Sound pressures generated by exploding eggs

Manufacturers of microwave ovens caution people to avoid re-heating certain food products because the rapid heating process can pose a danger to the user. Examples of such products are potatoes and eggs. Heating a potato in a microwave can generate steam under pressure. The internal steam pressure induces high tensile stresses in the potato skin, sometimes leading to its sudden (and unpredictable) bursting. A re-heated hard-boiled egg can also explode unpredictably but its bursting mechanism works differently than the potato. It is now believed that the egg yolk develops many small pockets of superheated water, leading to an increasingly unstable condition. When the egg yolk is disturbed by an internal or external stimulus, the pockets spontaneously boil, thereby releasing considerable energy (i.e., an explosion). An acoustical investigation was conducted using nearly 100 eggs that were re-heated under controlled conditions in a calibrated microwave oven. About a third of the re-heated, boiled eggs exploded outside the oven. For those eggs that did explode, their peak sound pressure levels ranged from 86 up to 133 decibels at a distance of 300 mm. The paper will describe the test protocols and discuss the results.