The conventional practice when cooking dried legumes is to soak them before cooking and to refrain from adding salt or acidic ingredients during cooking. One of the charms of lentils is that they don't need to be soaked. We wondered if this noncomformist status would also apply to the restrictions on salt and acid.
We wanted to determine the best cooking times and uses for three varieties of : French Le Puy, peeled red, and whole red masoor (red lentils with the brownish seed coat left on).
We found that the addition of salt to the cooking water significantly developed the flavor of each lentil. The use of vinegar, however, was not such a hit. It doubled the cooking time and gave the lentils a rather grainy texture. Subsequent tests with wine and tomato yielded similar results. This is because acids interfere with lentils' ability to bind with water. This in turn causes them to take longer to soften and to have a rather dry taste. So unless you're using lentils in a slow-cooking dish and want to lengthen their cooking time, avoid cooking them with acidic ingredients.