Food 101

A Bum Rap for Microwaving

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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.


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