The Best Way to Cook Vegetables

Today, we look at which method of cooking vegetables preserves the most antioxidants. Here’s our first story.

You may remember when I compared the effects of different cooking methods on certain phytonutrients in bell peppers, mushrooms, and a few other veggies. Well, food scientists have outdone themselves in this study. They looked at six different cooking methods on 20 different vegetables, comparing three distinct measures of antioxidant activity. That’s more than 300 separate experiments to figure out the best way to cook our vegetables.

First, though, let’s figure out the worst in terms of loss of antioxidant content. Consider boiling, microwaving, pressure-cooking in water, griddling, like cooking in a frying pan without oil, frying in a frying pan, and baking at about 400 degrees Fahrenheit (200°C). What’s your guess for the worst? The worst is boiling when it comes to losing antioxidants. The second worst? Boiling under pressure. When we use wet cooking methods, some of the nutrition is lost into the cooking water; when it comes to preparing vegetables with their antioxidants in mind, water is not a cook’s best friend.

But how bad is bad? It may be less than you think. Averaging over those 20 vegetables, boiling only removes about 14% of the antioxidants. So, if you really like boiled broccoli, fine—just eat one more floret. Seven florets of boiled broccoli have all the antioxidant power of six florets of raw broccoli. So, the best way to eat your veggies is really whichever way will get you to eat the most of them—with the exception of frying, which not only adds empty calories but also toxic byproducts that are produced when oils are heated to frying temperatures.

What’s the gentlest cooking method? The one that preserves the most antioxidants? Microwaving, which preserves more than 97% of the antioxidants.

One vegetable has antioxidants that can really get clobbered, no matter how you cook it, losing up to 75 % of its antioxidant capacity. What’s the one vegetable that’s really best eaten raw? Was it artichoke hearts, asparagus, beets, broad beans (also known as fava beans), broccoli? – I hope we don’t have to eat raw Brussels sprouts – cauliflower, carrots, celery, eggplant, garlic, green beans, leeks, corn on the cob, onions, peas, bell peppers, spinach, Swiss chard, or zucchini? The most vulnerable vegetable is the bell pepper. So, I try to eat those raw or add them right at the end to whatever I’m cooking, like pasta sauce, so they just kind of get warmed up without really getting cooked.

On the other hand, three vegetables are hardly affected by cooking at all. You could even boil them and still retain nearly all their antioxidants. Can you guess at least one of the three? They’re artichokes, beets, and onions. Boil away. Asparagus gets an honorable mention here. It’s pretty much unaffected by any cooking method except for frying, so you can boil asparagus, too.

Final question, and perhaps it’s the most interesting. There are two vegetables that, no matter what you do to them, tend to increase in antioxidant value. They may become even healthier upon cooking. Which two are they? First, the runner-up: green beans. With the exception of boiling and pressure cooking, they actually increase in antioxidant power when cooked, so microwaved green beans have more antioxidants than raw.

And, tied for first? Which two vegetables nearly always increase in value, no matter how you cook them? Carrots and celery. So, when we make a nice vegetable soup, we can actually boost their nutrition.

One vegetable that wasn’t covered was one of my favorites—kale. As I discuss in my video How to Cook Greens, kale can be cooked by blanching or steaming to boost antioxidants. It can even be boiled and not lose antioxidant nutrition.

When I used to teach medical students at Tufts, I gave a lecture about this amazing new therapeutic called iloccorB. I’d talk about all the new science, all the things it could do, excellent safety profile and just as they were all scrambling to buy stock in the company and prescribe it to all their patients, I did the big reveal, apologizing for my dyslexia, I had got it backwards. All this time, I had been talking about broccoli.

Sulforaphane, is thought to be the main active ingredient in broccoli, which may protect our brain, protect our eyesight, protect against free radicals, induce our detoxification enzymes, help prevent cancer, as well as help treat it. For example, I’ve talked about sulforaphane targeting breast cancer stem cells.

But then I talked about how the formation of this compound is like a chemical flare reaction, requiring the mixing of a precursor compound with an enzyme in broccoli, which is destroyed by cooking. This may explain why we get dramatic suppression of cancer cell growth from raw broccoli, cauliflower, Brussels sprouts, but hardly anything boiled, microwaved, or steamed, except for microwaved broccoli —that actually retained some cancer fighting abilities. But who wants to eat raw Brussels sprouts?

I shared a strategy, though, for to how to get the benefits of raw in cooked form. In raw broccoli, when the sulforaphane precursor, called glucoraphanin, mixes with the enzyme, called myrosinase, because you chopped or chewed it, given enough time—sitting in your upper stomach for example, waiting to get digested, sulforaphane is born. Now the precursor is resistant to heat, and so is the final product, but the enzyme is destroyed. And with no enzyme, there’s no sulforaphane production.

That’s why I described the hack and hold technique. If you chop the broccoli, Brussels sprouts, kale, collards, or cauliflower first, and then wait 40 minutes, then you can cook them all you want. The sulforaphane is already made, the enzyme has already done doing its job, so you don’t need it anymore.

When most people make broccoli soup, for example. they’re doing it wrong. Most people cook the broccoli first, then blend it, but now we know it should be done the exact opposite way. Blend it first, wait, and then cook it. What if we’re using frozen broccoli, though?

Commercially produced frozen broccoli lacks the ability to form sulforaphane because vegetables are blanched, flash-cooked, before they’re frozen for the very purpose of deactivating enzymes. This prolongs shelf life in the frozen foods section, but the enzyme is dead by the time you take it out of your freezer, so it doesn’t matter how much you chop it, or how long you wait, no sulforaphane is going to be made. This may be why fresh kale suppresses cancer cell growth up to 10 times more than frozen.

The frozen is still packed with the precursor—remember that’s heat resistant, and they could make lots of sulforaphane out of the frozen broccoli by adding some exogenous enzyme. Where do you get myrosinase enzyme from? They bought theirs at a chemical company, but we can just walk into any grocery store.

This is another cruciferous vegetable, mustard greens. All cruciferous vegetables have this enzyme. Mustard greens, grow out of little mustard seeds, which you can buy ground up in the spice aisle as mustard powder. So, if you sprinkled some mustard powder on your cooked frozen broccoli, would it start churning out sulforaphane? We didn’t know, until now.

Boiling broccoli prevents the formation of any significant levels of sulforaphane due to inactivation of the enzyme. However, addition of powdered mustard seeds to the heat-processed broccoli significantly increased the formation of sulforaphane. Domestic cooking leads to enzyme inactivation of myrosinase and hence stops sulforaphane formation, but addition of powdered mustard seeds to cooked cabbage-family vegetables provides a natural source of the enzyme and then it’s like you’re practically just eating it raw. So, if you forget to chop your greens in the morning for the day, or are using frozen, just sprinkle some mustard powder on top at the dinner table and you’re all set. Or some daikon radish, or horseradish, or wasabi—all cruciferous vegetables packed with the enzyme. Here they used just like a quarter teaspoon for seven cups of broccoli, so just a tiny pinch can do it. Or you can add a small amount of fresh greens to your cooked greens. Right—because the fresh greens have that enzyme that can go to work on the precursor in the cooked greens.

One of the first things I used to do in the morning is chop my greens for the day and so when lunch and supper rolls around, they’re good to go, as per the hack and hold strategy, but now with the mustard powder plan I don’t have to prechop.