December 6

Do Microwaves Cook From the Inside Out? | The Facts About How Microwaves Work


Author: Cornelia Suehr and its partners may earn a commission if you purchase a product through one of our links.

Does it leave your mind baffled when you notice the inner parts of your favorite pie are scalding hot while the pastry is still cold? I'm sure one question that lingers in your mind is, do microwaves cook from the inside out?

Microwave ovens (see also convection ones) are one of the lifesaver kitchen appliances to be invented. Unlike conventional ovens, microwave cooking is speedy and economical.

But, some myths surrounding how even the best microwaves cook create some doubts on whether microwaved foods are safe. So, let us break down how a microwave cooks to help you distinguish facts from fiction.

Cooked meat

Do Microwaves Cook From the Inside Out?

There have been so many myths about how microwave ovens cook food but cooking from the inside sounds incredible. Since the 1970s, after the discovery of microwave ovens, people have believed that microwave cooks food from the inside out. 

Cooking from the inside out is a myth that has no scientific basis. The truth is microwave ovens cook from the outside and pass the heat to the interior - of course, the higher the wattage the more heat an oven can generate. Here is what happens when you hit the start button.

An electron tube called a magnetron creates microwaves in the oven. The microwaves are then reflected on the metal walls, find their way into the food, and get absorbed. The microwaves then cause a vibration once they contact water molecules, creating heat that cooks the food. 

Therefore, meals containing a significant amount of water, such as fresh vegetables, can be prepared faster than dry ones. Microwave radiation is transformed to heat when it is absorbed by food. The radiation does not contaminate the food. So, the amount of water in food affects how it cooks in a microwave oven (see our guide on how long to boil water in a microwave).

Veggies in a mivrowave

What Happens When Cooking Dense Foods?

Microwave ovens - retro ovens are the latest craze by the way - never cook food from the interior, even though energy is generated directly after the contact between food and microwaves. The energy available for the next inch is reduced for every inch of dense food particles that the microwaves contact.

In other words, the power does not travel very far before being absorbed. The interaction with the microwaves heats just the very exterior layers of the meal. Heat conduction from the food surface to the inner parts is responsible for the rest of the cooking.

What Happens When Cooking Foods With High Water Content?

Cooking food with high water content in a microwave has a bit of science in it. Water contains polar molecules. Molecules with polarity have a positive and negative charge on both ends.

When microwave energy interacts with polar molecules, they initiate a rapid rotation. As you may have noticed, frozen foods heat slowly at first but heat up rapidly shortly after. The reason is, molecules in frozen foods are not free to rotate. 

Through the convention, free molecules heat up quickly, heating the frozen ones simultaneously. The rotation process continues until food is cooked evenly - for the less powerful microwaves cooking evenly might prove an issue, though.

Does that sound like cooking inside out? No. As we have seen, microwaves can't reach the inner part of the food without going through the surface. So, the myth that microwaves cook food from the inside out is wrong and misguided. 

Why Is Microwave Food Cold in the Middle?

Microwave ovens cook the same way conventional ovens do. A traditional oven passes heat from heating elements(either electric or gas burners) into the food. The food then cooks through conduction from the outside to the interior. 

So, during the initial cooking stages, the food is cold in the middle even when it is fully cooked on the exterior. A good example is when baking a cake. The heat edges while the inside is still soggy.

Similarly, microwave ovens (see ovens under 150 bucks) cook through microwaves that penetrate the interior through the food surface. Once the food particles in the surface contact the waves, heat is generated and penetrates deep into the interior through radiation. 

Why Don’t Microwaves Cook Evenly?

When microwaving frozen food, you might have noticed some parts getting well-cooked before others could even become warm, right? Some hot and cold spots occur because radio waves immediately excite fat and water molecules and penetrate the food chunk.

The areas with high grease and water particles generate heat and cook up very fast. For instance, the inner part of a pie cooks faster than the surface because of water molecule concentration. 

Also, if the microwave plate is no longer spinning, your food may not cook evenly. When microwaves are produced, they form wave patterns. Sometimes, the waves hit and cancel each other, creating a cold zone. 

So, if the plate fails to spin, some food sections remain in a cold position while others are in a hot area resulting in uneven cooking. Consult a qualified technician if you experience such a case. 

Food in a container in front of a microwave

What Should You Do to Ensure Food Is Cooked Evenly?

It can be frustrating to think your microwave is a lifesaver only to get unevenly cooked dinner after a long day at school or work. Luckily, there are things you can do to prevent such situations.

Allow Food to Rest

Because microwaves penetrate unevenly in thick chunks of food, ensure you let the food rest for a few minutes. Allowing food to rest is why microwave instructions advise one to let microwaved food sit for a few minutes before eating.

It is not to protect your mouth from heat; instead, it gives your food enough time to cook evenly. The heated parts pass heat through the convention to the frozen or uncooked ones until the heat is distributed evenly. 

Stir Regularly

For liquid foods, stop the microwave regularly and stir your food. Stirring speeds up the convention process and ensures even distribution of heat. 

Spread the Food

Piling the food in the middle may result in uneven cooking as it will be difficult for microwaves to penetrate the center. So, always spread the food to achieve an even density. 

Which Part of the Microwave Is the Hottest?

It is possible to find a microwave comprising specific areas with high temperatures. But this does not necessarily translate to thinking that those regions are the hottest. On the contrary, the microwave could be having a hotspot problem, and you should contact a technician. 

As mentioned earlier, microwaves bounce back and forth after hitting the reflective metal box before landing on the food item. It, therefore, means that the microwaves are all over the oven and are not concentrated in certain areas. 

Today, most microwaves (see ovens under 200 dollars) come with a technology that involves a spinning platter that ensures the food pass through the hot area for even cooking. You can quickly check the hotspots at home by placing several bowls of water to heat without the spinning platter. After heating for 3-4 minutes, check the bowl with boiling water. The place where it was positioned is the hotspot. 

Is the Food Cooked in a Microwave Safe?

Yes. Food cooked in a microwave is safe, and the allegations that it contains harmful radiation are false. Microwaves are absorbed by food and transformed into heat. There are no traces of the radio waves once you remove the food from the microwave.

Also, the microwave stops producing radio waves once you hit the stop button. 

Unlike what happens during ionizing radiation, a typical microwave oven does not directly alter the molecular sequencing in foods. Another wrong misconception is that microwave kills nutrients.

When cooking at high temperatures, either in a microwave or conventional oven, vegetable nutrients are lost. So, the trick is to avoid high temperatures and prolonged cooking to keep all the nutrients intact

Woman in a white shirt putting food in a microwave

The Final Word

Many myths and misconceptions about how microwave ovens work and food safety, but most are baseless. It is clear that microwaves cook from outside in and not the other way, as most people conclude. 

The waves penetrate to the middle of the food chunk through the surface and not vice versa. Similarly, no radio waves are acting upon microwave-cooked food. The waves are absorbed and transformed into energy, making your food safe. 

So, do not let the misconceptions cloud your mind; enjoy the convenience of preparing delicious oven-cooked meals today. 

About the author

Cornelia Suehr

A professional chef and a passionate writer on all topics related to cooking, baking, appliances and kitchenware. Her posts resonate with foodies, home chefs and professionals.


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