Healthy Vegetable Preparation
The nutritional content of vegetables, as well as their taste and texture, is affected by how you handle them, and especially by how you cook them. Here are some general rules to keep in mind:
Nutrient loss occurs when vegetables are exposed to light and air; therefore, don’t wash, chop, or slice vegetables until you are ready to use them.
While vegetables should always be washed before you cook or serve them raw, long soaking is not recommended, as it can leach out water-soluble vitamins. You can quickly but thoroughly rinse vegetables under cold running water, or dunk them in several changes of water in a basin. Use a soft brush to remove dirt that clings; lukewarm water also helps to release sand and grit from leafy vegetables.
Peeling and Chopping Vegetables
Remember that many nutrients are concentrated just beneath the skin. If possible, do not peel vegetables such as potatoes and beets; or, cook them in their skins and peel them after cooking, when their thin skins will slip off. (Even if you don’t eat the skin, leaving it intact during cooking helps preserve nutrients.)
In general, most vegetables should be cooked until they are barely tender or crisp-tender. Only then will they retain most of their nutrients, bright colors, and fresh flavors. Of course, this rule does not apply to every vegetable: Potatoes, for instance, need to be cooked until tender, or they will be inedible.
Because roasting and baking are dry-heat methods, they can be used successfully for vegetables with a thick skin that will retain the vegetable’s internal moisture. Examples of this are baked potatoes or winter squash. Other vegetables need oil, moisture, or a combination to keep them from drying out as they cook. Very wet vegetables, such as tomatoes, are sometimes roasted in order to partially dehydrate them, thus concentrating their flavors.
This high-temperature, dry-heat method cooks vegetables quickly and preserves their flavor and texture; but it also requires oil to prevent them from burning. Use a pastry brush or an oil spray for a light and even coating of oil.
Although traditional sautéing and stir-frying require a good deal of oil, you can adapt these techniques to make them more healthful: Cook the vegetables in a nonstick skillet with a bit of oil and some water or broth. The broth and the natural moisture from the vegetables will pan-steam the food.
Then, as the moisture evaporates, the small amount of oil that’s left (and is now coating the vegetables), will help the vegetables brown a bit.
Of the various cooking methods, boiling takes the severest toll on vegetable nutrients, causing most of the vitamin C and other water-soluble vitamins to leach into the cooking liquid. The heat can also destroy vitamin C, and if thiamin is present, it converts this B vitamin into a form that the body cannot utilize. So except where boiling is unavoidable (huge quantities of corn on the cob, for example), boiling is not recommended.
For simple vegetable side dishes, steaming is one of the best methods available. It is quick and keeps nutrient loss to a minimum. If you eat a lot of vegetables, it would be wise to buy a Chinese-style bamboo steamer. It is larger than most steamer inserts and will fit over most large saucepans or skillets (as well as woks).
This is a method very similar to healthful sautéing or stir-frying. If you cook vegetables in just a small amount of water or broth (not necessarily with oil) in a tightly closed pot, the added liquid will come to a boil and start to cook the vegetables. The vegetables will then give up their own liquid. The two liquids together will turn into steam and cook the vegetables. This works best for relatively soft vegetables (such as onions) or harder vegetables that have been thinly sliced or shredded. For leafy vegetables, such as spinach and other greens, the only additional moisture needed to cook them is the water clinging to their leaves after they have been washed.
Microwaving, like steaming, is a good, nutrient-preserving cooking method. Consult your microwave oven manual for vegetable-specific information.
In these wet-heat methods, vegetables are cooked in a flavorful liquid with little to no fat. In the case of stews and soups, the cooking liquid gets eaten as part of the dish, thus preserving a maximum amount of the vegetable's nutrients. For braised vegetables, the cooking liquid can be flavorful enough to serve along with the vegetable, as a sauce.
From The Wellness Kitchen, by the editors of the University of California, Berkeley Wellness Letter and the staff of the Wellness Kitchen