How starch granules thicken as they absorb water
Starch consists of amylose and amylopectin, two glucose polymers packed tightly inside granules. When starch granules encounter hot water, the heat breaks hydrogen bonds holding the granules rigid, allowing water to penetrate. The swelling granule absorbs liquid, gelatinizes, and thickens the surrounding sauce.
Each starch source has a characteristic gelatinization temperature. Potato starch swells noticeably at 60 degC, while corn starch requires hotter water around 65 degC, and wheat flour needs 70+ degC. This temperature difference explains why cornstarch slurries thickened at low heat may suddenly thicken more when the mixture finally boils.
Why starch thickness isn't permanent
Overheating continues breaking down starch. Once gelatinized granules rupture from excessive heat or mechanical agitation, they release their starch molecules into the liquid, which can actually thin the sauce. Professional cooks know that a slurry thickens its maximum during cooking, then slowly thins afterward if kept hot.
Reheating previously thickened sauces changes their consistency because the granules have already released their payload. This is why hollandaise and beurre blanc are made fresh to order: reheating breaks the emulsion and thins the sauce despite the original thickening from starch or egg.