Summer Squash
Yellow crookneck, yellow straightneck, zucchini)
Zucchini and the yellow summer squashes are high in dietary fiber: insoluble cellulose and lignin in the seeds and peel and soluble pectins in the vegetable itself.
Green and yellow summer squashes have small to moderate amounts of vitamin A derived from yellow carotenes (including beta-carotene) in the skin. Zucchini and the yellow crookneck and straightneck squashes also have some vitamin C.
One half-cup (four ounces) boiled zucchini slices has 1.2 g dietary fiber, 1,005 IU vitamin A (44 percent of the RDA for a woman, 34 percent of the RDA for a man), 18 mcg folate (5 percent of the RDA), and 4 mg vitamin C (5 percent of the RDA for a woman, 4 percent of the RDA for a man). A similar serving of yellow crookneck or straightneck squash has 1.3 g dietary fiber, 147 IU vitamin A (6 percent of the RDA for a woman, 5 percent of the RDA for a man), 15 mcg folate (4 percent of the RDA), and 5 mg vitamin C (7 percent of the RDA for a woman, 6 percent of the RDA for a man).
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Steamed quickly in very little water, to preserve the vitamin C.
Low-fiber diet
Look for: Dark green slender zucchini with pale yellow or white striping. Yellow crookneck squash should be brightly colored with lightly pebbled skin. Yellow straightneck squash may have either smooth or pebbled skin.
Choose smaller (and therefore more tender) squash. The best zucchini are four to nine inches long; the best crooknecks and straightnecks are four to six inches long.
Avoid: Limp squash. They have lost moisture and vitamins. Avoid squash whose skin is bruised or cut; handle squash gently to avoid bruising them yourself. Bruising tears cells, activating ascorbic acid oxidase, an enzyme that destroys vitamin C. Avoid squash with a hard rind; the harder the rind, the older the squash and the larger and harder the seeds inside.
Refrigerate summer squash, which are perishable and should be used within a few days.
Scrub the squash with a vegetable brush and cut off each round end. Peel older, larger squash, then slice them in half and remove the hard seeds. Younger, more tender squash can be cooked with the peel and seeds.
As the squash cooks, its cells absorb water, the pectins in the cell walls dissolve, and the vegetable gets softer. The seeds, stiffened with insoluble cellulose and lignin, will remain firm.
Chlorophyll, the pigment that makes green vegetables green, is sensitive to acids. When you heat zucchini, its chlorophyll reacts with acids in the vegetable or in the cooking water to form pheophytin, which is brown. The pheophytin makes cooked zucchini look olive-drab. To keep the cooked zucchini green, you have to keep the chlorophyll from reacting with the acids. One way to do this is to cook the zucchini in a large quantity of water (which will dilute the acids), but this increases the loss of vitamin C. A second alternative is to leave the top off the pot so that the volatile acids can float off into the air. Or you can stir-fry the zucchini or steam it in very little water so the vegetable cooks before the chlorophyll/ acid reaction can occur.
Yellow squash stays bright yellow no matter how long you cook it; its carotene pigments are impervious to the normal heat of cooking.
Canning. Canned zucchini has about as much vitamin C as fresh-cooked zucchini.
Lowering the risk of some cancers. According to the American Cancer Society, foods rich in beta-carotene may lower the risk of cancers of the larynx, esophagus, and lungs. There is no similar benefit from beta-carotene supplements; indeed, one controversial study actually showed a higher rate of lung cancer among smokers taking the supplement.
