I found this question on one of those Wired YouTube videos where experts answer various questions from the web. A microbiologist gave a less than satisfactory answer to this question: Can bacteria survive high temperatures and microwave radiation? Do we really need to clean our microwaves? First of all, yes, you need to clean your microwave, regardless of bacteria being present inside it. I’ll get to that. But, the main question is, does microwave radiation kill or disable bacteria and especially pathogenic organisms?
Yes, Microwaves Destroy Bacteria
Not only do microwaves inactivate or kill bacteria, they are purposely used in food preservation to do just that. Microwave “pasteurization” is a viable and powerful way to extend the shelf life of foods since the method requires very short heating times that do not damage the food itself as much as conventional methods, which require long heating times.
The use of microwave radiation to reduce microorganisms in food has been the subject of many studies. Many foods have been examined, like turkey, beef, chicken, frozen foods, and potatoes. The overwhelming conclusion is that microwave radiation works and successfully extends the shelf-life of foods. It does this by directly reducing the number of microbes present.
Microwave heating inactivates various pathogenic bacteria, such as Escherichia coli, Streptococcus faecalis, Clostridium perfringens, Staphylococcus aureus, Salmonella, and Listeria spp. Still others are known to be sensitive to microwaves. While it is not precisely known how microwaves destroy bacteria, there is no question that it does. 1Woo, I S et al. “Differential damage in bacterial cells by microwave radiation on the basis of cell wall structure.” Applied and environmental microbiology vol. 66,5 (2000): 2243-7. doi:10.1128/AEM.66.5.2243-2247.2000
Your Microwave Probably Isn’t “Sterile”
If bacteria are hanging around on the walls of your microwave oven and the high-energy part of the microwaves impacts them, they will be potentially destroyed or heavily damaged. However, microwave heating is uneven. Microwave cooking leaves cold spots in food when it is not heated thoroughly enough. This is why your microwave probably has a turntable, to help expose more of the surface of the food to the microwaves. It is also why frozen microwavable foods often tell you to let the food sit for one minute after heating, to allow the heat to spread throughout the food and eliminate cold spots, thus ensuring food safety.
There may be bacteria present in some parts of your microwave that are in a “cold spot” and are safer from microwave radiation than other areas. This may be especially true when the bacteria are present inside food residues or spills. So, while the inside of your microwave oven is probably one of the worst environments bacteria could find themselves in, it may not be sterile. You should not consider it good enough to “eat off of” simply because microwaves destroy bacteria.
It is important to realize that if there are bacteria within food the only way microwave cooking can effect the organisms is through heating. If you do not heat the food to a high enough temperature throughout, the bacteria will not be killed. Micrwaves do not penetrate deep into food.
Why You Should Clean Your Microwave
If the inside of your microwave looks filthy and gross, I would think this is enough motivation to clean it. But not everyone is as motivated by filth as others. For some folks, the presence of foods spills and other food debris is not enough. But, only is this unsightly and sometimes smelly, but the presence of food residues on the inside walls of your microwave can damage your oven over time.
One reason to clean your microwave, if not being filthy is not motivation, is that each time you microwave, you can loosen food particles that are adhering to the roof of the microwave and if your heating food is uncovered, those particles can end up on your food.
Also, a dirty microwave becomes less efficient, leading to longer cooking times, which wastes energy but also results in a shorter life for your oven. Food residue and food particles within the microwave oven can also lead to uneven heating and hot spots. Some dried-up food particles, like sugary or salty ones, can even lead to sparking, which could damage the oven’s components, including the main part, the magnetron.
So, yes, you should clean the inside of your microwave, regardless of whether you are concerned about the presence of bacteria.
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