When testing a leaf for starch why is it rinsed in cold water after it is boiled?

Rinsing a leaf in cold water after boiling it is done to stop the enzymatic activity that breaks down starch. Boiling the leaf softens the cell walls, allowing the iodine solution to penetrate and react with any starch present. Rinsing in cold water halts the enzymatic breakdown of starch, preserving the blue-black color reaction with iodine, indicating the presence of starch in the leaf.