Science of Everything: Microwavable Plastics

It’s been on your mind all day — that leftover pasta that’s waiting for you in the fridge. You get home right as your stomach starts rumbling. It’s dinner time. But wait, all your plates are in the dishwasher, and you can’t wait until the cycle finishes; you’re hungry now. You can microwave your food in the takeout container, right? What’s the worst that could happen?

Not all dishes are microwave safe. Plastic is a popular material for takeout containers, but each type of plastic is made up of different molecules. The identity of those molecules and how they are arranged often determines how the plastic will react to heat.

When molecules are arranged in a neat repeating pattern, they are more rigid and maintain their shape when heated. But plastics with molecules arranged in a less orderly way often have lower melting points. Heating them too much weakens the glue that links these molecules together and causes your container to deform.

Even if your container withstands the microwave’s high temperatures, heat creates stress on the plastic and causes some undesirable consequences. “The microwaved product’s temperature may rise high enough that its chemicals leach into the food at unsafe levels,” says Dr. Timothy Duncan, a chemist in the Human Foods Program at the U.S. Food and Drug Administration (FDA). 

I prefer that my leftovers be free of unsafe chemicals. Luckily, the FDA is working to make this the norm. They require that food packaging undergoes rigorous testing before being sold. As manufacturers develop new packaging that increases product shelf life and reduces food waste, we don’t know if the chemicals used to make those will leach into the food they hold. Tim’s job is to help the FDA understand how those chemicals migrate from the packaging to the food. 

Another safeguard is the recognizable wavy line logo, the microwave safe label. This logo is added by the manufacturers. “It helps people understand when microwave heating is less likely to deform or melt a plastic container,” explains Tim. The neat repeating pattern of molecules in the heat-tolerant plastics acts as a barrier to the chemicals. They take longer to travel past these structures, so these plastics are “less likely to leach unsafe amounts of chemicals into foods during short heating periods,” Tim adds. 

But this doesn’t mean your container will withstand any situation. The microwave safe label won’t tell you how long to heat it for or how many times it can be safely used. Just like high heat, extreme cold can also damage plastics so look for the snowflake logo if you want to use these containers in the freezer.   

These logos are incredibly helpful, but are not regulated by the FDA, so select your microwavable dish carefully. Avoid heating plastic wrap, plastic bags, and plastic that you purchase with foods like juice, cream cheese, and yogurt. These are primarily for storage and are likely not microwave safe. While it’s convenient to heat your leftovers in takeout containers, especially when you’re in a rush, many of these are not designed for repeated use. Also avoid using containers with visible chips, cracks, or scratches, as damaged containers are susceptible to deforming and leaching unwanted chemicals into food.  

Glass and ceramic have higher melting temperatures than plastic. While they are less likely to break in the microwave, some of these have decorative paint or metal. So be sure your container has the microwave safe logo.

After all, your leftovers are already perfect as is, and you don’t need your container adding any extra ingredients.