What Is a Roux? Flour and Fat Doing Two Jobs at Once

Flour thickens because its starch granules absorb water and swell when heated, bursting and releasing long molecules that tangle and trap liquid. The problem is that dry flour dropped into hot liquid gels instantly on the outside of each clump, forming a waterproof skin around a pocket of dry flour, which is exactly how lumps form and why they cannot be stirred out. Dispersing the flour in fat first coats each particle in something water cannot immediately penetrate, so when liquid is added the grains separate before they gelatinise and the thickening happens evenly throughout. Cooking the flour in the fat also does two further things. It hydrolyses some of the starch, which reduces thickening power as cooking continues, and it drives the Maillard reaction and caramelisation, producing colour and a substantial amount of flavour. Those two effects run together, which is the central trade-off of the technique: the darker and more flavourful the mixture becomes, the less it will thicken.

Cooking time determines both colour and thickening power, and the recognised stages are used for different dishes:

• •White, cooked only a couple of minutes until the raw flour smell goes, with maximum thickening power, used for bechamel and cream sauces
• •Blond, cooked a little longer to a pale gold with a faint nutty aroma, used for veloute and for sauces where slight colour is acceptable
• •Brown, cooked until the colour of peanut butter, with noticeably reduced thickening and a strong toasted flavour, used in espagnole and in meat gravies
• •Dark brown to chocolate, characteristic of Louisiana cooking, which can take forty minutes or more of constant stirring and thickens only weakly while carrying enormous flavour
• •The usual ratio is roughly equal weights of fat and flour, adjusted according to how thick the finished sauce should be
• •A dark version in gumbo may be made with oil rather than butter, because butter solids burn long before the flour reaches that colour

The practical rules follow from the chemistry rather than from tradition. Adding cold liquid to a hot mixture, or hot liquid to a cooled one, both work well, because a temperature difference gives time to disperse before gelatinisation begins, while adding hot liquid to a hot mixture thickens instantly and unevenly. Adding liquid gradually at first, whisking to a smooth paste before loosening it further, is more reliable than pouring everything in at once. The sauce must then be cooked out for several minutes after thickening, both to finish gelatinising the starch and to remove residual raw flour flavour, and a sauce that tastes pasty has usually been undercooked at this stage rather than badly proportioned. Lumps that do form can be removed by straining, which is inelegant and entirely effective. Alternative thickeners behave differently and are not simple substitutes, since cornflour thickens more strongly and gives a glossier, more translucent result that breaks down with prolonged cooking, and arrowroot and modified starches each have their own tolerances for acid, heat and freezing.

The technique underpins the classical French mother sauces codified in the nineteenth century and refined in the twentieth, and a large part of traditional restaurant cooking descends from it, which is why it is taught early in professional training. It also appears far beyond that tradition, in Louisiana cooking where the dark version is foundational, in Japanese curry roux sold as blocks, in Central European sauces and soups, and in the white sauce used in lasagne and in numerous baked dishes. Fashion moved against it in the later twentieth century, as reduction-based and emulsion-based sauces came to be seen as lighter and more modern, and flour-thickened sauces acquired an old-fashioned reputation that they have partly shed. The honest comparison is that reduction concentrates flavour and takes time and quantity of stock, while a flour-thickened sauce achieves body cheaply and quickly and carries its own flavour when browned, so the two solve different problems and the choice is not a matter of sophistication.

Coating starch grains in fat keeps them apart so they swell evenly instead of forming a sealed skin around dry flour, which is exactly how lumps happen. Cooking the mixture builds flavour and simultaneously destroys thickening power, so a dark version carries taste and thickens weakly. A temperature difference between mixture and liquid buys time to disperse, and the sauce must cook on after thickening to lose its raw flavour.