REDUCING
Reducing is the process of boiling down a liquid to thicken its consistency and concentrate its flavor. If you thicken a sauce by reducing, season it after it reaches the desired thickness; otherwise you may find it overseasoned or salty. Many tomato sauces demand long cooking and reducing; unless these sauces—or almost any thickened sauces—are reduced over very low heat, they will cook so fast that flavor and color will be impaired. Naturally, this technique is used only with sauces that contain no egg (which would curdle during cooking). Those that contain cream or flour must be stirred often to avoid scorching as they reduce. For perfectly smooth texture, many reduced sauces are strained before serving.