Adding Liquid to a Roux

Have the liquid very hot. As soon as the roux is cooked, remove the saucepan from the heat and whisk half the hot liquid into the hot roux until the mixture is homogeneous. It will thicken immediately. Whisk in the remainder of the liquid until smooth. Whisking hot liquid into a hot roux ensures constant temperature for the starch granules which have begun to swell and gelatinize. Return the mixture to the heat and, now using a wooden spoon, bring to a boil, stirring constantly. Stirring prevents the formation of lumps in the sauce, as well as precipitation of the not fully swelled starch cells to the bottom of the pot where they can burn; finally, it cools the progressively thickening starch sol by introducing some air into it, so that its granules are submitted as gradually as possible to the increase of temperature, which finally results in the sauce boiling.