A microwave oven is an electric oven that cooks food items through the use of radio waves.
These ovens can be used for melting, thawing, reheating, softening and cooking. They are mostly used for thawing, heating leftovers, and (in North America, at any rate), popping popcorn.
Those who dismiss microwave cooking as too “high tech” or “mechanical” don’t realize that the same amount of mechanics and engineering now goes into regular stoves.
Convection microwaves are ovens that are combination convection ovens / microwave ovens, so that they can be used either as a microwave or a small oven, or to first to cook something quickly, then brown it. Countertop models are very popular in the UK, to the point where it’s becoming hard to find microwaves that don’t have convection added to them, but in North America, such combination ovens are still very rare (as of 2020), mostly only found as part of a unit that also includes a draw fan, and meant to be built in and mounted over top a stove.
For lab use, you can buy top loading microwaves.
Microwaves are good for:
- most vegetables — both steaming and braising them;
- melting ingredients such chocolate, butter and lard;
- softening ice cream and butter (if you’re careful);
- making small batches of preserves such as jams or jelly;
- heating jams for glazes;
- making quick steamed puddings, though the flavour, some say, is never as nice as steamed ones that allow the flavour of the pudding’s ingredients time to marry;
- slowly simmering sauces on low power.
How a Microwave Oven Works
Electromagnetic waves such as microwaves go from a positive value to zero to negative and then back to zero. The microwaves pass through food, and as they do, they pass through the molecules that make up food. Most molecules have a positive charge at one end, a negative one at the other. As the radio waves pass through a food molecule, the negative part of the molecule rotates to align itself to the positive charge of the wave, and then as the radio wave changes to negative, the molecule rotates again, the positively charged end this time being attracted to the negative charge. This in effect causes the molecules in the food to rotate, which causes them to have kinetic energy, which they get rid off as heat.
The next part is dependent upon the assumption that all foods have some water in them, even if we’re not aware of it. The heat is picked up by the water in the food, and that heat is transmitted to other things making up the food item. To assist in this, the microwave oven frequency is tuned specifically to the length of the Oxygen-Hydrogen bonds in water.
Because the moisture in food is the first thing that gets heated in a microwave, it’s ready to start escaping from the food right from the start. This can mean that food cooked or heated in a microwave can get drier than when using other heating or cooking techniques. Consequently, it’s best to cover food to keep the moisture in: ideally, in such a way that allows a bit of the moisture to escape so that the food surface doesn’t go overly soggy from trapped moisture on it.
The length of a microwave is about 5 inches (12 cm.) The frequency of the radio waves is 2450 megahertz (2.5 gigahertz.)
At this frequency, radio waves get absorbed only by fat, sugar and water. This makes the process very energy efficient, because the radio waves will heat only the food: not the air, nor the cooking vessels. They will not get absorbed by such thing as glass, ceramic or most plastics, and they are reflected by metal.
In conventional cooking, the heat migrates from the outside to the inside. The air is also hot in conventional oven cooking, so that as moisture evaporates, it causes browning and crispiness on the surface. In microwave heating, everything gets heated all at once, both outside and inside. Heat inside the food migrates out, though of course in some thick places the microwaves might not make it all the way to the middle. In fact, the microwaves only penetrate 1/4 to 1/2 inch (1 to 2 cm) in. So in effect, it’s only the outside of things that get heated. This is why you have to stir soup or sauces halfway through, as the centre will still be lukewarm. If you made up a ball of mashed potatoes with an ice cube at its centre, and zapped it for a minute or two, then took it apart, you’d find the outside of the mashed potato hot, and the ice cube still an ice cube. Consequently, you could say that in effect, microwave cooking ends up being “outside in” cooking anyway.
The bottom line is that microwave ovens can heat food irregularly. If you have several dishes or a big bowl of soup in a microwave oven, you have to stir the food about a bit at intervals. This is why the turntable carousels in microwave ovens became popular, to move things around and help even out the unevenness.
