How Does a Microwave Oven Heat Food?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A microwave oven heats a cup of soup in two minutes without a flame, without a hot element and without the cup itself getting hot first. It does it by shaking the water molecules in the soup directly, using radio waves of a particular frequency, and almost every odd thing microwaves do, the cold spots, the exploding eggs, the sparks from a fork, follows from that.
A radio transmitter in a metal box
Inside every microwave oven is a magnetron, a vacuum tube invented for radar in 1940, which turns electricity into electromagnetic waves at about 2.45 gigahertz. That is a radio frequency, in the same general band as wifi and Bluetooth, with a wavelength of about 12 centimetres. The waves are guided into the cooking chamber, a metal box that reflects them, so they bounce around inside until something absorbs them.
The choice of frequency is a compromise. Water absorbs these waves strongly enough to heat but not so strongly that all the energy is soaked up in the first millimetre, which is what would happen at higher frequencies. It is also a band set aside internationally for industrial and domestic equipment, which is why the oven and the router use it and the radio station does not.
Twisting water molecules
A water molecule is electrically lopsided: its oxygen end carries a slight negative charge and its hydrogen end a slight positive one. In an electric field the molecule tries to line up with the field, and a microwave is an electric field that reverses direction 2.45 billion times a second. The water molecules twist back and forth to follow it, and as they twist they jostle their neighbours, and that jostling is heat.
This is called dielectric heating, and it works on any molecule with an uneven charge distribution, which includes fats and sugars to a lesser degree. It does not work on the glass, ceramic or plastic of a plate, whose molecules are not free to rotate, which is why the food is hot and the plate is warm only where the food touched it. The common claim that microwaves heat food from the inside out is not quite right: the waves penetrate a centimetre or two and heat that layer directly, and the middle of a thick piece of food still warms by conduction from there.
Why there are cold spots
The waves bouncing around the metal chamber interfere with each other, adding up in some places and cancelling in others, and the pattern of peaks and dead zones is fixed by the box's dimensions. Food sitting at a dead zone stays cold while food a few centimetres away scorches. The turntable exists to carry the food through the pattern, and ovens without a turntable use a rotating metal stirrer to move the pattern over the food instead.
You can map the pattern yourself by laying a slab of chocolate or a sheet of marshmallows in a stopped oven for a short burst: the melted spots are the peaks, and their spacing, about six centimetres, is half the wavelength. Dividing the wavelength by the time for one wave cycle gives the speed of light, and this is a standard school experiment for exactly that reason.
Sparks, explosions and superheating
Metal reflects microwaves rather than absorbing them, which is why the oven's walls are metal and why a smooth metal bowl does little harm. Trouble comes from points and edges. The electric field concentrates at a fork's tines or the crinkled edge of foil until it is strong enough to tear electrons from the air, and the result is a spark. A thin metal decoration on a plate can heat and crack the glaze the same way.
Eggs and whole potatoes explode for a different reason. Steam forms inside faster than it can escape through the skin or shell, pressure builds, and the food bursts, sometimes after it has been taken out. A cup of water heated in a very clean, smooth mug can go above its boiling point without boiling, because there are no rough spots for bubbles to form on, and then erupt violently when a spoon or a teabag is dropped in. Stirring before heating and letting the cup stand for a moment afterwards prevents it.
Safety, and what microwaves do not do
Microwaves are not ionising radiation. Their photons carry far too little energy to break chemical bonds or alter DNA, unlike X-rays or ultraviolet light, and they do not make food radioactive any more than a light bulb makes a table luminous. What they can do is heat tissue, which is why the door has a metal mesh with holes far smaller than the 12-centimetre wavelength, so light passes through and microwaves cannot, and why the oven switches off when the door opens.
Nutritionally, microwaving is one of the gentler cooking methods. Vitamins are destroyed by heat and by time in water, and a microwave uses little of either, so steamed-in-the-bag vegetables generally keep more of their vitamin C than boiled ones. The process, step by step:
- •The magnetron produces 2.45 gigahertz radio waves
- •The metal chamber reflects them until they are absorbed
- •Water molecules twist to follow the reversing field
- •The twisting molecules heat the outer layer of the food
- •Conduction carries the heat inward, helped by standing time
The takeaway
A microwave oven heats food by making its water molecules rotate billions of times a second in a rapidly reversing electric field. Plates stay cool because their molecules cannot rotate, cold spots come from the wave pattern in the box, and sparks come from metal points concentrating the field, while the waves themselves are far too weak to change the food chemically.