Starch

Starch is made up of thousands of glucose molecules linked up together. There are two structures of starch molecules: amylose and amylopectin. Amylose molecules are long and straight; amylopectin molecules are short and branched. Of the two, amylose is a more effective thickener than amylopectin as its long chains tangle with each other more readily and slow the motion of other molecules in the surrounding fluid.

When starch is mixed into cold water, its granules only absorb a limited amount of water and sink. Nothing happens. When starch is mixed into hot water, however, the granules absorb large amounts of water and swell up. As they do so, weak regions of the granules become disrupted; stronger regions lose their organized structure and become water-containing meshworks of long molecules. In other words, the granules become individual gels. A cloudy suspension of granules becomes more translucent as individual starch molecules become less packed and no longer deflect as many rays of light.

The temperature at which starch begins to behave in this manner is called the gelation range, usually around 120-140 degrees F/50-60 degrees C. 

Thickening of a sauce/dish with starch occurs as the granules become so saturated with water that they begin to leak amylose and amylopectin molecules into the surrounding liquid. The long amylose molecules form a fishnet of sorts that entraps pockets of water and blocks the movement of the swollen starch granules.

After reaching its thickest consistency, the starch-water mixture begins to thin out again. As more amylose leaks into the water, the starch granules break or otherwise become smaller. Heating close to boiling point, vigorous stirring, continued heating long after thickening, and addition of an acid accelerate the thinning process.

The cooling that follows allows amylose molecules to form stable bonds and water molecules settle in the pockets between the starch chains. As a result, the sauce/liquid gets thicker. If the temperature drops low enough, the starch particles begin to congeal. It is important to judge the consistency of a sauce/dish at serving temperatures, not at cooking temperatures. The best way to predict the final texture of a sauce is to pour a spoonful into a cool dish and sample it.