Only plastic containers labeled or identified as microwave-safe should be heated in a microwave. Margarine tubs, yogurt containers, takeout boxes, and other plastics intended for cold storage can warp or melt from hot food. If the container is not marked for microwave use, transfer the food to glass, glass-ceramic, or another compatible vessel.
Look for a clear microwave-use indication
A manufacturer symbol or wording is more reliable than thickness, color, or recycling number. Plastic that performs well in a refrigerator is not automatically suitable for the temperatures food reaches during heating.
Do not reuse single-use food tubs for heating
Margarine, cottage-cheese, and prepared-food tubs may be intended only for cold use. The U.S. Food Safety and Inspection Service (FSIS) notes that such containers can warp or allow substances to migrate when heated.
Vent lids and keep wrap off food when directed
Steam pressure can force a sealed lid open suddenly. Use vented microwave-safe lids and follow wrap instructions; approved films should be used as directed rather than tightly sealing very hot food.
Discard a container that warps or melts
If plastic changes shape, softens abnormally, or melts during heating, stop using it and follow the applicable food-safety guidance. Do not continue heating simply because the center of the food is still cool.
Sources
- USDA Food Safety and Inspection Service — Cooking with Microwave Ovens — Microwave-safe materials, containers that should not be heated, and safe paper/wrap use.