Enzymatic Hydrolysis of Starch
The enzymatic hydrolysis of starch is carried out under milder conditions: lower temperatures (up to 1000C), normal pressure, pH of the medium around 6 – 8. At the same time enzymatic hydrolysis is characterized by high reaction rate, high stability of the enzyme towards the denaturizing action of solvents, detergents, proteolytic enzymes, and a decrease in the viscosity of the reaction medium at higher temperatures, etc. Most often, enzymatic hydrolysis is carried out with the enzyme ? -amylase from different sources and less often ? amylase is employed. The bacterial ? -amylase enzymes attack the polysaccharide molecules in the inner part of the chain. They act on the amylose of starch so that they destroy the spiral of the polysaccharide chain and thus the characteristic blue color with iodine disappears. The viscosity of the starch solutions is quickly lowered. In the beginning dextrines are obtained, and if the enzyme acts continuously, maltose accumulates, in which one of the molecules of glucose has a free glucoside group and hence reducing properties. Kolusheva, 2007) The basic parameters which affect the hydrolysis process are temperature, pH of the medium, concentration of the substrate and concentration of the enzyme usually vary depending on the source of the enzyme. Most often the hydrolysis with thermally resistant – amylase is carried out at temperature 90–1000C, concentration of the substrate in the suspensions varying from 20% to 35%, pH between 6 and 8,and enzyme concentration 0. 03 – 1%.
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Three to four enzymes are used for the industrial hydrolysis of starch to D-Glucose. ?-Amylase is an endo-enzyme that cleaves both amylose and amylopectin molecules internally, producing oligosaccharides. The larger oligosaccharides may be singly, doubly, or triply branched via (1-6) linkages, since ? -amylase acts only on the (1-4) linkages of starch. ?-Amylase does not attack double-helical starch polymer segments or polymer segments complexed with a polar lipid (stabilized single helical segments). Fennema, 1996) Glucoamylase (amyloglucosidase) is used commercially, in combination with an a-amylase, for producing D-glucose (dextrose) syrups and crystalline D-glucose. The enzyme acts upon fully gelatinized starch as an exo-enzyme, sequentially releasing single D-glucosyl units from the non-reducing ends of amylose and amloypectin molecules, even those joined through (1-6) bonds. Consequently, the enzyme can completely hydrolyze starch to D-glucose, but is used on starch that has been previously depolymerized with a-amylase to enerate small fragments and more nonreducing ends. ?-Amylase releases the disaccharide maltose sequentially from the nonreducing end of amylose. It also attacks the non-reducing ends of amylopectin, sequentially releasing maltose, but it cannot cleave the (1-6) linkages at branch points, so it leaves a pruned amylopectin residue termed a limit dextrin, specifically a beta-limit dextrin. There are several debranching enzymes that specifically catalyze hydrolysis of (1-6) linkages in amylopectin, producing numerous linear but low-molecular-weight molecules.
One such enzyme is isoamylase; another is pullulanase. (Fennema, 1996) Fennema, Owen R.. Food Chemistry: 3rd edition. Marcel Dekker, 1996. Bemiller, James N. & Whistler, Roy L. , Starch: Chemistry and Technology. Academic Press, 2009. Kolusheva, T. & Marinova, A.. A Study of the optimal conditions for starch Hydrolysis through thermostable ? -Amylase. Journal of the University of Chemical Technology and Metallurgy, 42, 1, 2007, 93-96.