How to Cook Greens

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

“The main purpose of cooking vegetables is to make them more edible, palatable, and digestible.” The downside, though, is that “cooking may adversely affect the levels of nutrients, especially the heat-sensitive and water soluble ones.” But even if you boil greens for 10 minutes, the drop in antioxidant capacity, for example, which is a rough proxy for phytonutrient retention, isn’t that much. Yes, there’s a significant drop in each case—a 15 to 20 percent drop—but most of the antioxidant power is retained, even if you boiled lettuce for 10 minutes. The single nutrient that drops the most is probably vitamin C, but as you can see, collards start out so vitamin C-packed that even collard greens boiled for 10 minutes have twice as much vitamin C compared to even raw broccoli.

You can see the vitamin C in spinach really takes a hit. Even just blanching for five minutes can cut vitamin C levels more than half, with more than 90 percent dissolving away into the water after 15 minutes, though most of the beta carotene, which is fat soluble, tends to stay in the leaves. But just keeping it in a regular plastic bag, like you get in the produce aisle, can protect it. The refrigeration is important, though. Even in a bag, a hot day can wipe out nearly 50 percent. Not as bad as drying, though, which can wipe out up to 90 percent of the vitamin C, suggesting that something like kale chips may pale in comparison to fresh—though vitamin C is particularly sensitive. Other nutrients, like beta carotene, are less affected across the board.

Cooking by microwaving and steaming preserves the nutrition more than boiling, here measured in watercress. A little steaming or microwaving hardly has any effect compared to raw, though boiling even two minutes may cut antioxidant levels nearly in half. Watercress is a cruciferous vegetable, though—a cabbage- and broccoli-family vegetable—so it’s prized for its glucosinolate content, which turns into that magical cabbage compound sulforaphane. What does cooking do to it? Fresh is best, but steaming’s not bad, with microwaving coming in second, and then stir-frying and boiling at the bottom of the barrel. The glucosinolates in other cruciferous vegetables are also significantly affected by boiling. The researchers conclude that red cabbage is best consumed fresh, and look, not just in salads. As I talked about in How Not to Die, I always keep a red or purple cabbage in my crisper to cruciferize my meals, slicing off shreds and putting it on basically anything. But if you are going to cook it, steaming may be the best bet, “so as to retain the optimum benefits of the health-promoting compounds.”

Other nutrients we look to greens for are the eyesight-preserving nutrients like lutein, which I’ve talk about before, and folate, particularly important for women of child-bearing age; and vegetables are the main natural source. It’s been estimated that approximately half the folate is lost during cooking, which may be true for boiling broccoli, or stir-frying spinach or mustard greens. But the folate in stir-fried kale holds up better, only losing about a quarter, similar to steamed broccoli florets. But note that broccoli starts out so high that even boiled broccoli has more folate than raw spinach. But check out broccoli leaves. Not only do they start out with the highest levels, the levels actually go up a bit when you cook them. No one’s ever looked at the folate concentration in broccoli leaves, which ironically are commonly just cut off and thrown away, yet contribute great concentrations of this vitamin. Therefore, we should make sure to eat them.

Note they also compared thinly-sliced kale to kale just torn into larger pieces, to determine if a larger surface of exposure would promote greater losses of folate in kale. However, no effects were found, so slice away. Here, they just looked at stir-frying. What about the effect of other cooking methods on kale? There’s lots of studies on cooking cabbage and broccoli. However, very little information has been available on the queen of greens…until now.

First of all, fresh versus frozen. “The freezing process is generally regarded as destructive to antioxidant compounds.” One just assumes that frozen would have a lower antioxidant capacity compared to fresh, but kale breaks all the rules. The frozen kale showed a higher antioxidant capacity than fresh. And not just by a little; we’re talking 60 percent more. Wow! Okay, what happens when you cook it? If you start out normalizing the starting levels at 100 percent, blanching and steaming actually boost the antioxidant content, whereas microwaving or even boiling doesn’t seem to do much—so you can boil kale without losing out on its antioxidant punch. I told you kale’s a rule breaker.

But check out that blanching and steaming. Heat can disrupt the plant cell walls and all the little subcellular compartments, and spill out extra antioxidant compounds that may have been hiding. Now that’s usually counterbalanced by losses caused by high temperatures, but the kale compounds are looking pretty cruciferocious, and stood their ground.

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• Xu F, Zheng Y, Yang Z, Cao S, Shao X, Wang H. Domestic cooking methods affect the nutritional quality of red cabbage. Food Chem. 2014;161:162-167.
• Giallourou N, Oruna-Concha MJ, Harbourne N. Effects of domestic processing methods on the phytochemical content of watercress (Nasturtium officinale). Food Chem. 2016;212:411-419.
• Della Lucia CM, Mota ER, Montini TA, Ribeiro SM, Chaves JB, Pinheiro-Sant'Ana HM. Folates retention in brassica vegetables consumed in Brazil after different cooking methods. Arch Latinoam Nutr. 2014;64(1):59-68.
• Vinha AF, Alves RC, Barreira SV, Costa AS, Oliveira MB. Impact of boiling on phytochemicals and antioxidant activity of green vegetables consumed in the Mediterranean diet. Food Funct. 2015;6(4):1157-1163.
• Yadav SK, Sehgal S. Effect of home processing on ascorbic acid and beta-carotene content of spinach (Spinacia oleracia) and amaranth (Amaranthus tricolor) leaves. Plant Foods Hum Nutr. 1995;47(2):125-131.
• Zaraska M. Bitter truth: How we’re making fruit and veg less healthy. [online] New Scientist.
• Ilyasoğlu H, Burnaz NA. Effect of Domestic Cooking Methods on Antioxidant Capacity of Fresh and Frozen Kale, Int J Food Prop. 2015;18(6):1298-1305.
• Greger M, Stone G. How not to die: discover the foods scientifically proven to prevent and reverse disease. New York: Flatiron Books; 2015.

Image credit: pxhere. Image has been modified.

Motion graphics by Avocado Video

• antioxidants
• beta carotene
• boiling
• broccoli
• cabbage
• cooking methods
• cruciferous vegetables
• Daily Dozen
• folate
• greens
• kale
• lettuce
• lutein
• microwaving
• phytonutrients
• sulforaphane
• vegetables
• vitamin C
• watercress