For a roux, cook flour with an equal weight of fat over low to medium-low heat, stirring constantly: at least 2–3 minutes to remove the raw flour smell, 5–7 minutes for a nutty aroma and pale yellow color, and more than 10 minutes for a dark brown roux. As the roux darkens, its flavor deepens but its thickening power decreases.
Roux is a thickener made by cooking flour in fat. Because the fat coats the flour’s starch granules, they do not stick together when added to liquid, and they thicken soup without lumps. The first purpose of cooking the flour is to remove its raw taste. When flour meets fat, it first foams and looks like wet sand; the raw smell fades within 2–3 minutes. At this point the roux is considered white and is used in soups, béchamel, and cream sauces where a pale color is desired. In a white roux, the flour cooks without browning.
As cooking continues, the color and aroma change. After five to seven minutes, the flour turns pale yellow and develops a nutty aroma; this blond roux adds body and a lightly toasted flavor to vegetable soups and sauces. When cooked for more than ten minutes, it turns brown and its aroma becomes noticeably stronger. A dark roux adds deep flavor but has less thickening power, so more is needed for the same consistency because prolonged cooking changes the starch structure.
The ratio is simple: use equal weights of flour and fat. In household measures, one tablespoon of flour is equivalent to one tablespoon of butter or oil. Two tablespoons of flour are enough to give a 1-liter pot of soup a light consistency; increase the amount for a thicker sauce. If using butter, keep the heat at medium because butter’s milk solids brown at a lower temperature than oil and can give the roux a scorched flavor.
The way liquid is added determines whether lumps form. Add liquid to hot roux gradually, not all at once, while whisking constantly; the first addition combines with the roux into a thick paste, and each later addition thins the mixture. A temperature difference also helps: add warm or cold liquid to hot roux, and hot liquid to cold roux. If both are boiling, the starch swells all at once and forms lumps. If lumps remain, blend the mixture or pass it through a fine strainer. A whisk breaks up lumps much better than a wooden spoon.
After liquid is added, bring soup thickened with roux to a boil; starch reaches its full thickening power near the boiling point, and the remaining flour taste cooks off at this stage. Continue cooking over low heat for 5–10 minutes. Roux can be prepared ahead and stored in the refrigerator for a few days or in the freezer for months; when needed, stir a small piece into hot soup. You can prepare the roux in the same pot you will use to cook the soup. Adjust the soup’s salt after adding the roux and letting the consistency settle, because salt tastes more pronounced in thickened soup. Stay by the pot while making roux; flour can burn in seconds.