Roux thickens soup through gelatinization – a process that causes starch granules to absorb liquid and expand several times their normal size. Roux has applications in several preparations, but most notably in soups and sauces. Classic French roux uses butter as the primary fat, although other fats, such as rendered bacon and olive oil, perform the same function. Also, roux comprised of strong-flavored fats, such as rendered bacon, impart tastes to the soup redolent of the fat used.
Add 1 cup of fat to a sauce pan and place over medium heat for five minutes or until a sprinkle of flour causes bubbling action. Choose a fat that complements the soup preparation; for instance, if preparing a soup that contains bacon, use rendered bacon fat.
Add 1 3/4 cups all-purpose flour to the fat while incorporating with a whisk. Stir the roux consistently as it cooks – allowing the roux to stay on the heat too long results in scorched bits and uneven cooking.
Cook the roux to the color appropriate for the soup. For instance, use a white roux (five minutes cooking time) for white and cream-based soups, such as cream of mushroom; use a blonde roux (20 minutes cooking time) for tan or off-white soups, such as cream of chicken; use a brown roux (35 minutes cooking time) for dark soups or stews, such as beef; use a dark brown roux (45 minutes cooking time) for gumbo, or soups for which a deep, smoky flavor augments the taste. Store roux in an airtight container in the refrigerator until needed.
Whisk the cold roux into the soup. Adding hot roux to hot liquid results in clumps of flour that do not entirely dissolve or thicken unevenly. Use 3 tsp. of roux for every cup of liquid.
Simmer the roux and soup for 20 minutes, or until the soup has a smooth texture. The starch granules soak up moisture and expand during simmering, thickening the soup. Taste the soup 20 minutes after adding the roux to detect any starch granules (they typically coat and stick to the back of the tongue) and continue simmering if needed.
A.J. Andrews' work has appeared in Food and Wine, Fricote and "BBC Good Food." He lives in Europe where he bakes with wild yeast, milks goats for cheese and prepares for the Court of Master Sommeliers level II exam. Andrews received formal training at Le Cordon Bleu.