From light to dark, how important is it to cook a roux to the right color?
A roux, a cooked mixture of flour and fat, works primarily as a thickener for sauces and stews, but it also provides the dish with flavor and color. Notably, the flavoring and thickening properties don’t work independently.
Roux are always cooked to a specific shade that can range from white to blond to peanut butter—and even darker. The darker the color the more pronounced the roux’s flavor. But at the same time that a roux darkens, its thickening power lessens. This is because the intense heat from frying the flour in fat causes its starch chains to break down, and these smaller pieces are less efficient thickeners. So the longer a roux is cooked, the less effective at thickening it will be.
To quantify more precisely how cooking influences a roux’s thickening power, we borrowed a specialized tool for measuring viscosity from Brookfield Engineering Laboratories in Middleboro, Massachusetts. We prepared three roux cooked to white, blond, and peanut butter using 4 tablespoons of butter and ¼ cup of all-purpose flour for each batch. We then added 2 cups of water to each roux and simmered the mixtures for 20 minutes. Holding each batch at the same temperature, we then tested their viscosities with the borrowed viscometer. Using the white roux (cooked for just 1 minute) as our baseline, we found that the blond roux (cooked for 3 minutes) had 14 percent less thickening power. The peanut butter–colored roux (cooked for 5 minutes) had 26 percent less thickening power.
THE BOTTOM LINE: These aren’t small differences; it’s important to cook a roux to the right color. Cook the béchamel for a soufflé too long and it won’t have the same thickening power or structural integrity—and your soufflé won’t rise as much. And if you shortchange the cooking time for the roux in a stew recipe, you could end up with a gloppy, overly thickened dish.