Cornstarch Works Best When It’s Hot
When talking about cornstarch, there is a lot of emphasis on the importance of slurry. Swirling and suspending the starch in water or milk (or some other liquid) prevents it from clumping in your sauce and ensures everything thickens, looks pretty, and even looks similar. Suspensions are important, but they are only part of the equation when it comes to maximizing the thickening properties of cornstarch – you also need to pay attention to heat.
If you mainly use cornstarch to thicken your skillet sauces, you probably haven’t encountered this, as skillet sauces tend to be quite hot. But if you try to thicken a cold liquid, gravy, or sauce after it’s cooled a bit, adding cornstarch slurry won’t hurt. According to Harold McGee’s Food and Cooking , corn starch can require a gelling temperature (the temperature at which the granules begin to swell and thicken) up to 180. It’s not really boiling water, but it’s hot.
Luckily, you don’t need to heat the sauce to exactly 180 ℉; overdoing it a little with it will work fine. To thicken the starch to the maximum extent, bring the liquid to a boil, add a whisk to the slurry (two parts water for one part starch will do) and let it simmer for a couple of minutes. remember that your sauce or gravy on your plate will continue to thicken. (You can preview its final consistency by placing a small spoonful on a cold plate.)