Larousse Gastronomique defines roux as “a cooked mixture of equal amounts of flour and butter, used to thicken sauces.” Nevertheless, some recipes that borrow from classic French technique specifically call for making roux with oil (gumbo, for example). To find out if oil can be successfully used in a roux that calls for butter, we made a classic béchamel, or white sauce, with butter and compared it to béchamel made with neutral-tasting canola oil (we knew the distinctive flavor of olive oil would prevent it from being a viable substitute).
Tasters found the texture of the sauces identical. The sauces also tasted remarkably similar, though a few astute tasters thought the sauce made with butter was the richer of the two.
Next, we used these same sauces as part of a more complex dish, lasagna Bolognese. Here the flavor differences faded away, with the meaty tomato sauce overpowering the béchamel. Our recommendation: If the roux will be used in a dish that involves multiple flavors, go ahead and substitute canola oil for butter. If the roux will be used to thicken a simple sauce, it’s best to stick with butter.