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Does Cooking Boost or Bust Nutrients in Vegetables?

Fine Cooking Issue 57

Which do you think delivers more nutrients—a raw carrot or a cooked one? If you’re like most people, you’d probably pick the raw one. And you’d be wrong. Contrary to popular belief, some vegetables actually benefit nutritionally from cooking. It’s also true that cooking can destroy certain nutrients. And you might be surprised to learn that in some cases, cooking doesn’t make that much of a difference at all.  

Another generally unrecognized fact is that the nutrient content of a particular kind of vegetable (or fruit) can vary immensely. One grapefruit can contain 200 times more vitamin A than another. Genetic differences among varieties account for some of this range, but growing conditions, the time of harvest, and handling and storage conditions are also factors. For example, snap beans build up their stash of vitamin C and carrots develop more beta-­carotene as they mature. Some produce, such as spinach, can lose up to half its total vitamin C in 24 hours if it isn’t refrigerated. So be aware that even before you’ve settled on whether to steam or sauté, some significant nutrient loss may have already occurred.

The raw truth

Salads and fresh vegetable dishes are of course wonderfully healthy, but when I hear about people who adamantly eat only raw vegetables, I get worried. Some vegetables actually become more nutritious after cooking. For carrots, cooking softens its firm cell structure—we can access many more carotenes and minerals and more vitamin C in tender cooked carrots than we can by simply chewing raw, crunchy ones.

Corn is another example of how cooking can unlock nutrients. Corn contains lysine, one of the essential amino acids (which is a building block of proteins), but it’s in a form that our bodies can’t use. By cooking corn with a strong alkali (such as ashes, burnt shells, lye, or limestone), as some cultures that lived predominantly on corn did, the lysine converts to a usable form. Today, we get our lysine from other food sources.

Finally, I think it’s interesting to note that from an evolutionary standpoint, cooking vegetables has had important consequences. According to Dr. Richard Wrangham, a professor of anthropology at Harvard University, cooking allowed the development of a larger brain. Cooking made plant foods, particularly starchy tubers such as potatoes and manioc, softer and easier to chew and substantially increased their available energy. This greater food intake enabled Homo erectus to develop a larger brain.

How to preserve the most nutrients during cooking

So how much damage really occurs to nutrients when you boil or steam or sauté or roast vegetables? It’s difficult to say exactly, but the differences among various cooking methods isn’t as great as you might think. For minerals like iron, calcium, magnesium, potassium, and sodium, the loss is only in the range of 5 to 10 percent no matter how the vegetable is cooked. Vitamin loss, on the other hand, can be much higher, although many vegetables (such as tomatoes, eggplant, asparagus, and green beans) retain 80 percent or more of their vitamins regardless of the cooking method. 

Nutrients escape from vegetables in two ways: by getting dissolved in the cooking water or by getting destroyed by heat. 

Water-soluble compounds (which include vitamin C and some B vitamins) are the most vulnerable to loss from boiling, simmering, steaming, or braising. The simplest way to minimize the loss of these nutrients is to choose a cooking method that doesn’t involve water, or to cook vegetables in a way that the cooking liquid remains part of the dish, as it does in a casserole or a braise. 

For minerals, the loss is only in the range of 5 to 10 percent no matter how the vegetable is cooked

When you do choose to boil vegetables, adding them to the water after it has come to a full boil (as you would when blanching) minimizes the loss of vitamin C. If you start them in cold or warm water, enzymes in the vegetables become very active—at boiling temperatures, the enzymes are deactivated. Some of these enzymes are major destroyers of vitamin C, eliminating 20 percent of the vitamin within the first two minutes of cooking. Since blanching is a better way to cook many green vegetables, this method works out quite well.  

Another way to moderately limit nutrient loss is to cook vegetables in a minimum of water with a lid, which reduces the loss of water-soluble vitamins and volatile nutrients. But be aware that the crisp green color of some vegetables will become drab, and vegetables from the Brassica family (broccoli, cabbage, and cauliflower, for example) will become very strong tasting.  

Rapid cooking can help preserve some thiamine and vitamin C, which are the most susceptible to destruction by heat. Stir-frying and sautéing, both fast methods, are good techniques for minimizing nutrient loss as well as for keeping green vegetables bright. Another tip is to cut vegetables into smaller pieces to reduce the cooking time.  

Stir-frying and sautéing are good techniques for minimizing nutrient loss

Cooking has little detrimental effect on the nutritive value of carotenoids, which are precursors to vitamin A found in orange and yellow vegetables like carrots, sweet potatoes, squash, and rutabagas. Carotenoids (betacarotene is one example) are also found in red vegetables like tomatoes and in dark-green vegetables like broccoli and spinach. Studies suggest that cooking may even boost this nutrient in some cases

Carotenoids deteriorate with lengthy exposure to oxygen. Dried carrot chips, for example, lose their betacarotene when packaged in air-but retain it when packaged with nitrogen.  

Keeping it in perspective. When it’s time to start cooking, I’m thinking about food as more than just sustenance. For me, the decision of how to cook my vegetables includes things like flavor, appearance, time, and which method my family prefers. After all, it doesn’t matter how nutritious your vegetables are if you can’t get anyone to eat them.

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