Friday Favorites: Does Pressure Cooking Preserve Nutrients?

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In a review of more than a hundred articles about the effects of cooking on vegetables, they tried to find the sweet spot. On the one hand, heat can destroy certain nutrients; on the other hand, by softening the tissues, they can become more bioavailable. Researchers settled upon steaming as the best cooking method to preserve the most nutrition. You’re not dunking it in water or oil where the nutrients can leach out, and you’re not reaching excessive dry heat temperatures. But they acknowledge that of all the common cooking methods, we know the least about pressure cooking.

There are all these fancy new electric pressure cookers on the market, including the Instant Pot, with more five-star ratings than even How Not to Die—I’m jealous! These pressure cookers are great for cooking dried beans with just a touch of a button. But what happens to the nutrition? Here’s the antioxidant content of presoaked black beans boiled for an hour normally. Compare that to pressure cooking for 15 minutes…. Even more. In fact, six times more! Wow. Here I was pressure cooking just because I liked the texture better (the canned ones can be a bit mushy), and I was spending lots of money on cases of canned beans, whereas dried beans are just so dirt cheap. So wait, cheaper, tastier, and healthier? That’s quite a combo. Okay, but what about pressure-cooking vegetables?

Vitamin C is one of the more heat-sensitive nutrients. Sauté spinach or amaranth leaves in a pan for 30 minutes, and about 95 percent of the vitamin C is destroyed, whereas 10 minutes in a pressure cooker wiped out only about 90 percent. But who pressure cooks spinach for 10 minutes? And sautéing for half an hour? And even then, not much effect on beta carotene levels either way.

Vitamin C is but one of many antioxidants. What about the effects of pressure cooking on overall antioxidant capacity? Here’s the cooking methods they compared. So, for the carrots, for example: 12 minutes of boiling, compared to five minutes of pressure cooking, compared to six minutes of microwaving. Here’s what they found. Cooking carrots increased their antioxidant potential. In fact, pressure cooking nearly doubled their antioxidant value, whereas peas took a hit no matter how they were cooked.

I’m particularly interested in the greens. The chard wasn’t affected much across the board, but microwaving beat out pressure cooking and boiling for the spinach. Note that pressure cooking beat out boiling too, though, and pressure cooking is boiling (just at a shorter time at a higher temperature.) But the time appeared to trump the temperature. Significantly less nutrient loss pressure-cooking spinach for three and a half minutes, compared to boiling for eight.

Same for those magical cancer-fighting glucosinolate compounds in cruciferous greens—the healthiest greens like kale, collards, and turnip greens. Here’s where levels started out raw, with three-quarters wiped out by boiling—but less than half with pressure cooking. Now, both got beat by steaming, but that’s because you weren’t dunking the greens in water, which can leach out the nutrients. But even though the pressure-cooked greens were immersed just as much as the boiled greens, only half the nutrient losses, presumably because it was only half the time—seven minutes pressure-cooked compared to 15 minutes boiled.

Okay, so here’s my idea. This was after 10 minutes of steaming. What if you cut down that time by pressure steaming, like put a layer of water down at the bottom of an electric pressure cooker, drop in a metal steaming basket on top, and then put the greens in and steam under pressure? That’s how I cook the greens I eat every day. I’ve always loved collards in like Southern cooking, or Ethiopian cuisine, and I found I could get that same melt-in-your-mouth texture just steaming under pressure for zero minutes. What you do is set it for zero, so it shuts off as soon as it reaches the cooking pressure, and then quick release valve it immediately to release the steam. The greens turn out perfect—bright emerald green, cooked tender. Give it a try, and let me know what you think.

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• Natella F, Belelli F, Ramberti A, et al. Microwave and traditional cooking methods: effect of cooking on antioxidant capacity and phenolic compounds content of seven vegetables. J. Food Biochem. 2010;34(4):796 - 810.
• Palermo M, Pellegrini N, Fogliano V. The effect of cooking on the phytochemical content of vegetables. J Sci Food Agric. 2014;94(6):1057-70.
• Vieites-Outes C, Lopez-Hernandez J, Lage-Yusty M. Modification of glucosinolates in turnip greens (Brassica rapa subsp. rapa L.) subjected to culinary heat processes. 2016;14(4)536-540.
• 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):125-31.
• Xu BJ, Chang SK. Total phenolic content and antioxidant properties of eclipse black beans (Phaseolus vulgaris L.) as affected by processing methods. J Food Sci. 2008;73(2):H19-27.

Motion graphics by Avocado Video