Beware the microwave and its uneven heating, warn food experts

Zapping frozen meals in the microwave may be fast and easy, but it also can make you sick if it's not done properly.

That message has been slow to catch on, despite a spate of illnesses in the U.S. last year from improperly microwaved frozen foods. On Sunday, the U.S. government issued a new warning urging consumers to thoroughly cook frozen chicken dinners after 32 people in 12 states were sickened with salmonella poisoning.

"Given how people use microwaves, it's great for reheating, but maybe not so good for cooking," said Doug Powell, scientific director of the International Food Safety Network, based at Kansas State University.

The problem is that microwaves heat unevenly, and can leave cold spots in the food that harbour dangerous bacteria, such as E. coli, salmonella or listeria. Microwaving anything that includes raw meat, whether it's frozen or thawed, can cause problems.

"I think most food-safety experts probably would have said it's not a good idea to microwave anything that's from a raw state," said Michael Davidson, a University of Tennessee food microbiologist.

Many people wrongly assume all frozen meals are precooked and only need to be warmed. It's a misconception fostered in part by prepared meals that appear cooked, such as chicken that has been breaded or pre-browned.

Spotting raw ingredients isn't always easy because the only clue many companies offer is the two words "COOK THOROUGHLY" on the front of the package.

Several food companies have revamped the cooking instructions on their frozen foods to ensure they are sufficient for killing off any dangerous bacteria, says Leslie Sarasin, head of the American Frozen Food Institute trade group.

ConAgra and Nestle Prepared Foods, two of the largest frozen foods producers, have rolled out revised instructions on many of their brands, which include Stouffer's, Lean Cuisine, Banquet and Healthy Choice.

Experts say consumers must change their microwave habits, too. For one, many people microwave foods that really are not intended to be microwaved.

Consumers should also learn the wattage of their microwaves because certain foods require microwaves that have higher power levels. They also recommend that people use a thermometer to check food in several places when using the microwave.

However, Davidson, the Tennessee microbiologist, admits it is unlikely people will easily change their habits.

Most people don't know the wattage of their home microwaves, let alone thosethey might use to nuke their lunch at work, he said.

"Who's got a thermometer in their desk drawer?" Davidson said.