Anyone who has made gravy for a big holiday dinner knows that heating a bit of fat and some flour, and then adding broth will eventually create a thick sauce to cover your plate of mashed potatoes and turkey. But exactly what turns these three ingredients into a smooth sauce?
The answer is heat. If you put a bit of flour into a cup of cold broth, the flour would clump instead of mixing with the broth and forming gravy. That's because the starch molecule in flour is a group of thousands of smaller sugar molecules bonded together in the shape of a long chain.
When you put flour in cold liquid, it clumps. Without heat, the starch molecules continue to keep their shape as long chains of sugar molecules and remain separate from the water molecules that surround them.
Turn Up The Heat!
When you heat this mixture, however, the energy from the heat disturbs the starch molecules and causes them to break down into their smaller components. At this point, individual sugar molecules are set free and are able to bond with hydrogen molecules in the liquid base of the sauce.
As this happens, the clump of flour dissolves and the broth thickens and takes on the smooth, thick, texture characteristic of gravy.
Fat, in the form of butter or meat drippings, is included in the list of ingredients for a gravy mainly because it adds a rich flavor most people have come to associate with gravies and sauces.