roux
Flour cooked in fat, used to thicken -- the base of most classic cream and cheese sauces, of gravies, and of gumbo. Cooked longer a roux darkens and tastes deeper and nuttier, at the cost of some of its thickening power, so color and thickening strength pull against each other and which one matters more is the choice being made.
How to do it
Melt or heat the fat over moderate heat, add the flour, and whisk or stir until no dry flour remains. For a light roux, cook it for a couple of minutes -- just long enough to cook off the raw, cereal-like flavor without letting much color develop. For a darker roux, keep cooking while stirring frequently or constantly, depending on how high the heat is. When adding liquid, especially to a light roux headed into a sauce, add it gradually at first while whisking hard, then once the mixture is smooth and fluid, the rest of the liquid can go in faster. Bring the finished sauce to a proper simmer so the starch fully hydrates and thickens it.
Common pitfalls
- Not cooking the flour long enough -- raw flour leaves the sauce with an unpleasant, pasty taste.
- Adding a lot of liquid all at once -- isolated pockets of roux hydrate unevenly and leave lumps behind.
- Using too much heat -- the flour can scorch.
- Losing focus on a dark roux -- it can go from brown to burnt quickly.
- Expecting a dark roux to thicken like a pale one -- extended browning reduces its thickening power.
- Assuming every sauce needs to be thick -- a roux should give the dish the consistency it actually needs, not turn every liquid into paste.
Why it works
Flour holds starch granules that, once dispersed into liquid and heated, absorb water, swell, and release starch molecules that thicken the liquid -- this is starch gelatinization. The fat in a roux coats and separates the flour particles before any liquid arrives, which helps keep them from clumping together immediately. Cooking the flour also changes its flavor: as a roux gets darker, heat breaks down and transforms some of the starch, and Maillard and related reactions build toasted, nutty flavor -- but that same altered starch loses some of its ability to thicken. That's the central trade-off: a pale roux thickens more and tastes milder; a dark roux thickens less and tastes deeper and more toasted.
Equipment
A heavy saucepan or skillet with even heating is ideal. For a pale roux headed into a sauce, a whisk makes smooth blending straightforward. For a long-cooked dark roux, a flat-edged wooden spoon or heat-resistant spatula is often preferred, since it can scrape the whole bottom of the pan and keep flour from scorching in the corners. A light-colored pan interior makes the progression from cream-colored to blond, brown, deep brown, and finally burnt much easier to judge while learning.