Why do microwave ovens cook faster than conventional ovens?
Why do microwave ovens not heat to the center as effectively?
Why do conventional ovens burn food more easily?

I have guesses for the answers to all three questions, but I'd
like to get correct answers rather than good-sounding guesses.

-- Jeff, in Minneapolis

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Well, microwaves are not really micro.
That's why Microwave ovens have turntables. Because as the wave travels through the food, it's hitting unevenly.
Microwaves heat the moisture or water in the food (Or anything else) which is what, in turn, heats the food around it. This is why foods like pizza can get soggy.
Conventional ovens use outside heat in the air in the oven, so that will burn or singe food, whereas a microwave heats from within.

Microwave ovens are basically lasers-- they use coherent light, which is easier to bring to a much higher power because the amplitudes add coherently. So you can bring power into the circuit and channel it efficiently into the microwaves, getting a more rapid rate of heating.
The microwaves are absorbed in the outer layers of the food-- they get absorbed by water.
They heat the very outer layers, drying them out. At least microwaves can penetrate farther into the food, so the very outside surface eoesn't have to get too hot.

That's my understanding, no doubt there is more to the issue that other responders might bring to bear.

what health effect's does using a microwave oven cause if any?
I would imagine at some level there must be a health effect, as it is waves being emitted.
thats some form of radiation and it cant be good ?

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I don't think there's any evidence that a properly functioning microwave has any health effects at all. A leaky one could heat up metals or conductors outside the microwave, but a faulty wired electrical appliance of any kind can electrocute you, which is a far greater concern if we allow faulty devices.

ken i meant from the operation of the microwave oven.
micro-waves cant be good,am i wrong?
there is a element of radiation at play no?

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it shows you how much i know....not much i guess----"sohhfly"

KenG did answer the question.
There shouldn't be any.

The radiation is contained. We are not talking about high level radiation here. Stay out of the beam and you'll be ok.

Microwave ovens use radiowaves at a frequency around 2,45 GHz to induce rotation in certain molecules in the food, water is the most affected in foods, though other dipole molecules, like fats and sugars will also be affected. It is the interactions between the moving molecules that create the heat(compairing it to every day experiences with friction is sometimes used as an aid to understanding it). The reason it heat the food quicker than an conventional oven is simply that it doesn't rely on heating air or using heat conduction, but dumps quite a bit of energy directly into the food.

Well, there are two effects that can cause uneven heating, one is if the microwaves doesn't penetrate to the center of the food, that might happen if something thick and absorbing is cooked. The thing would have to be several cm thick though, you would have an attenuation of 3dB a every 2-3cm in meat and liquid water, while dry foods or fat would have a lower attenuation, at least, that is if IIRC...

The other effect is that the microwaves tend to set up standing waves inside the oven, so that there are areas where destructive interference prevent heating. This last is the reason many ovens have a turning table or plate inside.

Setting the microwave at a lower setting can help too, as this can allow time for heat to spread more from hot areas to cooler areas.

Well, as I see it, since the primary source of heat is from the water, you would hardly get a temperature high enough to burn in most foods, unless it started to dry out... However you can get superheated water or steam, so things can get hotter than 100C...

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When we got our first microwave many years ago my mother was worried that there would still be microwaves in the food lol.

My dad kept making baked potatoes, it was a novelty that you didn't need to stick them in the over for hours.

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I have often wondered what at the molecular level is actually happening. EM is converted into mechanical energy. When the EM radiation hits the molecule how is it converted. Maybe I am seeking a quantum level explanation. Any thoughts?

Ultimately the energy is converted to infra red EM, a higher frequency form of EM, which says to me an excited electron falling back down a level.

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"Quaerendo inventis"

Laser ???' Microwave ovens use a magnetron oscillator to produce a electromagnetic wave of 2,4 GHz. This frequency heats water molecules, and since most food have water in it, it will heat almost any kind of food more or less uniformelly depending of the water contents and distribution in the food.

There's still some debate about the safety of microwave ovens, not in shielding but on food changes. Since the food is heated at a molecular level, it is believed that proteins can decompose and turn to unknown compounds that may be toxic. By daily experience you have noticed for sure that some heated food behaves like rubber and it taste quite different when heated in thermal ovens.

