Food Production

Foods fermented with microorganisms have interesting flavors and improved digestibility. In Western culture, the majority of food fermentations involve yeasts (e.g., bread and wine) or bacteria (e.g., cheese and pickles). Aspergilli play only minor roles in Western cuisine (e.g., ripening salamis and country-cured hams). This is in sharp contrast to Asian food traditions, where filamentous fungi have been used for millennia, especially in complex two-stage fermentations. The best known of these processes include shoyu (soy sauce), sake (rice wine), and miso (fermented soybean paste). The initial step in each case is the production of a koji, a Japanese word that roughly translates as "bloom of mold." To produce a koji, Aspergillus oryzae or the closely related species A. sojae is grown on steamed rice or other cereal and incubated in warm, humid conditions. The hyphae infiltrate the grain, secreting hydrolytic enzymes that partially degrade the substrate, producing the koji, a fragrant, crumbly mass of mold and substrate. Perhaps because of the paucity of Western fungal fermentations, the English language does not have a word that similarly denotes a substance thus composed of living my-celia, extracellular enzymes, and partially degraded substrate. This mold-substrate mixture has different proper-

Table 2. Economically Useful Low Molecular Weight Products of Aspergillus

Product

Producing species

Organic Acids Citric acid

A. niger

Gluconic acid A. niger

Itaconic acid A. terreus

Pharmacologically active secondary metabolites Asperillomarasmine A. oryzae

Asperlicins A. alliaceus

Lovastatin A. terreus

Echinocandin A. nidulans/

A. rugulovalvus

Flavoring agency, antioxidant, chelating and cleaning agent, component of effervescent powders Manufacturing of toothphaste Copolymer in resin manufacture

Angiotensin-converting enzyme inhibitor Cholecystokinin antagonists Cholesterol inhibitor Antifungal agent ties than either alone and constitutes a form of matter not captured by the term solid-state fermentation.

Kojis are classified according to the degree of mycelial development and sporulation, their intended use, and the cereal substrate. Kojis are usually used as inocula for further fermentations. For soy sauce, the koji is mixed with soybeans and wheat. After fungal amylases and proteases have partially degraded the mixture, a salt solution is added. An anaerobic yeast-lactobacilli fermentation ensues; the final pressed liquid is clarified as soy sauce. For sake, a koji inoculum is mixed with steamed rice and water and incubated for 3 to 4 days in the moto stage. As the fungal enzymes partially saccharify the rice, a predictable change in the microbial flora takes place, with yeasts becoming the predominant microorganisms, to yield the moroni stage. After appropriate incubation, yeast metabolic activities yield alcohol. Miso is a fermented paste with the consistency of peanut butter, often made in Japanese homes. Widely used as a soup base, miso varies according to substrate (rice, barley, soybeans, mixtures), length of fermentation, amount of salt, etc. Similar products, based solely on fermented soybeans, are known as chiang in China, jang in Korea, tao-jo in Indonesian and Thailand, and tao-tsi in the Philippines (22,23).

Aspergillus oryzae and A. sojae are close relatives of the toxigenic species A. flavus and A. parasiticus. These domesticated species do not produce toxins; A. oryzae is on the GRAS list (Generally Regarded as Safe) by the U.S. Food and Drug Administration.

The first industrial enzyme patented in the United States was an amylase mixture purified from a koji by the early Japanese-American biotechnologist Jokichi Taka-mine in 1894 (7). Contemporary scientists have subsequently identified more than 50 different enzymes from kojL

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