Why Do Microwaves Heat Unevenly?

I couldn’t keep my household running without my microwave, but we all know how annoying it is to find cold spots in our food.

Why do microwaves heat unevenly though? There are a few simple reasons for this:

  • Food Density: Different ingredients absorb energy more efficiently than others.
  • Stirring: You’re not mixing the food regularly enough at the right intervals.
  • Power Level: You’re not using the right power setting for the job at hand.
  • The Appliance: If the turntable can’t turn freely, the food won’t heat well. 
  • Containers: Awkwardly shaped containers can affect heating performance.

That’s the quick summary answer!

In the rest of this guide, I’ll explain the problem in greater detail, and give you all the tips you need to eliminate this annoying problem.

Why Can’t Microwaves Heat Food Evenly?

To answer this question, you first of all need to know a little of the science around how microwave ovens work.

It’s nothing complicated, but it’s key to understanding how these hot and cold spots develop inside the appliance.

Microwave ovens are pretty simple devices, and they produce their energy via something known as a magnetron.

This is usually located at the top of the appliance, and the energy it generates passes into the main cooking area.

These microwaves then bounce around inside the appliance, into the food, off the sides, back into the food, and so on.

My own school days are a long time behind me, but you may remember that when waves collide, they can sometimes cancel each other out. 

When this happens inside a microwave, this means no heat is generated. As the magnetron is located in a specific part of the microwave casing, that means there’ll be areas more prone to this than others.

To solve this problem (mostly), most microwave ovens also feature a rotating platter that your food or container sits on.

It’s not a perfect solution, however.

If the item turns clumsily on the platter (perhaps because of the container shape), or if the platter isn’t turning smoothly, you’re going to find that some parts of the food are heated more than other parts.

You’ll know this yourself from reheating leftovers. If you cook them for a certain amount of time without stirring, you’ll quickly notice that some parts of the food are noticeably warmer than others.

All things considered, the typical microwave from a decent brand does a pretty good job of managing this – but the process can never be perfect.

Hence why we often have to keep stopping and starting when we’re reheating leftovers, and it’s why packaged meals often instruct you to stir the contents halfway through.

An old-fashioned steel microwave sitting on a gray countertop

Food Density Plays A Big Part

For foods that have a completely standard density, this stirring process is enough to get the job done simply enough.

Things get more complicated though when you have different ingredients – with different densities – in the same meal.

This difference in density affects the ease with which the microwaves can penetrate different parts of the meal.

(*This also answers the question Does a microwave heat from the middle?” It’s a commonly-held belief that microwaved food cooks from the inside out. It doesn’t, it’s just that any moist food in the middle of the dish is incredibly efficient at absorbing the energy!)

Here are two classic examples of how density can impact the heating of food in a microwave:

The Classic Slice Of Pie

We all know what it’s like to have a slice of pie go wrong in the microwave.

The outer crust is pleasantly warm, you take a bite, and then a terrifyingly hot molten center hits your mouth.

Although the microwaves hit the exterior crust first, the liquid center is much better at absorbing the energy that does get through.

You can read more about how microwaves heat water – and other liquids – elsewhere on the site.

Defrosting Meat

Most of us are also familiar with the problem of defrosting meat in the microwave.

Although water absorbs microwave energy very efficiently, ice is surprisingly dense stuff by itself.

The microwaves have to work really hard to break that solid water down at first. Once it’s turned into liquid though, it heats up much more efficiently.

This is a problem though, because while the exterior now heats more easily, the energy can only now start penetrating to the middle of the meat.

This is why it’s so common to find yourself with a very warm exterior to the meat, with a still incredibly icy middle.

Piling Your Food Too High

It’s not just about the type of food you’re heating either.

If everything’s piled up on the plate, you increase the risk of cold spots developing somewhere in that pile.

That means more stopping, stirring and starting the appliance.

It’s far better to spread the food out as evenly as you can across the cooking surface, as this will minimize the amount of stirring you have to do to get an even temperature across the dish.

A slice of fruit pie on a white china plate with a fork

Why Is Unevenly Heated Food A Problem?

Food that’s been unevenly heated in a microwave isn’t particularly pleasant to eat, but that’s not the worst thing by far.

The main danger of unevenly heated food is the risk of bacteria developing.