Since as far as I know all food is made from molecues, it all gets heated at the molecular level. Grilling on a stove or barbecue can produce significant quantities of highly tasty carcinogens.

EM at the microwave level is associated with molecular translations and rotations. Infrared radiation excites bond stretching and bending. Visible/UV are necessary for those nice big electronic transitions.

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If they can get you asking the wrong questions, they don't have to worry about the answers.

Yeah, but raw foods can contain all sorts of bacteria and other nasties. While many fruits and vegetables are better off raw, most foods, including some vegetables, are better for you cooked as the cooking breaks down certain structures within the food, making those minerals more easily accessible to your bodies digestive system.

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OK, we're getting somewhere. How does the bending and rebounding bond generate the IR?

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(By the way, I hate it that so many papers in the areas of planetary science and geology are not easily available to the dreaded "non-subscribers". It is like they are screaming at me: "YOU CAN'T HANDLE THE TRUTH". Good, I feel better now.)

"Quaerendo inventis"

Light is radiation of the same sort, but more energetic and more unevenly penetrating.


Fats are also good absorbers of microwave energy. And yeah, a magnetron is not a laser, though it operates in a vaguely similar way to a free electron laser...the output is not coherent, just in a spectrum where typical food has high absorbance and the interior of the microwave does not.


The taste difference is likely due to the absence of the reactive, unstable, and tasty substances produced by scorching and browning (caramelization, the Maillard reaction) reactions in conventional cooking. Its effects are identical to any other heat source, aside from timescale and distribution.

When a molecule absorbs microwave radiation, it then interacts with nearby molecules, transferring energy to the molecular lattice. The net energy content of the lattice as a whole increases, expressed as heat.

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If they can get you asking the wrong questions, they don't have to worry about the answers.

Sugars (in solution - not crystals) heat up fastest of all in a microwave oven. Probably the effect of all those hydroxyl groups interacting with the water. Put a tablespoon of syrup or jam in a microwave, blast it for 10 seconds and it is already boiling, whereas a similar quantity of water is still lukewarm.

Because of this, warnings are placed on (British) Christmas puddings. They are inclined to catch fire if cooked for too long in the microwave, and people just don't anticipate that they only require about a third of the time of other food items of a similar size/weight to heat up. They have a very high dried fruit content whose sugars heat up very quickly, and burn well!

Because frozen water and solid sugar don't accept microwaves very easily, there is a party trick whereby you enclose a bit of jam inside some ice cream and give it a short blast in the microwave. The jam heats up considerably and the ice cream is almost unaffected, creating hot jam inside cold ice cream.

I was taught a super-delicious and quick chocolate sauce for icecream by my mother which takes advantage of this effect. Take roughly a tablespoon each of butter, syrup (golden treacle) and cocoa powder (a little cinnamon or vanilla essence can be added). Blast it for 10 seconds, and the syrup is much hotter than the butter, so you then have to stir it to melt the butter into the syrup and mix in the cocoa powder, rubbing out any lumps. Another 10 or 20 seconds in the microwave and it is done. Pour it over your ice cream, and it solidifies in wonderfully toffee-like way.

Science is making me hungry.

Just a minor derail...

Years ago, some of my co-workers and I nearly convinced another fellow that if you reversed the polarity of a microwave oven's power cord, you could freeze food really fast.

The sad thing is, he was an avionics maintenance technician...just not a very good one.

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Brett
Peters Creek, Alaska
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Dielectric heating
http://en.wikipedia.org/wiki/Dielectric_heating

polar molecules like water having an electrical dipole, they align themselves in an electromagnetic field by rotating; as the field alternates in the form of a wave, the molecules reverse and oscillate with the electromagnetic wave. the heat generated is caused by the vibrational movement of the molecules i.e. kinetic energy.

http://en.wikipedia.org/wiki/Microwave_oven

the reason Microwave ovens work so fast is because they are very efficient.
and the energy that goes into heating say a cup of water goes into heating the water and not the cup.

it is not accurate to say that microwave ovens heat from the inside out
http://www.straightdope.com/columns/...the-inside-out
http://en.wikipedia.org/wiki/Water_absorption

the absorption frequency of water is complex
http://en.wikipedia.org/wiki/Microwave_oven
to paraphrase:
Microwave ovens work at usually at a frequency of 2.45 GHz (a wavelength of 12.24 cm)

which is the same as a WIFI wireless router
http://www.moonblinkwifi.com/2point4freq.cfm

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As in lycopene from tomatoes, which is much more bioavailable when they are cooked.

