Starch-hydrolyzing enzymes

FIGURE 2,44 Carbohydrafe-hydnolyzifig enzymes. Starch-hydrolyzing enzymes act prior to the oligosaccharide- and disaccharide-hydrolyzing enzymes. 

It is important to note that no evidence of maltase activity was found in the amylase preparations even when the highest concentrations used in these comparisons were allowed to react for twenty-four hours with one per cent maltose under the conditions for the hydrolysis of starch. Similarly, no evidence was obtained for the presence of any other contaminating or extraneous carbohydrases in the amylase preparations. Partial inactivation of the amylase under a number of different conditions failed to give any evidence of selective inactivation such as might be expected if more than one enzyme were present. The substrates used for measuring the activity of the partially inactivated amylase were starch and starch hydrolyzates that had already been extensively (58) J. Blom, Agnete Bak and B. Braae, Z. physiol. Chem., 250, 104 (1937). 

Glycogen and starch ingested in the diet are hydrolyzed by a-amylases, enzymes in saliva and intestinal secretions that break (al—>4) glycosidic bonds between glucose units. Most animals cannot use cellulose as a fuel source, because they lack an enzyme to hydrolyze the (fil—>4) linkages. Termites readily digest cellulose... 

Isoamylase is the name given to enzymes that hydrolyze the branch linkages of starch. These enzymes act by an endo-mechanism, hydrolyzing the branch linkages... 

For the non-solvent process,26 28 the pH of the starch hydrolyzate is adjusted to the optimum pH before adding the enzyme. Starch concentration is usually —15%. At higher concentrations, the yield of cyclodextrins decreases. At lower concentrations, the amount of cyclodextrins per unit weight of the starch increases, but the yield per unit volume of the reaction decreases. It is economically beneficial to utilize more starch than to recover a smaller amount of cyclodextrin per unit volume. 

In the solvent process,30,31 a solvent is used to direct the reaction to produce only one cyclodextrin. Higher starch concentrations can be used in this process than in the non-solvent process. (Concentrations of 30% are typically used.) Maximum yields per unit volume are achieved at 30% solids in the presence of solvents, compared to a maximum yield at 15% solids using the non-solvent process. The pH of the hydrolyzate is adjusted to the optimum for the enzyme, and the enzyme and solvent are added to the starch hydrolyzate. Solvents typically used for (3-cyclodextrin production include toluene, trichloroethylene and cyclohexane. As the reaction proceeds, (3-cyclodextrin precipitates as an insoluble complex. Yields per unit weight of starch are typically 30—45%, much higher than the yield obtained by the non-solvent process. 

Figure 13.4. Dry grind com process with raw starch hydrolyzing (RSH) enzymes. Source Wang et al, 2005.

Wang, R, Singh, V., Xue, H., Johnston, D. B., Rausch, K. D., and Tumbleson, M. E. 2007. Comparison of raw starch hydrolyzing enzyme with conventional liquefaction and saccharification enzymes in dry grind corn processing. Cereal Chem., 84, 10-14. 

The isomerization of glucose to fructose opened the way for starch hydrolyzates to replace cane or beet sugar (Dziezak 1987). This process is done with glucose isomerase in immobilized enzyme reactors. The conversion is reversible and the equilibrium is at 50 percent conversion. High-fructose com syrups are produced with 42 or 55 percent fructose. These sweeteners have taken over one-third of the sugar market in the United States (Olsen 1995). 

The enzymes that hydrolyze (1 — 6)-a-D-glucosidic linkages in starch and glycogen and in related a- and /3-dextrins are called debranching enzymes. The nomenclature for this type of enzyme was confusing, but has been clarified. 

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