The Vanishing Green

• The fresh green tint of vegetables, including cabbages, comes from chlorophyll, a pigment found in most plants. It plays a crucial role in photosynthesis, the process plants use to convert sunshine into fuel for growth. Chlorophyll is an unstable molecule, easily changed by heat and by exposure to hot water. It's water-soluble -- after blanching greens for the freezer, you'll find your water distinctly colored -- and prone to fading if it's exposed to acidic ingredients or even cooked in mildly acidic water.

A Base for Adjustment

• Baking soda's chemical identity as a base is why so many old-time recipes, and cooks who learned their craft in an earlier day, include baking soda in their cooking water for cabbages. Its alkaline nature can neutralize acidity from your tap water or from other ingredients, helping protect the chlorophyll in your cabbage from the dulling effect of the acid. This trick is effective with any green vegetable, from string beans and fresh peas to delicate greens such as spinach, but it carries a price tag.

The Downside

• Few cooks use this technique anymore, because despite its effectiveness it has a few significant disadvantages. For one, if you use too much soda in your water it gives the cabbages a distinctively soapy soda flavor. It also has a negative impact on the texture of your vegetables, giving them a soft and almost mushy character. That wasn't as important a half-century ago, when cooking vegetables very soft was the norm, but it's less acceptable now. Baking soda in the water also speeds the destruction of the many water-soluble vitamins in your cabbage, in part because of the role it plays in breaking down the tissues.

The Purple Cabbage Scenario

• If you still occasionally like to use baking soda with your cabbage, it's important to note that it applies only to green cabbage. If you cook purple cabbage from time to time, be aware that its pigments -- called anthocyanins -- have very different requirements. They're water-soluble, like chlorophyll, but soda has the opposite effect on them. It turns the cabbage to a disconcertingly drab blue, the sort of color used in horror movies for a zombie's lips. To retain the best color in purple cabbage, use acidic ingredients such as vinegar or lemon juice in your cooking water. Acidity can even restore an appealing color to purple cabbage that has already been discolored by soda or by hard, mineral-heavy tap water.