Cooling Reorganizes Starch Molecules and Firms Granules

Once the cooking is finished and the seeds cool down below the gelation temperature, the starch molecules begin to re-form some clusters with pockets of water in between, and the soft, gelated starch granules begin to firm up again. This process is called retrogradation. Some of the simpler amylose molecules start bonding to each other again almost immediately, and finish within a few hours at room or refrigerator temperatures. Sprawling, bushy amylopectin molecules take a day or more to reassociate, and form relatively loose, weak clusters. This difference explains why long-grain rices high in amylose have a firm, springy texture when served right after cooking and get inedibly hard when refrigerated overnight, while short-grain rices low in amylose have a softer, sticky texture and harden much less during overnight refrigeration. The hardness of all leftover grains can be largely remedied simply by reheating and so regelating their starch.