The whiteness of these vegetables, as well as that of salsify and cauliflower, is due to the pigment flavone. When this chemical compound is subjected to prolonged heat or alkali, a brownish-yellow tint develops. Several countermeasures can help keep your vegetables white. First and foremost, do not overcook them. Also, buy young, fresh vegetables in good condition. If your pipes carry hard water, try adding a touch of cream of tartar or lemon juice to help neutralize its alkalinity. The more of these acids you add, the whiter your boiled vegetables will be, but do not add so much that the flavor of the whitener becomes undesirably obvious.
** Asian Recipes