A good question from reader Mack. A roux (pronounced “roo”) is a thickener. More specifically, it is a thickener made from a 50-50 combination — by weight — of white wheat flour and fat (butter, oil, chicken fat, beef tallow, any fat will do). Roux-making is usually associated with French cooking, though the earliest reference to the technique is found in a German cookbook dating to 1533.
But I think the real question here is: what does cooking fat and flour together accomplish? First and foremost, the heat causes the individual flour granules to “gelatinize”, i.e. begin to shed some of their component starches. Though flour granules are very, very small, when you look at them under a microscope they resemble little bundles of sticks. These “sticks” are starch molecules. The benefits of partially unbundling them are twofold. First, the chalky mouthfeel and “starchy” taste created by big pieces of ground wheat endosperm coming into contact with the taste buds on the tongue is eliminated. Second, those long starch molecules, when introduced to liquid, get tangled up with one another. Water molecules get caught amid the tangles, lose their ability to flow, and the liquid thickens. A roux is thus a very handy thing for creating a smooth, lump-free white sauce like the béchamel that’s used in cheese soufflé batter.
Cook a roux a little longer and it starts to brown. This isn’t necessarily a bad thing, but it’s indicative that the hot fat is starting to break the starches down into their components sugars and those sugars are caramelizing. The result is a nice toasty flavor, however it comes at a cost to the roux’s thickening power, since short-chain sugars don’t tangle as well. You won’t notice the difference much if the roux is only slightly browned, but if you cook a roux to a deep caramel brown like those New Orleans gumbo makers do, you’ll need quite a bit of it to produce a noticeable change in a liquid’s viscosity.