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Microwave Oven Cooking 101: Have You Done the Marshmallow Test?

Julie Garden-Robinson, Ph.D., R.D., L.R.D., Extension Food and Nutrition Specialist and Professor Department of Health, Nutrition and Exercise Sciences, NDSU

Quick quiz:

Which of the following are safe containers for reheating food in a microwave oven?
Circle all that apply.
a. Foam take-out trays
b. Brown paper bags
c. Margarine tubs
d. Glass bowls

*Answer at the end of this article.

You can view the marshmallow test in
action and find out how to determine
the wattage of your microwave using a new learning tool Nebraska
Extension and NDSU Extension developed. Visit this link to see the videos and read the interactive online “Microwave It” magazine (zmag).
Complete the survey at the end to be eligible for prize drawings.

The first countertop microwave oven appeared in 1967. Today, most households have a microwave oven and use it nearly every day.

Microwave ovens cook food using a magnetron to convert electric power to very short radio waves of about 2,450 megahertz. At this frequency, water, fats and sugars absorb the waves and vibrate very fast to create high temperatures that cook the food.

Do you ever use margarine tubs, take-out containers, foam trays or brown paper bags to heat food in your microwave? These are considered unsafe for microwave oven use unless they are labeled “microwave safe.” The concern is that the container could melt and leach chemicals into your food.

Instead, reheat foods in glass, ceramic cookware and those labeled safe for microwave use. Microwave-safe plastic wraps, wax paper, cooking bags, parchment paper and white microwave-safe paper towels also are considered safe for microwave use. However, be sure to discard containers that hold prepared microwavable meals after you use them because they are meant for one-time use.

Wattage and Hot Spots

Do you know the wattage of your microwave oven? Cooking times vary depending on the wattage of your oven. The wattage may be listed on the oven. Follow the package label or recipe instructions to be sure you are heating microwaveable
foods properly.

Microwave ovens often have hot and cold spots. If you are curious about the “marshmallow test,” here is how to do it. Use a microwaveable plate or flip the turntable upside down so it does not rotate. Line the plate or turntable with miniature marshmallows. Place in the microwave oven, then set the timer for one minute.

Watch carefully. If the marshmallows begin to burn, turn off the microwave oven. The marshmallows that expand first show where your microwave oven’s hot spots are. Remember that uneven cooking (hot and cold spots) can lead to undercooked
food that is unsafe to eat.

Because of the uneven cooking that can occur, be sure to rotate the container during the cooking process and stir the food if possible, even if your microwave oven has a turntable. Covering the dish with a microwave-safe lid or plastic wrap also promotes even cooking.

After heating food in a microwave oven, be sure to allow a standing time of at least three minutes. During the standing time, the food continues to cook. Many microwaveable packaged foods list an endpoint temperature, such as 165 F. Measure the temperature using a food thermometer to ensure that the food has reached a safe internal temperature.

Answer: d

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