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A Bum Rap for Microwaving

By Robert L. Wolke
Wednesday, March 1, 2006

A magazine article I read about cooking fresh broccoli strongly advised against microwaving the vegetable. It said nutritional and health benefits are lost in this cooking method. Is this true? Is steaming broccoli better?

How about other fresh vegetables (Brussels sprouts, cauliflower, asparagus, etc.)? Microwaving is the only option for cooking vegetables at my office, unless we want to eat them raw.

I don't like raw broccoli or cauliflower.

This warning has been circulating ever since the November 2003 issue of the Journal of the Science of Food and Agriculture reported a study by F. Vallejo et al., a group of researchers at Spain's Center for Edaphology and Applied Biology in Murcia. (Edaphology is the study of soils and their effects on plants and other organisms.) To many people, this study seemed to indicate that microwaves kill the nutrients in broccoli and presumably in other vegetables as well.

The researchers cooked broccoli florets in four ways: microwaved, steamed, boiled and pressure- cooked. Among other things, they measured the percentage of healthful flavonoids removed from the vegetable by each cooking method. Flavonoids are phenolic compounds that are widely distributed in fruits and vegetables. In addition to giving these foods their colors, they behave as antioxidants; that is, they destroy the free radicals that can damage our DNA, possibly leading to cancer, stroke and other diseases. Flavonoids are therefore among the "good guys" in our foods.

The Spanish researchers reported that microwaving broccoli removed 97.2 percent of its flavonoids, boiling removed 66.0 percent, steaming removed 11.1 percent and pressure-cooking removed 8.8 percent.

Almost instantly after the publication of this research, a London-datelined news story trumpeted, "Microwave blasts out broccoli's health benefits." An article in Prevention magazine was headlined "Nuking Broccoli a No-No." And dozens of other media stories carried the study's purported finding that "microwaving destroys foods' nutrients."

Obviously, then, we should not microwave our vegetables if we want to retain their nutrients. A scientific study said so. Right?

Wrong. The study showed no such thing. In fact, as I'll show, microwaves can be the best way to cook vegetables to ensure minimal loss of nutrients.

Findings, Shortcomings

I obtained a copy of the original research report, which is hardly a document to be skimmed by the uninitiated reader. But I was able to analyze the experiments and determine exactly what the study did and did not prove. Here's what I found:

· The researchers measured the before-cooking and after-cooking amounts only of total flavonoids and two flavonoid derivatives. However, they did not analyze the many other kinds of antioxidants or for vitamins or minerals, so generalizing these results to all nutrients is completely unjustified.

· During microwaving, pressure-cooking and boiling, the broccoli was immersed in water, while for steaming the broccoli was placed on a rack above boiling water. The researchers reported that flavonoids in the broccoli were diminished to some extent by all four cooking methods. This is only to be expected, because all flavonoids are soluble in water. As borne out by the Spanish data, exposure to steam removes relatively little water-soluble material, compared with boiling in water.

· The microwaving was done at 1,000 watts for five minutes, boiling for five minutes, steaming for 3 1/2 minutes and pressure-cooking (at an unspecified pressure) for three minutes. According to the report, these conditions were selected by "an informal testing panel consisting of three trained people, who decided the best combination among different options for each of the four treatments," whatever that means. These seemingly arbitrary sets of conditions cannot be construed as universal characterization of the four cooking methods. In actual kitchen practice, the microwave power, pressure- cooker pressure, cooking times and amounts of water will vary widely, affecting the amounts of flavonoids extracted. One cannot simply say that "boiling does this" or "microwaving does that."

Because the Spanish study has been misunderstood and misreported, it has triggered undue mistrust of microwaving. But the Spanish research results have nothing to do with the effects of microwaves. As far as anyone knows, microwave radiation itself does not destroy or affect antioxidants, vitamins, minerals or any other nutrients in our foods.

A microwave oven is simply a high-tech way of heating food, and it is heat, whether from burning fuel, an electric heating element or absorbed microwave energy, that changes the properties of food in ways we call cooking. The more heat -- that is, the higher the temperature and the longer the food is held at that temperature -- the more cooking changes take place. And all cooking inevitably causes a certain amount of nutrient loss. For example, the B and C vitamins and various antioxidants, including flavonoids, not only leach out into the cooking water but also are unstable to heat.

Water-Free Cooking

The moral, then, is that we should cook our vegetables for as short a time and with as little water as we can get away with. We can't do without heat in cooking, but we can reduce the amount of water.

Well, how about using no water at all? After all, the vegetables are already about 90 percent water.

I have experimented with the water-free microwave cooking of broccoli, cauliflower and Brussels sprouts, with excellent results. Here is the procedure: Rinse the vegetables in cold water and dry with a clean kitchen towel. Place the vegetables in a small, covered, microwave-safe container. Do not add water. Zap at full power for three to four minutes or until vegetables are tender. Do not overcook.

And that's it. The vegetables will have cooked in their own steam, with no loss of water-soluble nutrients except, perhaps, as caused by the heat.

How, then, can I explain the Spanish researchers' finding that microwaving broccoli destroyed almost all of its flavonoids? I can't, and neither can they. As the authors themselves point out, "This flavonoid loss rate does not agree with that previously reported by other authors for microwaving."

All of which illustrates an even more important moral: Neither journalists nor health-conscious consumers should go running around like Chicken Little because of a single research study -- even if it has been carefully designed and accurately interpreted. We must remember that every "latest study" will someday be replaced by a later "latest study" that might either confirm or refute it.

That's how science progresses; it zigs and zags toward the truth.

Robert L. Wolke (http://www.robert (wolke.com) is professor emeritus of chemistry at the University of Pittsburgh. His latest book is "What Einstein Told His Cook 2, the Sequel: Further Adventures in Kitchen Science" (W.W. Norton, 2005). He can be reached at wolke@pitt.edu.

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