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Cornell computer models depict how, in an egg with its dense surface, the center is heated first and pressure from the developing steam can cause the food to explode.

ATLANTA -- Why do some foods, such as eggs, explode in a microwave oven? Why do microwave-heated TV dinners emerge with dried-out peas but frozen mashed potatoes? Why do microwaved French fries always come out soggy? For the first time, a Cornell University professor has explained the fundamentals of these processes, and his calculations, he says, could turn microwaving into a predictable science, resulting in more appetizing and nutritious foods in the future.

Ashim Datta, associate professor of agricultural and biological engineering, has developed mathematical and computer models to explain how different kinds of food, and properties such as such as shape and size, are affected by oven power and food placement during microwave heating.

"We believe that our models provide a quantum leap in understanding, simultaneously, how microwaves behave and their change in behavior as heating occurs," Datta says.

Datta and his co-researcher, Cornell graduate student Hua Zhang, will explain their findings on heating patterns in microwave ovens to food scientists at the Institute of Food Technologists (IFT) annual meeting here June 22 and 23. They believe their models will enable manufacturers to formulate foods according to the most successful size and shape for microwaving and to issue far more accurate and precise microwaving instructions. They also believe their findings will for the first time make microwave sterilization, to increase length of food storage, commercially possible.

About 95 percent of American homes own a microwave oven, and food products for the ovens generate sales of more than a $4 billion a year.

"Many products have failed in the past because foods tend to get heated unevenly and unpredictably," Datta says. About 90 percent of new microwaveable food products fail every year, he says. "Our comprehensive knowledge base should permanently help improve the U.S. food industry's ability to deliver safer, more wholesome and higher quality convenience foods that are demanded by today's busy consumer," he says.

The two researchers' models provide a comprehensive description of what goes on in a microwave oven by linking the electromagnetics, heat transfer and the biochemical changes in heated foods inside the oven. Their calculations also indicate what produces non-uniform heating patterns during microwaving.

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Cornell researchers Ashim Datta and Hua Zhang can now quantify and predict how foods heat when microwaved taking into account food characteristics, placement in oven and oven power.

For example, a frozen block of food does not absorb microwaves very well, but as it is heated and parts of the surface thaw, the thawed regions start absorbing more and more energy. "As a result, as the outer layer thaws and starts absorbing more and more energy, it serves as a sort of shield, preventing microwaves from getting to the inside of the food," says Datta. "We can now quantitatively predict why parts of the food may boil while other parts remain frozen."

In the case of an egg and other round foods, Datta explains that their shape and size may result in the center getting heated first; if the food is dense, the pressure from the developing steam near the center may not be able to escape fast enough and could cause the food to explode. "However, you would think a larger ball would merely take longer to heat, but that's not true," Datta points out. "A small and large ball get heated in completely different ways, and our models quantify these phenomena."

Datta believes the findings are likely to make large improvements in the microwave sterilization of foods, by making foods more nutritious and tastier. To date, he notes, microwaving has not been used on a commercial scale because heating patterns have been too unpredictable.

Two years ago Datta reported on how moisture, heating rate and a food's porosity interact during microwave cooking to produce sogginess or explosions. The updated models also take into account how the behavior of the microwaves and properties of the food constantly change during the process. Within a few months Datta expects to have final calculations computed that relate the properties and size of a food item and its placement in the oven with what to expect in terms of heating and how to ensure that heating is more uniform.

Zhang will present his paper on microwave sterilization at the IFT meeting June 22. That paper won the competition for the best graduate student research paper in the food engineering division of IFT. The paper on microwave heating of foods will be presented June 23.

The project on microwave heating patterns is funded by the U.S. Department of Agriculture, and the microwave sterilization project is funded by the U.S. Army.