White sauce, or béchamel, is made with three simple ingredients -- butter, flour and milk -- and forms the basis for a host of other dishes, including soups, casseroles and desserts. All-purpose flour is added to melted butter and cooked briefly. The addition of milk causes the flour molecules to swell, thereby thickening the sauce. You can substitute skim milk for whole milk to make a béchamel, but its lighter consistency and very low fat content may affect the final product.
The amount of fat contained in the milk used in a béchamel sauce affects its flavor and determines how rich the sauce is and how quickly it thickens during cooking. Because skim milk has significantly less fat and more water content than whole-fat milk, making béchamel with skim milk results in a thinner, less substantial sauce with less flavor. Skim-milk béchamel takes longer to thicken.
A béchamel sauce is done if it coats a metal spoon. You can avoid a thin sauce by slightly reducing the amount of skim milk in proportion to the butter and flour mixture, which is known as a roux. Add the skim milk slowly to the roux and stir constantly until it starts to thicken, and stop adding the milk when the sauce has reached the consistency you desire, even if it's less than a recipe calls for.
A white sauce gets its flavor from the butter used to make the roux and the milk fat, and your taste buds should be your guide when using skim milk. While butter does act as a thickener by suspending the fat particles in the liquid, it takes quite a bit of butter to thicken a béchamel. That said, if the sauce is thick enough after having decreased the amount of skim milk but it still doesn't quite measure up flavor-wise, add a bit more butter and blend it in until it is completely melted to enrich the flavo.
The Egg Solution
If skim milk is all you have on hand and calorie content isn't an issue, you can enrich your béchamel sauce with egg yolks. Beat the egg yolks vigorously into a creamy sauce and stir in some of the hot bechamel to warm the yolks slowly without cooking them. Return the egg yolk and bechamel mixture to the pan and incorporate completely into the sauce, stirring constantly. One caution: using uncooked or partially cooked eggs can introduce salmonella, a potentially dangerous bacteria, to foods; do not serve a béchamel thickened with egg yolks to very young children, the elderly, pregnant or nursing mothers or those with compromised immune systems.
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