Inside the Microwave

Today’s kitchens are increasingly filled with conveniences that help get food on the table fast. Bagged salad greens. Rotisserie chickens. Single-serving instant oatmeal. Grated cheese. And when it comes to appliances, nothing beats the ubiquitous microwave for quick and easy cooking.

Most of us use our microwaves without giving it another thought: Pop in a frozen dinner or last night’s leftovers, and lickety-split it’s a hot meal. Despite worrisome terminology such as “radiation” and slang expressions like “nuking,” microwave ovens have worked their way into most kitchens and practically every break room in this country. They’re a staple of our lives.

But does anyone really know what’s going on inside the oven once you shut the door? How healthy or unhealthy is “nuking” your food? And if we can’t live without our microwave ovens, what’s the best way to live with them?

Know Your Nuker

Let’s start with some history: Around the end of World War II, an engineer discovered that the radar waves used to detect planes could also heat food. The first beastly microwave oven, called the Radarange, debuted in the late 1940s. A more user-friendly consumer countertop model hit store shelves two decades later. Soon afterward, the microwave became a miracle, must-have household appliance.

People often say they are going to “nuke” their food, which is based on the common belief that microwaves reheat food by releasing radioactive energy. This is not, however, the case. Microwaves work by using a magnetron that converts electric power into waves of oscillating electromagnetic energy, often referred to as microwaves, that are similar to radio waves, explains Juming Tang, PhD, professor of food engineering at Washington State University in Pullman, Wash. “These waves permeate food, causing the agitation of water molecules and charged salt ions, which produces friction and a quick rise in temperature to warm the food rapidly,” he says.

Tang goes on to say that this is why the cooking times with microwave ovens are shorter than with conventional ovens: The latter oven sends heat through food relatively slowly, moving radiant heat inward from the outside.

“In a microwave oven,” says Tang, “the air in the appliance is at room temperature, so the temperature of the food surface is cooler to the touch than food placed in a conventional oven, where the items are heated by hot air or by radiative heat.” For this reason, to the angst and disappointment of many a home cook, food cooked in a microwave doesn’t generally become brown and crispy. The bigger concern about microwaves, though, is that while they are heating your food, they are also zapping valuable nutrients.

Where Nutrients Go to Die

According to microwave skeptics, the intermolecular friction created by the appliance reduces the bioavailability of essential vitamins and minerals, such as B vitamins and vitamin C. And it can even change the chemical composition of foods, including animal proteins and dairy.

“When food, including baby formula, is microwaved, it has been demonstrated that certain amino acids, which are the building blocks of protein, are converted from their natural, active forms into biologically inactive forms,” says Lita Lee, PhD, a chemist and enzyme therapist in Portland, Ore. Beyond the vitamins and minerals, she explains, foods contain a plethora of delicate and complex compounds, including antioxidants and enzymes, which could be negatively affected by microwaving.

Skeptics like Lee are buttressed by a number of studies suggesting that the nutritional impacts of microwaving food are not entirely rosy. Japanese researchers determined that microwaving may convert vitamin B12 (a vitamin vital to proper neurological functioning, among many other things) in meat, pork and milk into its inactive form, rendering it somewhat useless.

Studies also show that heart-healthy phenolic compounds found in extra-virgin and virgin olive oils can lose some of their antioxidant capacities when exposed to microwaving, and that microwaving garlic can destroy its most powerful medicinal compound, allicin.

Comparative Cooking

A batch of other studies show, however, that the news surrounding microwaving and nutrient retention may not be so unappetizing after all, at least when compared with some other cooking methods — particularly boiling.

United Kingdom researchers reported in the journal Food and Chemical Toxicology that cooking a range of cruciferous vegetables like broccoli by steaming, stir-frying and microwaving did not produce a significant loss of glucosinolates, which are powerful disease-thwarting antioxidants. But when researchers boiled these same kinds of vegetables, they reported major glucosinolate losses through leaching into cooking water.

A 2009 Journal of Food Science study tested the antioxidant capacity of 20 vegetables, including Swiss chard and zucchini, when exposed to different cooking methods. Researchers found that microwaving drained antioxidant power less than pressure cooking and boiling.

“It’s clear that water is not a cook’s best friend when it comes to nutrient retention, whether in the microwave or on the stovetop,” says Barry Swanson, PhD, a food science professor at Washington State University. This could be why a much-publicized 2003 European study found that microwaving decimated broccoli’s flavonoid antioxidant levels. “The study authors added way too much water and cooked the vegetable for longer than what would occur in a normal household situation,” notes Swanson.

He says that compared with more destructive methods like boiling, the relatively mild temperatures and short cooking times associated with microwaving can do a good job at retaining nutrients in produce — as long as you use little, if any, water and don’t cook the life out of them. “High amounts of water provide a sea into which nutrients can get washed away,” Swanson says.

“There are many factors and conditions such as time and amount of added liquid that affect the nutritional value of food when cooked in the microwave,” says Samer Koutoubi, MD, PhD, a professor in the Department of Nutrition and Exercise Science at Bastyr University in Kenmore, Wash. “Many of the losses of nutrients that have been reported, including vitamin C, were mainly due to excess cooking water and spending too much time at the highest heat setting.”

Lee disagrees that excess water or cooking time is mainly to blame for nutrient loss. “Microwaving for seconds or longer will destroy nutrients — by changing them to biologically inactive forms — and create toxins and carcinogens. Stovetop cooking cannot compare to the damage of microwaving.”

Given the conflicting evidence, it’s clear that a consensus on the nutritional impacts of microwaving is probably anything but imminent. So for now, it’s up to individual cooks and eaters to decide whether they want to ditch their nuker, or embrace it with cautious enthusiasm. If you decide on the latter, see “Microwave How-To” below, for tips. And if you decide on the former, think not just of the antioxidants you might save, but of the valuable counter space you stand to reclaim.