This risk is most present when it comes to either reheating leftovers, or cooking meat. 

Wherever there are cold spots in the finished meal, there’s the potential for bacteria to survive, and then thrive inside you.

For this reason, it’s incredibly important to follow the instructions provided on microwave meals. You should also check that any potentially risky food types are thoroughly cooked through before eating them.

A few years ago now I bought a cooking thermometer, something I thought was a bit of an extravagance when I just planned to use it for the occasional – but perfect – steak.

In fact, I found myself finding more and more reasons for using the thing! Even a very good one doesn’t cost that much money, and I’ve never regretted the investment.

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How To Microwave Food Evenly

So, now we know the science, what can you do to make sure your microwave heats food evenly.?

You can never remove the problem entirely as I’ve explained. These tips should help you out a lot though:

#1 Use The Right Setting For The Job At Hand

We all have a tendency to trash the owner manual after buying a new appliance. How hard can it be to use a microwave, after all?

The thing is, there’s probably some really good advice from the manufacturer, so do take a moment to properly familiarize yourself with your new microwave.

Even microwaves with very basic power settings usually come with useful advice about how to heat different foods. It pays to experiment with the advice that’s given for your specific appliance.

#2 Spread Your Food Out Properly

I touched on this earlier on in this article, but you can solve some of the density problems by properly smoothing the food out over your plate.

By starting off this way – then stirring regularly – you’ll heat the food in the most even manner possible.

#3 Check The Turntable And Position Your Plates Well

Check from time to time that your turntable still turns freely. It’s not unheard of for gunk to build up on the turning ring, which may introduce a bit of a judder to its movement.

Even the most disgusting microwaves can be brought back to life with a little bit of elbow-grease, and it’s worth taking care of your appliances.

I hate to replace an appliance before I have to, but I’ve made peace with the fact that even a very good microwave will only perform at its peak for around five years.

The better you take care of it though, the less often you’ll have to shell out for a replacement!

As for the plates you cook on, make sure they’re well-positioned in the center of the rotating platter. It’s not a fix-all by any means, but it will definitely reduce the amount of stirring you have to do.

#4 Stir Often

Talking of stirring, make sure you do it regularly throughout the cooking process.

There’s a reason so many packaged microwave meals instruct you to stir the contents halfway through. They’ve tested the process to death so you get a decent meal at the end of it!

When it comes to heating your own food – such as leftovers – just interrupt the microwave every minute or so and give the contents a thorough stir. Fold everything in from all sides, and redistribute evenly across the plate.

This helps to keep the heat moving evenly throughout the food. If there’s any ice left on freezer foods, this will also help break up that dense stuff.

#5 Wait A While Before Eating

It’s worth letting your microwaved food rest for a minute or two before you eat it. 

Not only does this stop you from burning your mouth, it will also help the temperature even out throughout the whole dish.

In other words, some of the really hot bits do some extra work to heat the cooler bits, making the whole thing much more enjoyable to eat.

Frequently Asked Questions

Before I finish, there are a few other questions that commonly crop up around this topic. 

I’ll provide some quick answers here before wrapping everything up!

Can A Microwave Heat Two Things At Once?

You can heat two things in a microwave at the same time, but if they’re packaged items you can expect to do a lot more stirring.

You’re trying to get the same amount of power to do twice as much work, after all, and so it’s going to take longer.

For non-packaged food, just follow the instructions that come with the produce. Increase the cooking time accordingly.

Where Is The Hottest Place In A Microwave?

The hottest place in a microwave is the area closest to the walls that the energy is bouncing off.

In terms of the food you’re heating, that’ll be the circumference of the rotating platter.

This is why you so often find yourself having to fold in leftovers from the outside as you stir the contents!

Why Should You Let Food Stand After Microwaving?

Without getting too technical about things, you should let food stand after microwaving due to the simple principle of thermodynamics.

In non-techie terms, all you need to know here is that heat tends to flow naturally from hot areas to cold areas.

Letting your food stand for a while helps the temperature equalize throughout the whole dish.

Wrapping Up

Now you know why microwaves tend to heat food unevenly!

Follow the tips I’ve included in this guide though, and you should experience far fewer problems with hot and cold spots forming in your microwaved food.

Happy cooking!