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But...come on...a thick slice of beefsteak tomato with a little salt and pepper...

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Or an heirloom tomato with a drizzle of EVOO and sea salt.

You should try my mothers spaghetti sauce. Tomatoes, shredded carrot, celery, onions, oregano, basil, thyme and chili powder. Absolutely deliciousness, rich and hearty, without any meat. Add some olive oil for some added smoothness. And a great source of lycopene to boot. Not that tomatoes aren't great any way you use them, it is just that that specific nutrient, which I hear is good to help prevent prostate cancer, is particularly abundant. If you want more vitamin C, have them raw, as vitamin C tends to break down with long cooking, and any good tomato sauce needs time to simmer and meld to be really good.

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That's not correct. Different cup materials will retain more or less energy.

Not inside out, but within


Yes, but microwave ovens do not radiate into free air, unless you disassemble one to make a radar or a HERF weapon

Energy does go into the cup, often from the heated food through conduction. However, some plastics can absorb microwave energy too. In fact, that used to be a rule-of-thumb for the plastics company I used to work for with regard to RF welding of plastic packaging.

Oh, and IIRC, most microwave oven's can't reduce the actual power. What happens, when you set it to less than max power, is the unit turns off the magnatron intermittently so as to let the food rest and let the heat spread out through the food. Thus, the food seems to warm up more slowly and with less "cooking".

I like to combine microwave and oven/stove cooking for maximum effect in minimum time. I can drastically reduce the cooking time of some items by steaming them in the microwave, or by partially heating them then browning them in the oven.

The next thing I want to try is an induction cooktop. They're expensive in the US, if you get an integrated model, but portable induction cooktops are common in asia and fairly inexpensive by mail order, or so I hear. I so want to boil water in 90 seconds. I can never get the water temperature quite right for tea by microwave, and an the electric stove takes too long and is too inefficient.

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Why not just use your heat vision? A second or two at the most... ?

Seriously, an induction top would be sweet.

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If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.

We have a combined Microwave/Halogen cooker, with fan assist. It will warm a cup of tea or crisp the top of your Crumble.

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this statement makes no sense in context.
while yes, the material (food) that is being heated transfers part of its heat energy to the container (the bowl)
the microwave radiation does not directly heat the container, the energy goes into heating the food.
unlike a conventional oven where everything inside gets hot at the same time.
it is clear that the energy needed to heat one cup of water in a cup is far less in a microwave than it is in a conventional oven.
try this, take a toaster oven and a microwave oven and place a cup of water in each appliance. then set the toaster oven to say 300 degrees and turn on the microwave oven at the same time. now i would be willing to bet that the cup of water in the microwave oven will boil far faster than the cup of water in the toaster oven, and if they were both hooked up to an amp meter, i would bet that the toaster oven will use far more energy to boil the water. now when the microwave oven is finished boiling the water, you can stick your hand inside and the sides will be cool to the touch.
I have to ask, after the toaster oven is finished boiling the water, will the insides of the oven be cool to the touch?

what is stated is correct.



i thought i said that
due to a mechanism called Dielectric heating...


all i said was that they work on the same frequency,
it should be self evident that microwaves do not radiate into free air unless yours is disassembled or faulty.
although i was thinking that rather build a weapon, one could build a WIFI amplifier by modulating the output of the magnetron.

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Well, if you're curious, you can always reach in and find out...



Seriously - I think microwave ovens are the next best thing to sliced bread. I've been using them since 1976, and would never want to live without one.

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If I set the budget, we'd have Ares and more. Unfortunately, I don't set the budget, and Ares is just too expensive and too far out for us to accomplish our goals within the budget we were given.

If we halt the ISS, all versions of Ares, and transport Orion and Altair aboard DIRECTv3's Jupiter family of Shuttle-Derived Launch Vehicles, we just might make it back to the Moon by 2020.