What It Is - Microwaves are a form of radiant energy that can generate heat. In 1945, an engineer named Percy Spencer was working near a magnetron (a device that generates microwaves) and noticed that the candy bar in his pocket had melted. Sensing a cooking breakthrough, Spencer grabbed another food and entered the history books as the first person to make microwave popcorn. By 1947, the first microwave ovens, called Radaranges, were sold for restaurant use. They were 6 feet (1.8 m) tall, weighed 750 pounds (340 kg), cost about $5,000, and had to be connected to water pipes for cooling. By the 1970s, smaller top-of-the line microwave ovens were available for home use and cost only about $50. Today, microwave ovens are found in more than 90% of North American homes.

What It Does - Microwave ovens heat food faster than most other appliances. They quickly thaw and reheat frozen foods. They handily melt butter and chocolate. They rapidly cook fish and vegetables. And they can soften the tough skins of produce like butternut squash to make peeling easier. Microwave ovens have revolutionized the popcorn industry, although many connoisseurs prefer the crisper texture of popcorn cooked in hot oil or with hot air.

Until recently, microwave ovens cooked mostly by heating up the water molecules in food, which helped food cook faster. But the resulting steam and lack of browning were big drawbacks. For instance, in older models, whole potatoes cook about 75% faster than in a conventional oven, but they cook with more steam than dry heat and consequently taste more steamed than baked. The speed also causes microwaved meat to cook so quickly that its juices tend to flow out before the lean protein is cooked through, making the meat dry. Like many foods, meats consist of 70% water or more. Even if that water reaches the boiling point (212°F/100°C), the meat won’t brown because browning doesn’t begin until well past 250°F (120°C). The meat has to lose its water and dry out before it can begin to brown. Browning in a microwave oven is more effective with drier foods like nuts. When put on a ceramic or glass plate, nuts will brown on the bottom after a few minutes from the heat conducted by the plate, just as nuts toasted in a metal pan will brown from the hot metal.

To circumvent the lack of browning, manufacturers of microwave ovens have added halogen lightbulbs or other broiling elements and fans to brown meats and other foods with dry infrared heat and with convection heat blown over the food’s surface. Newer combination microwave ovens may even include rotisseries, making the most of microwave, radiant, and convection cooking in one appliance. Food packaging can aid in browning as well. In some frozen foods intended for microwave cooking, thin sheets or patches of metal (called susceptors) concentrate the heat near the surfaces of food to help brown them. This packaging works well in foods like microwavable frozen french fries, which turn out fairly crisp and brown.

Newer ovens also attempt to overcome the other major weakness of microwave cooking, uneven heating. Microwave ovens cook food unevenly because the microwave energy is unevenly distributed inside the oven and because a food’s components (water, fat, and protein, for example) absorb the microwave energy at different rates. Using a rotating carousel, or manually rotating the food now and then, helps to promote even heating, as does frequent stirring and cutting foods into uniform pieces. To melt chocolate evenly, break it into similar-size pieces and stir every minute or so. To evenly cook foods with both thick and thin ends, like fish fillets and chicken breasts, position the thick ends toward the walls of the microwave oven (which heat up sooner) and the thin ends toward the center (which heats up more slowly). The same goes for foods with both tough ends and delicate ends, such as broccoli florets. Avoid square containers when you can because heat collects more readily in the corners; round containers distribute the heat more evenly. Lastly, cover the container with microwave-safe plastic wrap or waxed paper to trap steam, speed heating, and help prevent foods from drying out.

Microwave ovens range in wattage (a measure of the microwave energy they emit) from about 500 to 2,000 watts. Combined with the volume of the oven, the watts measure the oven’s cooking power, as higher watts in a smaller space cook food faster. Microwave recipes tend to be written for 700-watt models, so if your oven is a higher or lower wattage, adjust cooking times slightly down or up. For more gentle heating, cook at 50% power, which makes the oven cycle on and off so that it is emitting microwaves only 50% of the time.

Microwave Safety
Microwave ovens produce non-ionizing radiation, which doesn’t have the cancer risk associated with the ionizing radiation of X-rays and ultraviolet light. Nonetheless, ovens are designed to keep the microwaves secured inside. Government regulations allow leakage of only 5 milliwatts per square centimeter when measured 5 centimeters from the surface of the oven, which is far below the exposure level that is harmful to humans.

Use only microwave-safe cooking containers made of glass, ceramic, or microwave-safe plastic (made of glass-like phenolic resin). Softer plastics like those used for food storage and takeout containers can leach adipates and phthalates into the food when heated, and these compounds have been linked to the disruption of normal hormone function. Use only microwave-safe plastic wrap and try to keep the plastic from touching the food.

When cooking whole foods with a casing or skin, such as sausages, whole potatoes, or squash, prick them with a fork to prevent steam from building up and cracking the skin or causing blowouts. Because air and steam are trapped inside, shell eggs will explode in the microwave. To melt butter, do not cut up, and melt it at 30% or 50% so it won’t splatter.

Prevent sparks by keeping small or thin pieces of metal such as cutlery or aluminum foil out of the oven. These can produce high voltage at the edges and create an electric arc. Sparks are more likely with small bits of metal like the decorative edging of a plate but are less likely with large pieces of metal, such as the metal racks in some large microwave ovens.

How It Works - Microwaves are called “micro” because the wavelengths are shorter than radio wavelengths. Shorter wavelengths have higher frequency and more energy, but microwaves are still weaker than infrared energy and carry only 1/10,000 the energy of a gas flame. Microwaves are only strong enough to cause polar molecules (those with a positive charge at one end and a negative charge at the other), such as the water molecules in food, to move. That’s why microwave cooking works best with foods that are high in water such as fish and vegetables. During microwave cooking, the oven’s wave generator (magnetron) emits microwaves and creates an electromagnetic field that reverses its polarity about 4.9 billion times every second. That’s a frequency of 2.45 gigahertz or 2.45 billion cycles per second. The microwaves penetrate the food about one inch (2.5 cm) from the outside in, and the polar food molecules rotate frantically (4.9 billion times per second) as they try to align themselves with the constantly alternating electric polarity, which causes the molecules to move and bump into other molecules. Moving molecules generate heat, and that heat is transferred from molecule to molecule throughout the food. Voilà! The food cooks. Air in a microwave oven remains unaffected by microwaves because it consists of nonpolar molecules. The air warms up only from heat coming off the food.

Fast Facts
• Microwave ovens lose 30 to 40% of their heat, making them approximately 65% energy efficient. According to the American Council for an Energy-Efficient Economy, microwave ovens are two to four times more energy efficient than gas ovens, toaster ovens, electric convection ovens, and slow cookers.

• A 2006 study at Cornell University found that microwaved bacon contains significantly lower levels of carcinogenic nitrosamines than pan-fried bacon.

Kitchen Wisdom
Put Something Inside

Avoid operating an empty microwave oven. If there’s nothing inside to absorb the microwaves, the microwaves can return to the magnetron and damage it.

Dr. Phil Handel is the Program Director for Drexel's Hospitality Management, Culinary Arts, and Food Science program.

Article photograph by AndYaDontStop via Flickr (Creative Commons); "Science of Good Food" photograph by Horia Varlan via Flickr (Creative Commons); "Plate" photograph from FoodCollection/Getty Images.