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Starch Hydrolysis with Amylase

The products of bacterial and fungal a-amylases are in the a-configuration and the products of b-amylases are in the b-configuration, although all these enzymes cleave between a-1,4-linked glucose residues.

The a-amylases (1,4-a-D-glucan glucanohydrolases) are endohydrolases which cleave 1,4-a-D-glucosidic bonds and can bypass but cannot hydrolyse 1,6-a-D-glucosidic branchpoints. Commercial enzymes used for the industrial hydrolysis of starch are produced by Bacillus amyloliquefaciens (supplied by various manufacturers) and by B. licheniformis (supplied by Novo Industri A/S as Termamyl). They differ principally in their tolerance of high temperatures, Termamyl retaining more activity at up to 110°C, in the presence of starch, than the B. amyloliquefaciens a-amylase. The maximum DE obtainable using bacterial a-amylases is around 40 but prolonged treatment leads to the formation of maltulose (4-a-D-glucopyranosyl-D-fructose), which is resistant to hydrolysis by glucoamylase and a-amylases. DE values of 8-12 are used in most commercial processes where further saccharification is to occur. The principal requirement for liquefaction to this extent is to reduce the viscosity of the gelatinised starch to ease subsequent processing.

Various manufacturers use different approaches to starch liquefaction using a-amylases but the principles are the same. Granular starch is slurried at 30-40% (w/w) with cold water, at pH 6.0-6.5, containing 20-80 ppm Ca2+ (which stabilises and activates the enzyme) and the enzyme is added (via a metering pump). The a-amylase is usually supplied at high activities so that the enzyme dose is 0.5-0.6 kg tonne-1 (about 1500 U kg-1 dry matter) of starch. When Termamyl is used, the slurry of starch plus enzyme is pumped continuously through a jet cooker, which is heated to 105°C using live steam. Gelatinisation occurs very rapidly and the enzymic activity, combined with the significant shear forces, begins the hydrolysis. The residence time in the jet cooker is very brief. The partly gelatinised starch is passed into a series of holding tubes maintained at 100-105°C and held for 5 min to complete the gelatinisation process. Hydrolysis to the required DE is completed in holding tanks at 90-100°C for 1 to 2 h. These tanks contain baffles to discourage backmixing. Similar processes may be used with B. amyloliquefaciens a-amylase but the maximum temperature of 95°C must not be exceeded. This has the drawback that a final 'cooking' stage must be introduced when the required DE has been attained in order to gelatinise the recalcitrant starch grains present in some types of starch which would otherwise cause cloudiness in solutions of the final product.

The liquefied starch is usually saccharified but comparatively small amounts are spray-dried for sale as 'maltodextrins' to the food industry mainly for use as bulking agents and in baby food. In this case, residual enzymic activity may be destroyed by lowering the pH towards the end of the heating period.

Fungal a-amylase also finds use in the baking industry. It often needs to be added to bread-making flours to promote adequate gas production and starch modification during fermentation. This has become necessary since the introduction of combine harvesters. They reduce the time between cutting and threshing of the wheat, which previously was sufficient to allow a limited sprouting so increasing the amounts of endogenous enzymes. The fungal enzymes are used rather than those from bacteria as their action is easier to control due to their relative heat lability, denaturing rapidly during baking.

Note:Please look at the page of ‘’The Enzymes That Used In  Starch Industry’’