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.
When 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 of both to keep them from drying out as they cook. Very wet vegetables, such as tomatoes, are sometimes roasted in order to partially dehydrate them, which concentrates 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.
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