For starch and glycogen hydrolysis, two major classes of amylases have been identified in the microorganisms: a-amylase and glucoamylase. In addition, ^-amylase, which is generally of plant origin, has also been reported from a few microbial sources. a-Amylase (endo-a-1,4-D-glucan glucohydrolase, EC- is an extracellular enzyme, which randomly cleaves the 1,4-a-D-glucosidic linkages in the interior of the amylose chain. Amylases which produce free sugars are termed as saccharogenic amylases, and those which liquefy starch without producing free sugars are known as starch-liquefying amylases. ^-Amylase (a-1,4-glucan maltohydrolase, EC- is an exoacting enzyme, which cleaves the non-reducing chain ends of amylose, amylopectin, and glycogen molecules, yielding maltose. Glucoamylase [also known as amyloglucosidase, glucogenic enzyme, starch glucogenase, and gamma amylase (exo-a-1,4-D-glucan glucanohydrolase, EC-] produces single glucose units from the nonreducing ends of amylose and amylopectin in a stepwise manner. Unlike a- and ^-amylase, most glucoamylases (GA) are able to hydrolyse the a-1,6 linkages at the branching points of amylopectin, although at a slower rate than 1,4-linkages. Thus, glucose, maltose, and limit dextrins are the end products of GA starch hydrolysis.

Extensive studies have been done on the production of amylases, in particular for a-amylase and GA in SSF, employing several microorganisms and various kinds of agro-industrial residues, among which starchy substrates have been preferred (Table 4.1) (5,7,12,13,28,32,38,39-50). Reports on microbial production of ^-amylase are scanty and very few in SSF. A mutant strain of Bacillus megaterium, B6 mutant UN12, was described for the comparative production of beta amylase in SmF and SSF. The starchy wastes used as substrates were from arrowroot, arum, maize, potato, pulse, rice, rice husk, tamarind

Table 4.1

SSF processes for production of food enzymes



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