|MadSci Network: Biochemistry|
In considering reversible reactions, you have to consider the overall equilibrium constant, which is dependent upon the change in free energy from reactants to products.
The hydrolysis of starch, whether spontaneous or catalyzed by amylase, has a substantial decrease in free energy, and thus the equilibrium is very much towards hydrolysis. So while in principal the hydrolysis of starch by amylase is reversible, for practical purposes the reaction can essentially goes to completion. In other words, the equilibrium (if sufficient time is afforded to reach equilibrium) is very much to the right, toward the hydrolysis products. Insufficient starch remains to give the colored product with iodine.
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