All starches work by absorbing water (or other cooking liquid) into individual starch grains. The amount of liquid the particular starch is able to absorb and how concentrated the starch grains are in the liquid affect the thickness of the final dish. At the most extreme, starches can completely set a liquid into a solid, jello-like block!
However, simply stirring starch and liquid together isn't quite enough to get the liquid to thicken. The catalyst is actually heat. Without it, the starch grains simply sink to the bottom without taking on enough liquid to thicken.
As the liquid heats, its molecules begin to move around very rapidly. These molecules bump into the grains of starch, disrupting their structure enough to cause the granules to take in water. At a certain point during heating, the solution reaches a balance where the starch grains are still mostly intact but have absorbed as much liquid as they can.
If you continue heating, the starch will become too disrupted and the grains will actually lose their ability to hold water and thicken a sauce. This is what happens over long cooking or if you forget to turn down the heat on your dish after it comes to a boil. Luckily, you can re-thicken your soup or sauce by adding starch at the end of cooking with a beurre manie or by tempering in more starch.
You may also have noticed that dishes thickened with starch will thicken even more once they're off the heat and have cooled down. This happens because without the constant disruption from the all moving molecules, the starch will set into a stable structure with water trapped in between. Gentle re-heating will return the sauce to its original thickness.
Next week, we'll talk more about the different kind of starches we have available to thicken dishes and when we might want to use one over another. Stay tuned!
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