Enzymes

Enzymes are organic catalysts. Long before they were purified by scientists, their properties had been used in baking, brewing, tanning, and other processes. Because of their high specificity, their biodegradability, and their ability to work in watery solutions at moderate temperatures, enzymes are becoming increasingly important in today's environmentally conscious industries. In many processes, enzymes have replaced the strong chemicals and high temperatures formerly used in commercial catalysis.

The major industrial enzymes from fungi are hydrolytic, and the major producing species are A. oryzae and A. niger. Amylases and glucoamylases are used for turning starches into sugars and oligosaccharides. Pectinases are applied in fruit juice clarification. Aspergillus proteases are used in bread making and in chill-proofing beer, and a thermostable phytase is used widely in the animal feed industry as an additive. Representative commercial enzymes produced by Aspergillus are listed in Table 3.

With the advent of modern molecular techniques, the genes for many native Aspergillus enzymes have been cloned, sequenced, amplified, and engineered to improve yields, stability, and other properties (24,25). Genetic manipulation also has allowed Aspergillus to become the host for the production of various nonnative (foreign) proteins, the best known of which is chymosin. Rennet is a substance extracted from the stomachs of slaughtered calves that is used in cheese making. The principle active ingredient of rennet is chymosin, a protease that coagulates milk. Structural genes for the calf stomach enzyme can be transferred to fungi. The mammalian enzyme is then expressed and secreted by the fungus in fermentation broths. The yeast Kluyveromyces lactis and A. niger have both been developed as production hosts for chymosin production. Given the economic advantages of fungal fermentations, it can be

Table 3. Commercially Important Aspergillus Enzymes

Enzyme

Applications

Amylases Hydrolysis of starch; bread and beer production;

removal of sizing from fabrics; high-fructose syrups

Invertase Confections; soft center in chocolates

Lactase Hydrolysis of lactose; production of syrups for sweetening agents Pectinases Pretreatment of fruit juices to remove turbidity, reduction cloudiness in wines Phytase Animal feed additive for liberation of phosphate from plant material Proteases Meat tenderizing; removal of bitter flavors and chill proofing of beer; reduction elasticity of gluten proteins in bread predicted that Aspergillus will be the production host of choice for many other nonfungal enzymes that traditionally have been extracted from higher plants and animals.

Descriptions of the use of enzymes in fermentation, diagnostics, baking, brewing, cheese manufacturing, chemical biotransformation, detergents, effluent and waste treatment, leather, textile, starch conversion, and the like are presented in Godfrey and West's Industrial Enzymol-ogy (26). This work also gives detailed information on the legislation and regulation of commercial enzymes, toxico-logical and regulatory considerations, and the scientific and regulatory aspects surrounding the use of genetically engineered production strains (26). The long history of safe use and the GRAS status of A. oryzae and A. niger will ensure that they will remain the species of choice for industrial enzyme production, especially in the food industry.

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