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Dirty Microwave? Does It Reduce Cooking Efficiency?

Joan Young By Joan Young on
Badge: Editor | Level: 34 | Other Home & Garden Expertise:
a dirty microwave oven

It seemed to me, the other day, that I was needing to heat something longer than usual in the microwave. Then I looked at the interior with objective eyes. It was filthy! Was that the problem?

The inside of a microwave oven can become really grungy faster than we realize, especially if a family uses it. The likelihood that anyone other than the primary kitchen keeper is going to notice when things splatter, and wipe them up, is pretty low.

So, chagrined, and with a cleaning project on my list, I went to the internet to find out if there are consequences to having a dirty microwave. It turns out that there are.

When there are food particles clinging to the walls and tray of the microwave, these will be heated each time the appliance is turned on. We all know that, unlike conventional ovens, the more there is to heat in a microwave, the longer it takes. So crud in the microwave oven will “steal” energy from the food you want to heat.

My search for information yielded the interesting Efficiency of a Microwave Oven (a pdf). This is a lesson plan for a technical school unit on microwaves. It’s probably more information than you want. But it did propose an interesting experiment to test efficiency, and I adapted that to see if cleaning my microwave would boost its effectiveness.

First I filled four identical custard cups with the exact same amount of water, 4.0 ounces, and let them sit on the counter until all had equalized at room temperature, about 68 degrees Fahrenheit.

Then I did two tests in the dirty oven. I placed one dish in the microwave. Mine has a revolving tray to reduce hot spots, and I placed the dish about halfway between the center and the side. Then I zapped it on high for one minute, and took the temperature of the water with a candy thermometer. It registered 155 degrees F. A second trial gave the exact same result.

Since I only wanted to compare before and after cleaning, it didn’t matter to me what the exact temperature change was, I just wanted to see if there was any improvement.

Then I cleaned the microwave (eventually those icky jobs just have to be done). I used a suggestion I found online, and microwaved a bowl of water with one tablespoon of vinegar in it, for five minutes. This loosens and softens the cooked-on crud. Then I removed the glass tray that revolves, and washed down the inside walls, ceiling and floor, and the inside of the door. The tray was washed and dried, and returned to the oven.

After this was done I did the one-minute, water-heating test again. This time, the recorded temperatures after a minute of heating were 162 and 167 degrees, for an average of 164.5 degrees F. This is 9.5 degrees warmer than the result in the dirty oven.

While, to be a truly scientific test, I should have used more samples, I think the implication is clear: a clean microwave oven works better because it only has to heat the things you want heated and not left-over garbage. My simple test seems to bear that out.

Conclusion: a clean microwave oven will not only be more sanitary, it will save both time and energy use.