For performing enzyme assay dissolve the starch in hot water both, avoid direct heating.

boiling or less because less than boiling, is not dissolved well. 

thanks

Keep the content flask in a water bath with at ~80 degree of temperature with gentle shaking at magnetic stirrer, prior to this use a high quality starch.

Solving starch in hot water or heating the water cause starch gelatinization in which the inter-molecular bonds of starch molecules break down and allow the hydrogen bonding sites to engage more water. The best way for solving starch, is using the starch in cold water (4- 8˚C). After solving completely, you can use it at 25 ˚C for doing your test.