Page 206 - Turkey Book from Meathead

As proof of its goodness, when you chill the leftovers the gravy will solidify into a jelly. That's what happens to melted collagens: they turn to gelatin, and collagens bring flavor and texture to the table. See Meathead's article on meat science for more on the subject.
Try to resist the temptation to thicken this gravy with flour or cornstarch. If the idea is to moisten meat, starchy sauces just don't get the job done. Starches are large molecules and they can't penetrate the tiny openings in the meat. Starchy gravy just floats on top of the meat like a life preserver after the ship has gone down. Our thin gravy will soak into the meat and add much more flavor. After you taste it, you won't serve the thick high school cafeteria stu again. But if there are hardened traditionalists in the house, we have included instructions on how to satisfy them with Granny’s Gravy below.