Why don’t microwave ovens get moldy?

Asked by Kirk Hings of Idaho Falls

Answered by Nathan Taylor, Eastern Idaho Public Health, REHS Environmental Health Supervisor

Mold needs three things to grow: A food source, water, and oxygen. The most common forms of mold we see are found on bread, cheese, or any other food that has been left too long in the fridge, except for salad dressings or pickled products. Most commercial salad dressings contain vinegar which lowers the pH to a level that prohibits mold growth.

Other types of mold we see are on the walls or rafters of a home that has water damage or where moisture is constant over time in places like bathrooms, crawl spaces, or mechanical rooms. The food source for the mold in these cases is cellulose found in wood or the paper on sheetrock.

We are lucky in the Western United States in that humidity is naturally low and even minimal ventilation of these spaces will usually keep mold from growing. We usually only see mold growth when there is an additional source of moisture like a broken pipe in the summer or condensation on a cold pipe in the winter. With the increased use of plastic water pipes (PEX pipes) over copper and steel, we see even less moisture and subsequent mold issues from condensation. Single pane windows, where we used to always see mold growing around the bottom edges due to winter condensation, have almost all been replaced now by double pane windows.

Now, to answer the question about why we don’t see mold in microwave ovens. Remember, mold needs three things to grow; oxygen, a food source, and water. There is definitely oxygen inside a microwave oven, but what about food and moisture.

The inside of a microwave oven usually consists of aluminum that is either painted or coated with a plastic polymer (polyethylene or polypropylene) and glass. These surfaces do not absorb microwave energy nor do they provide a food source for mold. The food source for mold must come from the food that is cooked inside the oven that happens to splatter on the walls. Usually, this food source is very small and dries out quickly after the microwave is in use that it can’t support the growth.

The third ingredient for mold is moisture. Microwaves will create moisture as they excite water molecules to speed up and produce steam, but they also do a good job of venting that moisture out to the atmosphere and not letting it build up within the oven. Even opening the door to retrieve your food will allow most of the remaining moisture to escape. Any residual moisture will seep out around the door as microwave ovens are not air-tight.

So, in summary, if you keep your microwave oven clean, there is no risk of mold growing inside. If you don’t keep your microwave clean, that is just disgusting.

Try this at home!

Before you start get permission from your parents to do this experiment and work in a spot where pets and little kids won’t be able to get to your work.

Take some small paper plates and smear samples of different types of food on them. Make sure the smears don’t touch and even label them by writing on the plate. Make up 4 identical sets of sample plates.

  1. Leave one plate out just as you made it.
  2. Take another plate and mist it lightly with water and leave it out.
  3. Take another plate and seal it inside of a large plastic baggie.
  4. Take another plate and mist it lightly with water and seal it inside another large plastic baggie.

Let the plates sit and check on them every day and record if you see any mold growing on the various samples. After 2 weeks compare the results of the various plates and samples. You can try the same experiment with disposable plastic plates and see if it makes a difference.

BE SURE TO ALWAYS WASH YOUR HANDS AND NEVER TOUCH THE SAMPLES AFTER YOU START. BE SURE TO THROW EVERYTHING AWAY WHEN YOU ARE DONE.

To learn more about all aspects of public health visit: https://eiph.idaho.gov

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