Flour and milk can form the base of a great white sauce or roux, or can become a cooking disaster. However, just as there is more than one reason why flour can turn hard and lumpy when you mix it with milk, there is more than one way to ensure it does not. The right timing, ingredients and instructions are all you really need to prevent this from ever happening again.
Dissolve the flour in a saucepan by stirring it into in an equal amount of fat, such as melted butter, margarine or meat drippings, before adding in the milk. Dissolving flour before mixing it with milk prevents the starch in the flour from forming lumps.
Add milk straight from your refrigerator. The colder the milk, the less chance it will cause the flour to become lumpy.
Pour milk into the flour paste following a three-step process, stirring or whisking constantly during each step. Add about one-third of the milk to begin dissolving the flour and create a thick paste, add half the remaining milk to finish dissolving the flour and dilute the paste and then stir in the remaining milk.
Heat the sauce slowly over medium-low heat stirring constantly to evenly distribute the starch granules in the milk. As the sauce heats, the starch granules will burst, causing the sauce to thicken without becoming lumpy.