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
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!