You might think that Starbucks cups are microwave-safe since they’re made from paper, but, unfortunately, they’re not. While the bulk of the cup is made from a thick cardboard-like material, a plastic cup liner inside makes the cups dangerous to heat in a microwave. Most paper cups have some kind of liner, meaning that virtually all paper cups aren’t safe to heat in the microwave, not just Starbucks cups.
If you want to reheat Starbucks coffee, you should transfer it to a microwave-safe container first. It might seem counterintuitive that a cup designed to hold hot beverages can’t be heated in the microwave, but it’s true. In this article, we’ll explain why Starbucks cups aren’t safe to use in the microwave.
Microwaves, How Do They Work?
We won’t make this into a physics class, but a little background knowledge about how microwave ovens work will help us understand why heating paper cups in the microwave is a bad idea.
Unlike traditional ovens that use heated air for cooking food, microwave ovens use microwave radiation to heat things from the inside out. Radiation is a scary word for many people, but microwave radiation is completely safe. The light we see with our eyes is radiation, and microwave radiation is less energetic than visible light.
When an object—like a piece of food—is exposed to microwave radiation, the water molecules in the food vibrate in response to the microwaves and heat up. The wavelength of the microwaves used in microwave ovens is specifically tuned to excite water molecules. Anything that contains water can be heated in the microwave, and, conversely, if something contained no water at all, it wouldn’t get hot in the microwave.
Why Paper Cups Aren’t Safe for Microwave Use
Now that we know a little bit about how microwaves work, we can start to see why reheating coffee in a paper cup isn’t a great idea. Paper itself has some water content, making it susceptible to heating and catching fire in the microwave, but the more pressing concern comes from other parts of the cup.
Even though Starbucks cups are primarily paper, they also contain a plastic liner, wax, and glue. Paper cups aren’t waterproof, and without a liner, they’ll get soggy and leak after only a few minutes. To combat this, companies add plastic liners that hold the liquid inside. Heating a cup with a plastic liner in the microwave is dangerous as the liner could melt, causing physical damage if melted plastic touches your skin or chemical damage if it leaks into your coffee.
You might think that as long as you don’t overheat your Starbucks cup, you’ll be fine, but that’s not true either. Even a quick 10-15 second stint in the microwave can heat the glue holding your cup together enough for it to start to fail. You’ll have a small leak to deal with at best, and at worst, you could wind up burning yourself with hot coffee.
What About Reusable Starbucks Cups?
Starbucks sells reusable coffee mugs and travel cups that you can use at their stores for a small discount. These mugs are usually made from plastic or are ceramic, making them more likely to be microwave safe. However, it is important to check the cup for a microwave-safe label before using it in your microwave.
Even some cups that seem like they would be microwave-safe based on the material actually are unsafe for other reasons. If your cup has a rubber gasket or metallic paint, it can’t be used in a microwave, and it can be hard to tell just by looking at the cup. Your best bet is to look for a label and choose not to use it if you can’t find one.
Most people know that metal is unsafe for microwave use, but they don’t realize that paper cups also aren’t safe. It is natural to assume that a cup meant for holding hot beverages could withstand the heat generated in a microwave, but due to the inside-out heating design of microwave ovens, even containers designed to hold hot beverages can break down in a microwave.
The bottom line is that you should not use paper cups from Starbucks to reheat coffee in the microwave. Consider purchasing a reusable Starbucks cup if you regularly need to reheat coffee, but be sure to look for a microwave-safe label before buying.
Featured Image Credit: kevser, Unsplash
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