Which Containers Can You Use in a Microwave?

A microwave oven sends waves into the water molecules in food, making them vibrate and heat up. The food itself is what heats up, not the container; the container warms only from heat transferred from the food. Metal does not let these waves pass through; it reflects them from its surface. The reflected waves heat food unevenly and return to the oven’s electronics. Electric charge can build up at the tips of thin, pointed metal objects, causing sparks. For this reason, forks, aluminum foil, metal clips, and dishes with gold-trimmed edges should never go inside.

Heat-resistant glass is at the top of the safe list; borosilicate glass containers do not overheat or retain odors and colors. Glazed, undecorated ceramic and porcelain are also suitable. Paper towels and paper plates sold for microwave use can be used for short periods of heating. For plastic, the rule is clear: only containers with the microwave-safe symbol should go inside. Plastics without this symbol, especially yogurt, margarine, and prepared-food containers, can deform when heated. Hot, oily food can exceed the softening point of the plastic faster than expected. Disposable foam containers should not be used either; hot, oily food can melt them easily.

Use only containers that their manufacturer identifies as microwave-safe; do not use disposable yogurt, cheese, or margarine tubs. Lids are a separate matter: if a container is sealed completely, steam pressure can build up inside and force the lid off. Leave the lid ajar, use a vented lid made for microwave use, or cover the container with a paper towel. Covering the opening with plastic wrap is also not appropriate unless the wrap is marked as microwave-safe.

Finally, be aware that microwaves heat food unevenly. A microwave does not heat food from the outside in; it heats according to the food’s water content, so hot and cold spots can occur on the same plate. Stir the food halfway through heating, then let it stand for 1 to 2 minutes after heating so the heat can distribute. Foods with shells or membranes, such as whole eggs, chestnuts, and potatoes, can burst from steam that builds up inside; pierce them with a fork first. Liquids heated in deep, narrow containers can suddenly boil over without showing signs of boiling.