Roux and Flour-bound Sauces
Roux, a combination of butter and flour cooked on top of the stove, is the thickening agent for many important French sauces, notably the sauces mères or mother sauces from which all flour-bound sauces derive. The sauces mères are classified by colour - béchamel and velouté, which are white sauces, and sauce espagnole, a brown sauce. Béchamel is made by adding milk to a white roux (roux blanc), in which the flour and butter are cooked just long enough to eliminate the taste of raw flour without colouring the mixture. A velouté is made with a lightly coloured roux (roux blond), to which a white stock (veal, chicken or fish) is added. Sauce espagnole is made with a brown roux - in which the flour and butter are cooked until well browned - and a brown veal or beef stock. Nowadays, sauce espagnole is rarely used and brown sauces are more likely to be made without the roux; the sauce is thickened simply by the reduction of the stock.
The sauces mères are the basis for many variations: béchamel with cheese becomes a sauce mornay. Velouté made with chicken stock and enriched with cream is a sauce suprême. A brown sauce with red wine, shallots and herbs is called a sauce bordelaise; with the addition of port wine, cognac, truffles and foie gras a brown sauce becomes a sauce Périgueux. Both béchamel and velouté sauces may be further thickened and enriched with a liaison of egg yolk, or cream and egg yolk (as in the Rouelles de Veau Bourgeoise). Brown sauces may be enriched with a small amount of butter for added flavour and sheen.
In any roux-bound sauce, care must be taken to avoid lumps. To this end it is necessary to add either a cold liquid to a hot roux or a hot liquid to a cold roux. (If both liquid and roux are hot, it suffices to remove the roux from the heat and let it cool a few minutes before adding the hot liquid.) The mixture must be whisked constantly while the liquid is added.
A béchamel sauce and its variations may be used to coat vegetables or for gratins, for egg dishes (Oeufs à la Tripe), fish or chicken. A béchamel is also used as a base for other preparations such as soufflés or terrines. The thickness of the sauce may be adjusted to the dish. Velouté sauces are appropriate for vegetable, fish, veal, or chicken dishes. A velouté sauce may be made separately from the dish it embellishes or it may be made with the liquid in which the food is cooked. Brown sauces are used to coat red meat and game.
Béchamel, velouté and brown sauces may be made ahead and kept warm in a water bath or reheated over low heat. (Do not boil if the sauce has been bound with egg yolk.) Rub the surface with a little butter to keep a skin from forming.