why does a microwave oven heat food unevenly?

Question

some is burning hot, some is still ice cold!

Answer 1

A microwave oven heats food by bombarding it with Microwave energy (a portion of the electromagnetic spectrum, much like visible light or radio waves). When water molecules are hit by a photons in the microwave frequency, they become excited, and move around faster - this is what causes them to heat up.

Since the outer surfaces of the food are the ones exposed to the most microwaves, they tend to heat up faster than the insides.

Basically, for a microwave to excite a molecule on the inside, it has to 'miss' all the molecules on the outside to get to it. The thicker your food is, the less likely that is to happen.

What ends up happening is that the outside of the food gets heated by direct radiation, while the inside has to be heated by conduction - the transfer of heat from the outer surfaces to the inner ones directly. This process is much slower, and so you end up with food that is hot on the outside, and cold on the inside.


That was longer than I anticipated, but hopefully it helps!

Answer 2

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Answer 3

The answer has been said on the replies, just not simply enough. A microwave oven passes microwave radiation through the food. Water, sugar, and fats in the food absorb energy from the microwave beam produced by a magnetron. Ever wonder why the food is super hard on the outside sometimes after being in there a long time? Because most microwaves have power settings that you can change but they are set (mostly) to cook at a 100 percent. Adjusting the settings can give you a better cooking atmosphere, but it will take longer. If the food is a liquid, put celephane on the top and it will cook faster. if it needs to be crunchier, just set it at 75 percent and make sure you turn it if it doesnt have a turntable. Hope this helps.

Fun Fact: First microwave was made in 1947. It weighed and was 6 feet tall. Its output was 3000 watts!!

Answer 4

In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. No heat has to migrate toward the interior by conduction. There is heat everywhere all at once because the molecules are all excited together. There are limits, of course. Radio waves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference, but you get the idea. The whole heating process is different because you are "exciting atoms" rather than "conducting heat."

In a microwave oven, the air in the oven is at room temperture, so there is no way to form a crust. That is why microwavable pastries sometimes come with a little sleeve made out of foil and cardboard. You put the food in the sleeve and then microwave it. The sleeve reacts to microwave energy by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.

Answer 5

Microwave oven uses microwaves to heat food. Microwaves are radio waves. In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz). Radio waves in this frequency range have an interesting property: they are absorbed by water, fats and sugars. When they are absorbed they are converted directly into atomic motion -- heat. Microwaves in this frequency range have another interesting property: they are not absorbed by most plastics, glass or ceramics. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.

You often hear that microwave ovens cook food "From the inside out." What does that mean? Here's an explanation to help make sense of microwave cooking.
Let's say you want to bake a cake in a conventional oven. Normally you would bake a cake at 350 degrees F or so, but let's say you accidentally set the oven at 600 degrees instead of 350. What is going to happen is that the outside of the cake will burn before the inside even gets warm. In a conventional oven, the heat has to migrate (by conduction) from the outside of the food toward the middle (See How a Thermos Works for a good explanation of conduction and other heat transfer processes). You also have dry, hot air on the outside of the food evaporating moisture. So the outside can be crispy and brown (for example, bread forms a crust) while the inside is moist.
In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. No heat has to migrate toward the interior by conduction. There is heat everywhere all at once because the molecules are all excited together. There are limits, of course. Radio waves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference, but you get the idea. The whole heating process is different because you are "exciting atoms" rather than "conducting heat."
In a microwave oven, the air in the oven is at room temperature, so there is no way to form a crust. That is why microwavable pastries sometimes come with a little sleeve made out of foil and cardboard. You put the food in the sleeve and then microwave it. The sleeve reacts to microwave energy by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.

Answer 6

When the microwaves are bounncing around in the oven, there are areas that repeatedly get the crests of the wave more than the rest of the ove. These spots get hortter and result in uneven heating, there is actually a small fan (it looks like a ceiling fan) above the cavity, the Xrays (oops, ment microwaves) are shot at the fan which promotes some randomization of the hot spots. The other thing is that sugar, water and other polar molecules are affected very much do by microwaves, whereas nonpolar fats do not respond to microwaves very much. Thats why your corn gets really hot (the water and sugar and starch) and the meat takes longer (has less water and little sugar).

Answer 7

A mw oven generally needs to have some way of getting the microwaves to reach all sides of the food. This is usually done either by A) having a rotating platform to turn the food or B) by having a fan whose metal blades reflect the microwaves in different directions.

What kind does your mw oven have...? If it doesn't have a lazy susan inside, the fan might have burned out. If it does have a turntable or rotisserie, or whatever they call it, either it is not rotating, or the clutch that engages the dish in the bottom to the motor is not engaged. You might be able to re-seat them together by fidding with them, but first run the oven empty (on LOW power) for a minute, watch through the window to see if the drive for the turntable works or not.

