Explain why a negative iodine test for starch would indicate a positive result for the enzymatic degradation of starch by amylase.

If iodine solution reacts with starch, it will change color into black.

If iodine solution is put on the fruit, according to the coloration, it can be concluded if there is starch in the fruit or not. If the yellow-orange iodine solution stayed the same color when it was put on the fruit, it can be concluded that there is no starch in the apple. On the other case, the black color of the fruit suggests that there is a great amount of starch in the potato. What is actually detected is alpha-amylase.

Alpha-amylase breaks down starch into di - and trisaccharide. If iodine solution reacts with starch, it will change color into black. But, a presence of clear halos in the medium indicates a presence of alpha-amylase because it will break down starch.