Modern varieties of vegetables, the ones you see for sale in the produce section of the supermarket, are generally sweeter, starchier, and less fibrous than their wild ancestors. They are also far less nutritious: wild dandelion leaves, for example, have eight times more antioxidants than spinach and forty times more than iceberg lettuce.
So doing what you can to maximize the nutrients in the vegetables you eat is important—but it turns out that many common cooking habits are actually making vegetables less nutritious. Did you know you should wait 10 minutes before cooking chopped garlic? Or that broccoli is one of the most perishable vegetables in your crisper? Investigative journalist Jo Robinson spent 10 years combing through the latest research on nutrients in vegetables and fruits for her book, Eating on the Wild Side, and her evidence-based tips for storing and preparing vegetables will change the way you cook.
When you hear “nutrients,” you may only be thinking of vitamins, such as vitamins A and C, or minerals like calcium and iron. But vegetables are also a source of phytonutrients, the powerful antioxidants that plants produce in order to protect themselves from harmful UV light or damage from scavenging insects. “What [the scientific community] is discovering is that consuming these phytonutrients plays the same role for us, ” Robinson told me. “It protects us from external and internal threats.” Lycopene in tomatoes, resveratrol in red wine, and anthocyanins in blueberries are just a few of the phytonutrients scientists are excited about, and their names may be familiar to you.
Research on phytonutrients is relatively new, which is why tips about how to make the most of them in the kitchen are not yet common knowledge. Time to change that! Here are 10 ways you may be unknowingly making your vegetables less nutritious.
1. Buying fresh tomatoes instead of canned.
Cooking tomatoes makes them more nutritious, and the longer you cook them, the better. Heat changes the lycopene into a form our bodies can more readily absorb and—surprise!—canned tomatoes are much higher in phytonutrients, thanks to the heat of the canning process. Tomato paste, being more concentrated, is even better.
2. Storing lettuce wrong.
You might think that damaging your vegetables before storing them is a mistake, but when it comes to lettuce, tearing the leaves triggers a protective blast of phytonutrients that you can take advantage of by eating the greens within a day or two. Lettuce that is torn before storing can have double the antioxidants of whole lettuce leaves.
3. Boiling spinach—or any vegetable really.
You may have heard that boiling vegetables is a no-no because water-soluble vitamins like vitamin C leach out of the food and into the cooking water, but you might not know that boiling also reduces the antioxidant content. The difference in spinach is especially dramatic: after 10 minutes of boiling, three-quarters of its phytonutrient content is in cooking water, not in the vegetable itself. (Of course, if you consume the cooking liquid, as you do when making soup, you consume all the nutrients in the water as well.)
Steaming, microwaving, sautéing, and roasting—cooking methods that don’t put vegetables in direct contact with water—result in more nutritious vegetables on the plate.
4. Eating your salad with fat-free salad dressing.
We’ve known for a few years that you absorb more of the nutrients in salad when you eat it with fat, but the type of fat can make a difference. Most commercial salad dressings use soybean oil, but extra-virgin olive oil is much more effective at making nutrients available for absorption. Unfiltered extra-virgin olive oil is even better, as it contains double the phytonutrients of filtered.
5. Cooking garlic right after chopping it.
If you mince a clove of garlic and quickly throw it in a hot pan, you consume almost no allicin, the beneficial compound that makes garlic such a health star. That’s because the enzyme that creates allicin is not activated until you rupture the cell walls of the garlic—and is quickly inactivated by heat. Just two minutes in a hot pan or 60 seconds in the microwave reduces the allicin in just-chopped garlic to almost nothing.
Letting the chopped garlic sit for 10 minutes before exposing it to heat gives the enzyme time to do its work, so your finished dish contains the maximum amount of allicin. Using a garlic press is even better than mincing, as it releases more of the compounds that combine to create allicin.
6. Throwing away the most nutritious parts of the vegetable.
Most American recipes call for only the white and light green parts of scallions, but the dark green parts have a higher concentration of phytonutrients. Instead of throwing out the nutritious tops, you can ignore the recipe instructions and toss in the green parts as well, or explore recipes from elsewhere in the world—such as Chinese scallion pancakes—which utilize the entire green onion.
Beet greens are another often-discarded vegetable part that we would be better off eating; they have more antioxidants than the beet roots, which are already high in phytonutrients. Try cooking and eating the greens alongside the roasted roots in recipes like Warm Golden Beet Salad with Greens and Almonds.
And don’t forget vegetable peels, which often contain a higher concentration of antioxidants than the rest of the vegetable. Try roasting them and eating them like chips!
7. Eating potatoes right after cooking them.
Many people avoid white potatoes because they are a high-glycemic vegetable, spiking blood sugar after eating. But chilling potatoes for about 24 hours after cooking converts the starch in the potatoes to a type that is digested more slowly, making them a low-glycemic vegetable. So potato salad chilled overnight is a low-glycemic food, as is a cooked, chilled, and reheated baked potato.
8. Cutting carrots before you cook them.
Cooking carrots whole and cutting them up after they are cooked keeps more nutrients in the vegetable. And speaking of cooking, carrots are one vegetable that is better for you cooked than raw—cooking helps break down the cell walls, making the nutrients easier to absorb.
9. Buying broccoli florets, instead of a whole head.
Broccoli looks like a hardy vegetable, but from an antioxidant standpoint, it is shockingly perishable, quickly exhausting its stores of powerful phytonutrients after harvest. “I call it one of the ‘eat me first’ vegetables,” says Robinson. One study found that after 10 days—the time it took to get the vegetable from field to supermarket produce section—broccoli lost 75 percent of its flavonoids (a type of antioxidant) and 80 percent of its glucosinolates, the compounds in cruciferous vegetables that are associated with numerous health benefits.
Cutting the broccoli into florets doubles the rate of antioxidant loss, so in addition to buying the freshest broccoli you can find and cooking it right away, you should choose whole heads rather than the bags of pre-cut florets.
10. Cooking beans from scratch and discarding the cooking liquid.
Dried beans are some of the most phytonutrient-rich foods out there, but the big surprise is this: canned have more antioxidants! If you prefer from-scratch beans, let the beans sit in the cooking liquid for about an hour after cooking to reabsorb some of the nutrients that have moved into the liquid. And try using a pressure cooker to cook beans; one study found that beans cooked in the pressure cooker had more antioxidants than those cooked with other methods.
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