Microwave ovens
Opinion is highly polarised when it comes to microwave ovens — either they are disparaged or they are embraced, and often with passion either way. Coincidentally, microwave ovens work because water is polarised too (a water molecule has a positive end and a negative end) and can interact with microwaves to be heated.
Microwaves are used in radar, and the microwave oven developed out of Second World War radar research in the United Kingdom. It is usually attributed to an engineer by the name of Percy LeBaron Spencer, who noticed a chocolate bar in his pocket melt when he was working on a magnetron and immediately saw a cooking application (as opposed to realising that chocolate melts at body temperature). This is the urban legend, but that it came out of WWII radar research is true. A patent was in place by 1945.
The first commercial microwave oven was available by 1947, and appropriately called the Radarange. It was 2m high, weighed 340kg, needed water-cooling and cost about $20–30,000 in today’s terms. Curiously, it didn’t sell well, although some were installed in submarine galleys (where they added ballast too, I presume).
It took until 1967 for magnetron redesign to bring the price down to about a tenth and a little more interest was shown.
Then it went crazy and now nearly every kitchen has one.
Here is my attempt to answer 10 questions about microwave ovens:
1. Are microwaves radiation?
Yes, they are.
Food is grilled by radiation (infra-red). Sunlight is radiation.
2. Are microwaves safe?
Yes under domestic conditions.
The longer the wavelength, the safer the radiation is.
Microwave oven radiation has a wavelength of 12 cm, and the levels outside the oven are strictly regulated and vanishingly small (the waves are too big to get through the tiny mesh screen on the door). The wavelength of visible light is 0.000038 to 0.000070 cm.
Radiation doesn’t start to get troublesome until the wavelength falls below about 0.0000125 cm. The wavelength of X-rays is in the range 0.000000001 to 0.000001 cm.
3. What distinguishes safe from unsafe radiation?
Mainly, whether it is non-ionizing or ionizing.
To be unsafe, the radiation has to carry enough energy (short wavelength means high energy) to disrupt chemistry by ionizing (or charging) atoms. When it does it is called ionizing radiation, and this starts around the far UV. The near UV, while technically non-ionizing, can burn skin by other mechanisms and lead to cellular damage. We are protected from far UV radiation by the ozone layer. Without the ozone layer, land-based life as it exists now would not be. We are protected from the ionising effects of still-shorter wavelength radiation by the Earth’s magnetic field. We survive in a tiny niche.
4. Are there other uses for microwave radiation?
Yes, many.
The following technologies all use the same wavelength as the microwave oven: mobile phones, Wi-Fi, Bluetooth, GPS, cordless phones, radar and some amateur radio. The cosmic background radiation is in the microwave band. The strength of all these signals is low.
5. How does a microwave oven heat food?
Mostly (but not only) by agitating water.
Most natural food is made up of water (especially vegetables, see carrots). The water molecule has a positive and negative end, and when placed in a microwave it begins to oscillate at the frequency of the microwave radiation, which is 2.4 GHz (2.4 billion times a second). Movement is thermal energy, so the water heats. This heat is then transferred by conduction in the normal way to other food molecules (fats, proteins and carbohydrates).
The food is neither ‘nuked’ nor ‘zapped’.
6. Why don’t microwaves brown?
Microwaves don’t heat food beyond the boiling point of water (100C) no matter how long the food is cooked.
The water on the surface of the food eventually approaches its boiling point and turns to steam that escapes the food. However, now there is less water in the food, so the microwaves become less effective at heating. As more water is lost, so is the microwave’s ability to heat and browning temperatures are not reached. Browning reactions only begin to develop at any useful rate when temperatures get to around 130C and above. Microwave temperatures are self-limiting.
In a conventional oven, as the surface dries, its temperature continues to increase above 100C and browning occurs.
Some specialized microwave containers or materials are designed to heat above 100C and brown by heat transfer.
7. Are microwaves bad for the nutrient content of food?
No more than other methods. All cooking involves a trade off.
Some nutrients can be lost to cooking. But then again, many nutrients are locked up in food cells, and only become available to our digestive system when the food is cooked. This is why cooking is such a valuable step in the digestive process.
At least by cooking fast, microwave ovens can strive for a balance between nutrient release and nutrient loss.
8. Do microwave ovens cook food from the inside out?
No. Microwaves cook like conventional ovens do — from the outside in.
Microwaves are strongest at the surface of food, but they can penetrate more deeply than the infra-red radiation of a grill, or the air-heat transfer in a conventional oven.
Typically they penetrate up to 3cm, but their effectiveness declines with depth, and practically it is more like 1cm. The reason their strength declines is that their energy is absorbed by the water molecules as they are heated. Wi-Fi etc. work in the home because the internal walls are dry and the microwave signal passes through unhindered.
But the microwave penetration of food is still greater than with conventional methods, so heat is more readily conducted into the interior, and microwave ovens generally cook faster.
But not always. It takes the same time to cook 2 potatoes as it does 1 in a conventional oven, but it takes longer to cook 2 than 1 in a microwave oven because the energy has to be distributed across twice the volume of food.
Likewise, if the food is smaller than say 1 cm, it takes a long time to heat because the microwaves are too ‘large’ to interact with small items (remember their 12cm wavelength). As a rule of thumb, food should be larger than about a quarter wavelength (3cm), or small items grouped together.
9. Can metal go into a microwave?
Yes, sometimes. The interior of microwave ovens is made of metal.
What matters are sharp points. The tines of a fork are not a good idea! Notice that the interiors of most microwaves are somewhat rounded (this is also to reflect the microwaves more evenly). Aluminum foil can survive so long as it is very smoothed out. Some microwaves have metal shelving. Generally to be avoided though.
10. Is microwaving the best way to defrost?
No.
Perhaps counterintuitively, the best and most food-safe way to defrost food uses an ice-water bath.
