The roasted turkey sizzles on its platter.
You've spooned the fat from the drippings in the pan, and now all that's left is to thicken the remaining golden juices to make gravy. To do this, the recipe calls for flour or cornstarch as the thickening agent. Does it make a difference which you choose?
How do thickeners thicken, anyway?
Both flour and cornstarch use the carbohydrate, starch, to thicken sauces. Mixed with cold liquid, starch isn't too thrilling, but add a little heat to the mix and the individual starch granules get to work, absorbing liquid and swelling.
By the time the mixture nears boiling, the starch granules will have grown to about ten times their size at room temperature. These swollen starch granules form a thick but tender matrix for the flavorful turkey drippings in your gravy, which thickens even more as it cools.
Flour and cornstarch
Although both flour and cornstarch owe their thickening powers to starch, cornstarch is pure starch, while flour contains starch plus protein. What difference does a little protein make? Protein takes up volume but contributes little to the thickening power of flour. As such, you need about twice as much flour as cornstarch to thicken a sauce.
That means flour could be more likely to add an undesirable pasty flavor. While pure starch becomes transparent as it swells with liquid, the protein in flour reflects light, making a sauce look cloudy. Cloudy, velvet-textured gravy can be delicious, but cornstarch is used more often to thicken fruit pie fillings, creating the gem-like transparency around the fruit.