OMAHA, Neb. — 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 last year from improperly microwaved frozen foods. New warnings were recently issued 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 harbor dangerous bacteria, such as E. coli, salmonella or listeria. So 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 foods prepared to appear cooked, such as chicken that has been breaded or pre-browned.

Even some meals designed to be microwaved can be unsafe if they are not heated thoroughly enough, or are cooked using directions meant for a microwave with different voltage.

The government doesn't track microwave-related food-borne illnesses, but every year more than 325,000 people are hospitalized for food-related illnesses. Last fall, hundreds became ill when Banquet pot pies made by ConAgra Foods were linked to a salmonella outbreak and frozen pizzas made by General Mills were tied to an E. coli outbreak. Both products were recalled.

Since then, 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 Nestlé 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.

But preparing frozen foods safely may require a change in consumers' microwave habits, too. In the latest outbreak, some of the meals were microwaved even though the products weren't intended to be.

Microwaves produce short radio waves that penetrate food about 1 inch and excite water, fat and sugar molecules to produce heat. Food safety experts say that method poses more risk than a stove or oven because it heats food unevenly.

To be safe, they suggest getting a food thermometer and using it to check the temperature of microwaved food in several places, especially if the product includes raw ingredients.

"If you were going to make one of these things for a kid, you'd definitely want to be checking the temperatures on the things or using your (conventional) oven," Davidson said.

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

Consumers also need to become better acquainted with the technical specifications of their microwaves. The unit's wattage — how powerful it is — influences how well it heats food, and cooking instructions are written for specific wattages.

But microwaves lose power over time, and some smaller microwaves may not produce enough power to safely cook some products. Banquet pot pies, for example, now include a warning that the product shouldn't be cooked in microwaves with less than 1,100 watts output. If most people don't know the wattage of their home microwaves, forget the ones they might use to nuke their lunch at work.

While following directions helps, experts say the bottom line is that cooking raw food is still a job best left to stovetops, grills and ovens.