Why does cornstarch or other grain slurries thicken sauces?

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I’ve seen Chinese dishes use cornstarch paste for thickening sauces and flour base used in French dishes. What do these grains do that makes the liquids viscose?

In: Chemistry

Starch is made up of long chains of glucose sugar molecules joined together, to form a huge string. There are actually two types of starch; amylose and amylopectin. In amylose, all the glucose molecules are joined together in one long line. Amylose is sleek and streamlined; the racing car of the starchy world. Amylopectin, however, isn’t so smooth. The glucose molecules attach to one another at different angles, producing shorter bulkier chains, with branches and bristles sticking out; more of a 4×4 than a racing vehicle.

The most important thing about starch, though, is the way in which it reacts to heat. Starch from corn or wheat, or indeed any plant, is stored in the form of tiny solid granules, containing a mixture of amylose and amylopectin (the precise ratio depends on the plant). These granules are starch molecules packed together in a tight rigid structure. When mixed into cold water, the granules absorb a little bit of water, but not much; they are rigid, they don’t like outsiders, thank you very much. At this point, if you add too much water the granules simply clump together at the bottom of the pan, rejecting the water entirely. I think we’ve ALL been there. Lumpy gravy anyone? Gradually adding water distributes the granules throughout the liquid, but they’re still distinct granules just floating about in water, they don’t want to socialise, and they have no effect on the thickness of the liquid.

When you add heat, though, you see changes. Gradually the water molecules gain energy from the heat; they start to jump around in excitement, leaping and bounding around the starch granules. Soon, this jumping and pouncing starts to affect the granules, the weaker sections start to break down, they want to play, and they start to think about interacting with all that energetic water. As soon as one region of the granule breaks down and interacts, this creates room for more water to be absorbed. The more water that the granule absorbs, the more strain it is under to give way and interact with the water. Starch molecules begin to leak out into the water, and flap around joyously. If they happen to bump into another starch molecule, they hold hands and skip together. More and more chains bump together, and hold hands; they start to form a meshy network of chains throughout the liquid, this is when your sauce starts to thicken.