A Concentration of Flavors

• When you roast a cut of beef, the heat causes several physical changes to the roast's proteins. First, the heat causes them to shrink and become firm. When that happens, some of the moisture that normally fills the muscles' cells is squeezed out and collects at the bottom of the roaster. It's filled with salt, proteins, free amino acids and a number of other compounds that lend flavor to the beef. Secondly, if the temperature is high enough, the surface of the beef -- and perhaps, some of the free juices -- sear and turn brown. This process, called Maillard reactions, breaks down the amino acids in the beef and converts them into savory, complex flavor molecules. These also collect in the bottom of the roaster.

Classic Gravy

• The classic technique for making beef gravy uses flour. Heat a small quantity of butter in a pan and add flour, cooking it until the flour's starches begin to swell and approach the gelling stage. Then, mix the roux into the cooking juices and heat the mixture until it thickens. Some cooks reserve the beef fat skimmed from the roast's drippings and use that to make the roux, increasing the beefy flavor and also -- unfortunately -- the saturated fats. The resulting gravy is opaque in appearance, with a rich, hearty body and mouthfeel.

Cornstarch Gravy

• The use of rich, saturated butterfat or beef fat to make a roux is one reason why many diners shun gravy, considering the added fat and calories to be prohibitive. The beef's cooking juices themselves are relatively virtuous, if the fat is diligently skimmed away, so health-conscious cooks sometimes use cornstarch to make their beef gravy. Whisk the starch into a small quantity of cold water or broth, then stir it into the cooking juices and heat until they thicken. Cornstarch makes a clear gravy, which can look very attractive when it's made with darkly browned juices. However, cornstarch doesn't give the heartiness many diners expect in a beef gravy.

Instant-Mixing Flour

• One alternative thickener provides the best of both worlds. Quick-mixing instant flour, colloquially called gravy flour, is available in supermarkets under various brand names. It's made by pre-cooking the flour until its starches reach the gel point, just as you would with a roux, but it's done without fat. Then the flour is dried and milled back into a powder. When it's added to your beef drippings, it thickens as quickly as cornstarch, without the use of a roux, but providing the same rich and hearty texture you'd expect in a flour gravy. Whisk it into cold water first, as you would with cornstarch, or simply sprinkle the flour over your gravy and whisk it in.