Microwaving microorganisms: salty shield?

Microwaving microorganisms: Salty shield?

Microwave mania has turned frozen dinners into fast food. Pop the entree into the microwave, set the time and temperature, and presto -- in minutes, you get a hot meal that might have taken hours to prepare from scratch.

But just how hot is an important safety question. British researchers suggest microwaving may not heat the centers of foods -- especially heavily salted foods -- enough to kill toxic bacteria.

Richard W. Lacey and Stephen F. Dealler at Leeds University in England parceled out 300-gram servings of refrigerated mashed potatoes both with and without added salt, then microwaved them for 1 minute and measured their core temperatures. They found that increasing concentrations of the salts sodium chloride, potassium chloride, ammonium chloride or monosodium glutamate led to lower core temperatures. Mashed potatoes containing 600 milligrams of these salts failed to reach the core temperatures of unsalted samples, which heated uniformly all the way through.

This suggests that if a food contains Salmonella or Listeria bacteria -- major causes of food poisoning -- microwaving may heat the microorganisms to a temperature that spurs their growth rather than kills them, Dealler says.

In measuring the core temperatures of frozen dinners microwaved according to package directions, the researchers noted similar results. The core temperature increase of dinners containing 200 to 1,000 mg salt averaged 62 percent that of comparable unsalted foods they report in a letter in the April 5 NATURE.

The team proposes that microwave radiation induces the flow of an ionic current on the surface of foods with high salt concentrations. The ions may absorb the microwave energy and act as a shield, reducing the waves' penetration, Dealler says. This would explain why microwaved food often boils on its surface but remains cool on the inside, he adds.

Theodore Labuza, a food scientist at the University of Minnesota in St. Paul, says other research indicates significant amounts of salt and sugar tend to change the way food absorbs microwave energy. He suggests the mechanism may be more complex than the British team postulates. Both Labuza and Dealler recommend microwaving foods longer and at lower power than the package instructs, then letting them sit for a few minutes to help ensure that harmful bacteria have been killed.