A microwave oven cooks with high-frequency radio waves that cause food molecules to vibrate, creating friction that heats and cooks the food. Microwaves travel so fast (and therefore cook food quickly) because they're extremely short. Nonmetal containers are used in these special ovens because microwaves pass through them (unlike metal), thereby cooking the food from all angles (top, bottom and sides) at once. The fact that the waves pass through glass and ceramics means that the containers stay relatively cool while the food they contain becomes quite hot. The exception is when, during long cooking periods, the food can make the container very hot. Ideally, containers and products like paper towels and paper plates suitable for microwave cooking should be labeled "microwave safe." Because microwaves penetrate only about one inch into food, the center of most foods is cooked by heat conduction. This also means that thin pieces of food cook faster than those that are thick. Some microwave ovens have turntables for even microwave distribution. Others have revolving antennae for the same purpose. Microwave ovens use relatively little energy and do not heat up the kitchen. Microwave ovens range in power from about 500 watts to about 900 watts. Knowing the wattage of your oven is vital to following microwave oven recipes, most of which are written for 700-watt models. Factors that affect how fast food cooks in a microwave oven include: the temperature of the food when cooking begins; the volume of food being cooked at one time; the size and shape of the food; the amount of fat, sugar and moisture in the food (fat and sugar speed the cooking; moisture impedes it); bone distribution; and food density (carrots, for example, are much more dense than eggplant).
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.