All starch, whether it’s made of straight or branched chains, is composed of crystals, which don’t dissolve in cold water. Think of a grain of rice, a piece of raw potato, or a strand of spaghetti — put it in cool water and it stays the same. But heat breaks down those crystals so that starch can dissolve. As a result, when you cook a starchy food, it absorbs water and becomes easier to digest.
The more overcooked rice or pasta is, the faster it makes your blood sugar rise. When starch is heated and then cooled, it can return, in part, to its crystal form; that’s why hot potatoes have a high gylcemic load, while potato salad’s is slightly lower.