A roux is a smooth paste, made from flour fried in fat, that is added to sauces, soups or gravy to make them thick, smooth and rich. The invention of the technique in the 17th century at the court of the Sun King was a true revolution in French - and thereby European - cuisine, with four out of the five Mother Sauces of classical French cooking based on a roux. The French colonists who resettled in America a century later took their roux to the extreme through browning, to achieve the deep nutty richness that has become characteristic of Cajun cooking.
How do you make a roux?
French cooking traditionally utilises butter, but in Cajun cooking, where the roux is cooked for much longer, oil, lard or bacon grease are favoured because they don’t burn as easily. Traditional ratios call for equal parts fat and flour.
Heat up the fat over low heat, then stir in the flour until a paste forms. From there, the length of cooking time determines the darkness of the roux and thus the flavour of the final product. Béchamel sauces for lasagne or macaroni and cheese use a blonde-coloured “white roux”, cooked just long enough to eliminate the taste of raw flour.
The roux for sauce espagnole is browned to a light tan (known in French as roux), and roux for Cajun and Creole dishes are cooked to a dark chocolatey brown paste that can easily end up charred in the hands of an inexperienced cook.
The science behind it
Two things happen when starch molecules come into contact with water. They absorb the water and swell up (which helps to thicken sauces), but this swelling also makes them bind together, forming small clumps of dough - not ideal for a smooth sauce.
Frying the flour helps keep the individual particles separated by a coating of fat, so that when the water is added, the starch molecules can swell independently, aided by constant stirring. The starch particles then slide past each other rather than clumping up, giving rise to a smooth consistency.
Usage: "A smooth and creamy béchamel starts with a good roux."