How Dishwashers and Microwaves Differ in the Way They Heat Plastic

How Dishwashers and Microwaves Differ in the Way They Heat Plastic

How Dishwashers and Microwaves Differ in the Way They Heat PlasticIt may not be the most popular recommendation I've ever made, but at this point in my research, I don't have an issue with washing dishes on the top rack of a non-commercial dishwasher.  I do, however, recommend against microwaving plastic.

I've been asked hundreds of times why in the world I'm okay with dishwashers, but not microwaves. Now I don't claim to be a scientist, but the answer stems from my understanding of the fundamental differences in the way each appliance creates and applies heat.

How Do Microwaves Heat?

The magnetron inside the oven converts ordinary electric power from a wall socket into very short radio waves, which are then transmitted at a frequency of about 2450 Megahertz. At that frequency, power is readily absorbed by water, fats and sugars, resulting in very fast vibration and high temperatures that cook the food.

Every microwave oven has an individual pattern. Some areas may be bombarded by more microwaves than others and heat faster, and as EWG explains, hot spots are created where the plastic is more likely to break down.

Evil Mad Scientist put together a really great visualization of the microwave hot spots from several different microwaves using extremely heat-sensitive raw Appalams (see below).


How Are Dishwashers Any Different?

Dishwashers use an electric heating element that creates a low-level of heat for a longer period of time, so the risk of hot spots is low.  The most common plastic used in dishes (and the only one, besides silicone, that I recommend placing in the dishwasher) is polypropylene.  Polypropylene is chosen for dishes because its melting point is very high compared to many other plastics, at 320°F.  This means that the hot water used won't cause the plastic to warp or degrade quickly.

To underline this point, let's look at the common myth that plastic baby bottles can be sterilized in the dishwasher.  The water in home dishwashers only reaches 130 to 150 degrees, and the truth is that it would need to reach 250 degrees to sterilize.  Even commercial dishwashers (which wash a full rack in under 3 minutes) only reach temperatures of 180°F.

I recommend you only wash products labeled as “dishwasher-safe” – and even then – only on the top rack, because most dishwashers have a heating element in the basin of the dishwasher.

Also keep in mind that using harsh cleaners, such as chlorine bleach, in the dishwasher can create conditions where additives and monomers might leach from plastics.  For this reason (and many others), it's important to use a non-toxic, mild detergent.  My friend Leona of My Healthy Green Family just posted her borax-free DIY recipe, so you might give it a try.

A Special Note About Microwaving in Glass Containers

Glass and ceramic food containers are the best options for heating food in a microwave, but you should be careful to choose dishes specifically labeled for microwave use.

The issue with glass that is not microwave-safe arises when micro-air bubbles are present in the glass.  As the glass heats up in the microwave, the bubbles can expand to the point where the glass breaks or shatters.  Pyrex glass bakeware is an excellent example of microwave safe, heat resistant glass that can also be baked.

But what if you have an older piece of glassware that isn't marked or the marking has worn off? I'd recommend checking with the manufacturer before microwaving.  If that's not possible, there is a way to test whether it's microwave-safe or not:  just heat the container empty for one minute, and if it comes out hot, then it is NOT microwave safe. If the container is warm, it should be fine for heating food, and if it remains cool, you can cook in the microwave with that container.

Photo Source: Evil Mad Scientist

  1. Well written article. I appreciate your writing skills. It’s
    great. You have done a great job by sharing this post with us. I would like to
    read your more updates. Keep it updates.

  2. Another big difference between microwave and dishwasher-you don’t heat up food in the dishwasher! 

  3. Also, the dishwasher is applying the heat from the outside, not directly agitating the molecules of the object itself in the course of applying the heat.

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