one of several Rhizopus species used to create foods such as tempeh that are traditionally made in Asia.

Background

Fungi of genus Rhizopus form rhizoids, which are branching filaments that grow downward and anchor the fungus in much the same way roots anchor plants in soil. Rhizoids release digestive enzymes and absorb digested organic substances. While found on a variety of organic matter, including fruits and soils, rhizoids primarily use glucose, a simple sugar, as their carbon source. They have also been found to use xylan, cellulose, sucrose, and other sugars as carbon sources. Urea is the preferred nitrogen source. R. oryzae is commonly used in solid-state fermentation (SSF), which uses culture substrates or bases that have low moisture levels.

Use of R. oryzae and other fungi in fermentation of foods dates to about two thousand to three thousand years ago, when it was known in China as koji, meaning “moldy grains.” The practice of inoculating grain with mold traveled to Japan during the Yayoi period (300 BCE–300 CE). It is an important step in the fermentation process in Asia.

Koji can refer to both the mold and the grain it is growing on. The fungus consumes the starch in the grain to produce enzymes. Several types of fungi and many varieties of grain are used to produce a variety of foods and beverages. For example, R. oryzae is one of the fungi used to make tempeh, a fermented food made of soybeans or cereal grains that originated in Indonesia. Some soybean-based fermentation, such as soybean sauce, uses a mixture of several fungi to develop a range of flavors and characteristics. Fermentation changes the available nutrients in soybeans by increasing the protein content, releasing vitamins, and developing antioxidants. Foods fermented with R. oryzae develop a velvety and juicy texture that many consumers prefer.

Industrial applications of R. oryzae developed after the discovery in 1936 that the fungus produced significant quantities of lactic acid from glucose. Its tolerance of a wide range of conditions and substrates meant that it could be readily used in industry. Two strains of R. oryzae have been identified and are divided by the primary organic acid they produce when grown on D-glucose, or dextrose. Both groups have as the primary fermentation product L-(+)-lactic acid. The other group also produces fumaric acid.

Overview

Fungi are capable of producing enzymes that degrade the cell walls of plants. Some enzymes also change the chemical composition of materials. Fermentation using fungi can make foods more digestible and increase the availability of proteins. R. oryzae is generally recognized as safe (GRAS) by the U.S. Food and Drug Administration and is often used in food production. It is one of the species used to make textured meat substitutes from substrates including beans and cereals; the texture is produced by the mycelium binding together particles.

Another application of the fungus is in developing new food sources from agricultural waste. By-products such as brewers’ spent grain (BSG) and waste such as stale bread can be converted to new edible products that provide significant protein. For example, Gmoser et al. found that the protein content of stale sourdough bread fermented with R. oryzae and Neurospora intermedia, another edible filamentous fungi, increased from 16.5 percent to 21.1 percent. The resulting product also contained greater quantities of essential amino acids, dietary fiber, and several nutrients. Combining the bread and some BSG produced products with a more palatable texture, similar to commercial soybean burgers.

R. oryzae is able to use sugars from biomass to produce large quantities of ethanol, fumaric acid, and lactic acid, which are known as sustainable platform chemicals in industrial applications. Platform chemicals are necessary for making many chemicals and materials. The demand for ethanol has increased consistently in the twenty-first century due to its many uses. It is used as an additive to automotive gasoline, a solvent, and an industrial chemical, for example. Fumaric acid is used in the food industry as a flavor enhancer, a preservative, and a pH adjustor. It can also be used in producing materials such as polyester. Lactic acid is added to foods and animal feed but also is used in manufacturing renewable plastics and other products.

Biodiesel has become increasingly important as a clean energy source. Production has mostly used edible oils to create first-generation biodiesel. Governments and organizations have been moving away from using edible oils to make fuel for ethical reasons. Second- and third-generation biodiesels are produced using non-edible oils such as waste cooking oil and microbial oils, which contain high levels of free fatty acids (FFA). FFAs form soap in the process used in biodiesel plants, so they must be neutralized before the oils can be transformed. Researchers have been studying the use of R. oryzae to produce enzymes that efficiently neutralize FFAs in the production of second- and third-generation biodiesels. Researchers have also looked at the effects of fermentation on biomass, or organic material used as fuel, to find alternative transportation fuels. Because fungi such as R. oryzae can use multiple carbon sources, they could be useful in developing these fuels.

Bibliography

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