When placing food on a microwave turntable, place it a bit off centre so that it will actually travel through different parts of the oven — if you just place it in the centre, it will just spin in one place, which won’t do as good a job in heating the item evenly.
Microwave Oven Parts
Magnetron tubes are still in use to produce the microwaves, though the current holy grail amongst product developers is to replace its use with transistors. This would both reduce the weight of the oven, enabling portable ones to be made, and reduce the amount of electricity used. It might also mean that transistors could be placed around the inside of the oven, to allow more uniform cooking.
The magnetron tube is usually housed in the side of the oven where the control panel is, above the control panel. These tubes are about $10 US (2006 prices) of the manufacturer’s cost in producing the machine. The tubes should provide 10 to 15,000 hours of normal household use (10 to 15 years.)
The microwaves enter the actual oven chamber through top vents (called the “waveguide section”) on that side. The fan at the top of the microwave, called a “mode stirrer”, is designed to stir up and distribute the microwaves evenly. The idea is that the microwaves are directed out at the fan, which then distributes them throughout the microwave.
Microwave oven doors have glass windows with metal screen in it. The metal screen reflects the microwaves back in.
One of the most common things to go wrong with a microwave is breaking the glass plate that sits on top the turntable.
Microwave Heating Power
Precise temperature control per se is missing in a microwave.
Instead, cooking calculations are based on time, and energy output of the oven.
Sadly, energy outputs per microwave oven vary wildly. In microwave ovens for home use, the figure ranges from 300 to over 1,200 watts (as of 2006.) In commercial ones, the power is about 2,000 watts.
In general the ranges are based on bringing to a boil at high power 1 cup ( 8 oz / 250 ml) of water whose starting temperature is 75 F / 24 C:
|3 to 4 minutes||Low wattage||400-650 watt|
|2 to 3 minutes||Regular wattage||650-850 watt|
|Less than 2 minutes||High wattage||850-1000 watts|
It can be hard to know what the wattage output is on a microwave oven if it doesn’t say on it and you’ve lost the booklet or your microwave oven is a hand-me-down. Here is a simple test that should be accurate within 50 to 75 watts:
- Put 1 litre (34 oz) of water into a microwave-safe jug or container. (Ideally, the water will be somewhere around 70 F / 21 C — it doesn’t matter if it is somewhat over or under.) Record the starting temperature in Fahrenheit;
- Zap on high in the microwave for exactly 2 minutes and 3 seconds — 2:03;
- Measure the temperature that the water is now;
- Subtract the end temperature from the temperature that you started with;
- Multiply that result by 19.4.
When you lower the power on a microwave through its user interface, it doesn’t mean that you lower the “heat of the microwaves.” Rather, it means that the microwave generation is cycled on and off. For instance, 40% power means that the microwaves are only being generated for about 40% of the cooking time.
For instance, the thaw setting of a microwave oven operates at a low power setting. It cycles the microwaves being emitted off and on, to allow the warmth being generated on the outside to migrate inside, instead of just blasting the outside and cooking it.
In Europe, there has been since 1992 a standard microwave oven rating scheme, based on heating 350 g of water. Ovens get assigned a heating category ranging from A to E. This allows people to give you more precise cooking directions in recipes for your microwave.
The energy output in microwave ovens actually drops after 10 to 15 minutes of usage.
One advantage of microwave ovens is particularly apparent in summers: the heat they cause to be generated stays in the oven, and doesn’t heat up the kitchen.
All microwave ovens are designed to shut themselves off at the end of cooking time.
Safe Microwave Oven Usage
Don’t put any tightly sealed dish in the microwave — just as on the stove, you need to allow steam to escape.
It’s recommended by most health authorities to microwave food in plastic — whether a bag, a container, or plastic wrap — only if the plastic in question has been labelled as safe for microwave use. The issue is that foods, particularly high fat foods (e.g. even cheese) may absorb chemicals from some plastics during microwave heating.
