Microwave ovens are fast, but they do have a few annoying kinks. Eggs can explode. French fries tend to be soggy. About 90 percent of the new microwavable food products unveiled each year are failures. To find out why, some food industry representatives recently consulted Ashim Datta.
"There were a lot of interesting things about microwave food processing that we did not have very good answers to," says Datta, a food processing engineer at Cornell. He and his graduate student Hua Zhang created mathematical and computer models to quantify exactly what happens to various types of food in a microwave.
Microwaves bouncing around inside an oven cook food by vibrating molecules in the food, primarily water, which generates heat. Usually the microwaves penetrate the surface of a food but peter out before they get to the center. This doesn't always hold true, however. The heating pattern depends on the shape of the food. In round or oval objects, like an egg or a cup of soup, says Datta, the microwaves are focused into the center, "much like light waves through a lens."
Heat and pressure build up internally in such foods, which is why eggs can explode in microwave ovens. Sogginess, on the other hand, is caused by excess water vapor. When microwaves vibrate food molecules, the heated food releases water vapor. Microwave ovens generate more moisture than do conventional ovens. Moreover, in a conventional oven, hot circulating air dries up water vapor very effectively. "But in a microwave oven the surrounding air is cold," says Datta. "It cannot remove that moisture, so it starts piling up at the surface of the food."
Datta suggests that adding infrared heating coils to microwave ovens will dry up excess water vapor. He's using his precise mathematical models of microwave heating to develop guidelines for microwave manufacturers and the food industry. "We want these guidelines to show what shapes, sizes, and properties of food make it heat in certain ways," says Datta, who uses his own microwave mostly for reheating. "It's not the best for Indian food."
