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" How does cornstarch work ? "

Dear Anne and Sue,

OK, so what's the deal with cornstarch? Why, when it's supposed to thicken, does it sometimes seem to go the opposite way and make stuff runnier? How does cornstarch work?

Thank you,
Dallas R.
Calgary, AB


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Hi Dallas,

As you've discovered, starches are sometimes noted for their idiosyncrasies! Cornstarch is one of the most commonly used thickeners, but it has some unique traits. So thanks for the good question!

Cornstarch must be cooked to 95°C (203°F) before thickening begins. At that point, it usually thickens fairly quickly and the sauce turns from opaque to transparent. When cornstarch thins after it's thickened, it's usually due to continued stirring. Once the thickening network forms, any agitation interferes with the setting process. The sauce thins when the starch network that sets and traps the liquid is broken. Liquid is released and thins the sauce.
While cornstarch thickens as it heats, it also sets as it cools, so it's particularly useful as a gelling agent for desserts that must hold their shape such as lemon pie filling. Also, as cornstarch becomes clear when thick, while flour remains somewhat opaque, the color of fruit sauces is deeper and more appealing when those sauces are thickened with cornstarch. Cornstarch also sometimes appears to thin as it stands. This is due to a process called syneresis (commonly referred to as weeping). What you'll see is a fluid seeping from the gel. This problem is more evident if the gel (often a pudding or pie filling) also contains eggs or has a high sugar concentration. People often wonder what the difference is between cornstarch and flour. Both are cereal starches, but cornstarch is pure starch while flour contains gluten. The gluten reduces the thickening power of flour. One tablespoon of cornstarch thickens one cup (250 mL) of liquid to a medium consistency. It takes two tablespoons of flour—twice as much—to thicken the same amount of liquid.

Your query really brings up the point that there's a lot to learn about the thickeners that cooks use every day. Wishing you successful sauces!

Anne & Sue


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