Microwave ovens can be useful for cooking or reheating a variety of foods, but they're inconsistent in how they heat. Ovens vary widely in power, and some heat more evenly than others. As long as the cook follows the right steps to ensure the food's heated properly, microwaving is no more hazardous than any other form of food preparation. Foods can be cooled and reheated later, following the same precautions used with any other cooking method.
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Microwave ovens work by generating powerful, short-frequency radio waves. The food's molecules absorb that energy and convert it to heat, cooking the food. Fats and liquids absorb the waves' energy best, with muscle and bone heating more slowly. The oven's microwaves don't heat evenly to begin with, and the different foods that make up your meal don't heat evenly, so there's a risk that part of the food will be overcooked while part doesn't reach a food safe temperature. You can avoid that risk by following the right techniques for cooking and reheating food in the microwave.
Cooking from Frozen
When you're cooking or reheating foods from frozen, it's best to begin by thawing the food. Use your microwave's timed defrost or defrost-by-weight feature, or reduce the power setting to 30 percent and microwave for a few minutes at a time until the food is thawed. If you're heating a commercially prepared microwave product, follow the directions on the packaging. If you're heating or cooking food you've prepared yourself, microwave it on high at intervals of one or two minutes, until the food is steaming or noticeably hot to the touch. Flip large pieces of food, or stir small pieces and sauces, to distribute the heat.
Once the food is well heated, let it rest for a few minutes while the heat equalizes throughout the portion. Test the food in several places with an instant-read thermometer and verify that it's reached the USDA's recommended food safe temperature. For most mixed or sauced foods, that's 160 degrees Fahrenheit. Poultry needs to reach 165 F, and but most other meats are safe at 145 F. For your foods to remain food safe after cooking, they must be properly cooled and reheated.
Cooling and Reheating
Your goal is to reduce the temperature of your food to 40 F or below, as quickly as possible. Divide large portions into small, flat containers, which cool more readily. Resting the food on a freezer gel pack or cooling sealed containers in an ice water bath can provide faster cooling, without straining your refrigerator's capacity. Food should be refrigerated within two hours and reach 40 F within another two hours. Reheat your food on the stovetop, oven or microwave by heating it until it returns to a food safe temperature of 165 F when tested in several places with the instant-read thermometer.