SCIENCE / MEDICINE : Scientists Close to Solving the Case of the Exploding Soup

Scientific research is about to blow the lid off the mystery behind a common household horror: why thick soups and stews explode and splatter the inside of microwave ovens.

“It’s a consumer problem: The housewife or kid comes back and here’s lunch all over the inside of the microwave,” said David W. Cable, a mechanical engineer at MetriCor Inc. in Woodinville, Wash.

He said the researchers want to know why some microwaved foods splatter as they cook--and how to prevent the messy problem.

Cable presented preliminary results of the study last week during the annual meeting of the Institute of Food Technologists, which drew more than 15,000 food scientists to the Anaheim Convention Center.

“What causes cream of mushroom soup to attack the walls of a microwave oven while chicken bouillon remains cautiously in its bowl?” the institute asked in a news release. “Scientists are coming close to an answer for that question.”

Cable said a food being cooked in a microwave oven is injected with microwave energy “at a rate similar to lightning striking a tree. The phenomenon is the same, except for its magnitude.”

“As water and sap (in a tree) flash to steam, the tree literally explodes. . . . This is what happens in the microwave to foods that are trapping the steam.”

Food scientists call the splattering phenomenon “popping” or “bumping,” Cable said.

“Some people have developed a sort of Richter scale of these explosions,” he said.

Cable and the institute said that so far, the food industry sponsored study has provided the following solutions, some of which may seem obvious:

* Foods should be centered in the microwave because heat is more evenly distributed there. The edges contain hot spots that are more likely to make water in thick soups flash into exploding steam bubbles.

* Thicker soups are much more likely to splatter than thinner liquids because heat concentrates more easily in a viscous liquid, allowing steam bubbles to expand until they are large and create a messy explosion.

Cable suggested cooking thick soups or stews at lower power, stirring them before putting them in the microwave oven, and covering the cooking container with a paper towel to contain the splattering.

* If a cooking container contains ridges or some other rough surface, bubbles can form more easily and in greater numbers. And lots of small bubbles distribute heat more evenly, while a few large bubbles are more likely to send the soup flying.

The Rutgers study may result in improvements in packaging for microwaveable foods, allowing them to be cooked with less mess, Cable said.