Worst case, if the fan or turntable does not work and you can't afford a new mw oven right now, rotate the food 1/4 turn (90 degrees) halfway through the cooking time.

8 NOV 06, 2109 hrs, GMT.

Answer 8

Ok so after all the science experiments and informatoin (true and untrue) about how a microwave works, how about a real answer to your problem?

Microwave ovens are not like regular ovens. In a regular oven you turn on the power, put the food in and set a timer. Not so with Microwaves. For many of the reasons noted above microwave ovens heat in odd spots, and the cook must learn their oven and how to use it.

First, the food must rotate. If your oven doesn't have a turntable, you must get in and turn the food regularly so that the microwaves can work on it all.

Second, depending on the food being cooked, the dish should be sealed or covered with a plastic wrap. This keeps moisture from being lost too quickly and the food drying out.

Lastly, learn the power settings. High is great for heating a cup of coffee quickly, but if you want to pentrate a frozen meatloaf, the outside will be dry adn crusty before the inside is even warm.

Answer 9

your microwave must turn for it to heat evenly...heres my best explanation:



the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. No heat has to migrate toward the interior by conduction. There is heat everywhere all at once because the molecules are all excited together. There are limits, of course. Radio waves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference, but you get the idea. The whole heating process is different because you are "exciting atoms" rather than "conducting heat."

In a microwave oven, the air in the oven is at room temperture, so there is no way to form a crust. That is why microwavable pastries sometimes come with a little sleeve made out of foil and cardboard. You put the food in the sleeve and then microwave it. The sleeve reacts to microwave energy by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.

Answer 10

Microwaves cook food from the inside out. Though many microwave ovens do not have cold spots, some do and thus some cold spots. Purchasing a go-around for the microwave helps, I know because I always use it except when making popcorn.

If you do not have a go-around, then remember half way through the cooking to turn the item 180 degrees. And, if you can turn it over. Oh, rememer that when you take something out of the microwave, the food is still cooking for at least one minute!

Answer 11

Microwave ovens use various combinations of electrical circuits and mechanical devices to produce and control an output of microwave energy for heating and cooking. Generally speaking the systems of a microwave oven can be divided into two fundamental sections, the control section and the high-voltage section . The control section consists of a timer (electronic or electromechanical), a system to control or govern the power output, and various interlock and protection devices. The components in the high-voltage section serve to step up the house voltage to high voltage. The high voltage is then converted microwave energy. Basically, here is how it works: Electricity from the wall outlet travels through the power cord and enters the microwave oven through a series of fuse and safety protection circuits. These circuits include various fuses and thermal protectors that are designed to deactivate the oven in the event of an electrical short or if an overheating condition occurs If all systems are normal, the electricity passes through to the interlock and timer circuits. When then oven door is closed, an electrical path is also established through a series of safety interlock switches . Setting the oven timer and starting a cook operation extends this voltage path to the control circuits . Generally, the control system includes either an electromechanical relay or an electronic switch called a triac. Sensing that all systems are "go," the control circuit generates a signal that causes the relay or triac to activate, thereby producing a voltage path to the high-voltage transformer . By adjusting the on-off ratio of this activation signal, the control system can govern the application of voltage to the high-voltage transformer, thereby controlling the on-off ratio of the magnetron tube and therefore the output power of the microwave oven. Some models use a fast-acting power-control relay in the high-voltage circuit to control the output power. In the high-voltage section the high-voltage transformer along with a special diode and capacitor arrangement serve to increase the typical household voltage, of about 115 volts, to the shockingly high amount of approximately 3000 volts! While this powerful voltage would be quite unhealthy -- even deadly -- for humans, it is just what the magnetron tube needs to do its job -- that is, to dynamically convert the high voltage in to undulating waves of electromagnetic cooking energy. The microwave energy is transmitted into a metal channel called a waveguide , which feeds the energy into the cooking area where it encounters the slowly revolving metal blades of the stirrer blade . Some models use a type of rotating antenna while others rotate the food through the waves of energy on a revolving carousel. In any case, the effect is to evenly disperse the microwave energy throughout all areas of the cooking compartment. Some waves go directly toward the food, others bounce off the metal walls and flooring; and, thanks to special metal screen, microwaves also reflect off the door. So, the microwave energy reaches all surfaces of the food from every direction & hence a microwave oven heat food unevenly. All microwave energy remains inside the cooking cavity. When the door is opened, or the timer reaches zero, the microwave energy stops--just as turning off a light switch stops the glow of the lamp