Swelling and Gelation

What makes starch so useful is its behavior in hot water. Mix some flour or cornstarch into cold water, and nothing much happens. The starch granules slowly absorb a limited amount of water, about 30% of their own weight, and they simply sink to the bottom of the pot and sit there. But when the water gets hot enough, the energy of its molecules is sufficient to disrupt the weaker regions of the granule. The granules then absorb more water and swell up, thereby putting greater and greater stress on the more organized, stronger granule regions. Within a certain range of temperatures characteristic of each starch source but usually beginning around 120–140°F/50–60°C, the granules suddenly lose their organized structure, absorb a great deal of water, and become amorphous networks of starch and water intermingled. This temperature is called the gelation range, because the granules become individual gels, or water-containing meshworks of long molecules. This range can be recognized by the fact that the initially cloudy suspension of granules suddenly becomes more translucent. The individual starch molecules become less closely packed together and don’t deflect as many light rays, and so the mixture becomes clearer.