To test if older glass dishes such as Pyrex are microwave oven safe, heat the dish empty for 1 minute.
- if the dish gets warm, it’s not safe;
- if the dish is somewhat warm, it can be used for quick reheating;
- if the dish stays cool, it can be used for cooking.
At first, the focus on microwave-safe dishes was on making them safe in both regular ovens and microwave ovens. This is still the focus for frozen dinners, to give the consumer buying them a range of choices as to how to heat them, but for home use, many manufacturers now make dishes exclusively for use in microwaves.
*Never* ever attempt to deep fry in a microwave; the oil would over heat very quickly.
Do not attempt home canning in a microwave (link opens on our site dedicated to home canning.) The results have been documented as unsafe.
Microwave safety — exploding water in a microwave
Water being heated in a microwave can explode — it’s not an urban myth.
The process of bubbles forming in water is called “nucleation”. It occurs when water molecules break apart, releasing gas. This happens naturally at the bottom of pots or kettles, where the heat source is concentrated.
Water in a microwave, however, can heat up to its boiling point without bubbles forming, thus, it doesn’t appear to be “boiling.” In fact, microwave heating allows water to “superheat” past its boiling point.
But, when the water is finally moved or disturbed in some way, such as adding something to it, nucleation occurs all at once and the water appears to “explode.”
To avoid this, stir the liquid before heating, and, keep zapping the water in the microwave until it actually does boil for 5 to 10 seconds.
It’s a myth that this can only happen to distilled water — it can happen to any water.
When making mashed potato, some prefer to cook the potatoes in the microwave as opposed to in water. They say they are less waterlogged.
Purists sniff that you can’t actually bake a baked potato in the microwave — you steam the potato. A microwave baked potato, they point out, is missing both the crispy crust and the truly fluffy interior that comes from baking in a dry heat environment. Some people start the potato in the microwave, then finish it in an oven.
Vegetables can be braised on low power with a small amount of stock. Many people feel that a microwave can braise vegetables better than a pot on the stove top can.
Many people swear by cooking rice in microwaves to produce fluffy rice every time.
To microwave veggies, wash them, but don’t shake the water off them or drain them very well. Let the water clinging to them be the water you use to cook them with. Cover, and microwave, which then in effect steams them.
Fish and seafood can toughen when over-reheated, so you may wish sometimes to remove them from the food being reheated, reheat that food most of the way there, then pop the fish or seafood back in for the remaining time.
Quick onion sauté: put chopped onion in a bowl or measuring jug, with half as much oil as you’d think you’d use in a pan. Zap. No stirring is necessary, and there is no need for a frying pan taking up a burner on the stove.
Microwaving, some studies contend, tends to cause a somewhat higher loss of vitamins B1, B2 and B3. Some microwave fans don’t accept these findings.
Microwave cooking was the first completely new method of cooking food that man had come across in thousands of years.
Magnetron tubes were invented in 1921 at the General Electric Company’s Research Laboratory in Schenectady, New York by Dr Albert W. Hull (19 April 1880 to 22 January 1966) for use as an amplifier, power converter, etc. Magnetrons were vacuum tubes to produce microwave radiation.
In 1940, Henry Albert (aka “Harry”) Boot (29 July 1917 to 8 February 1983) and John Randall (23 March 1905 – 16 June 1984) at the University of Birmingham in England adopted the design of magnetron tubes to create a device called a “cavity magnetron” to produce microwaves for wartime use in radar devices.
The main person is the story is a Dr Percy LeBaron Spencer (19 July 1894 to 8 September 1970.) He was born in Howland, Maine. When he was 18 months old, he and his older brother John (“Al”) Alby lost their father when he died. His mother then abandoned them in the care of an aunt and uncle, who raised them. When he was 7, that uncle died. Consequently, he was only able to stay in school until he was 12 years old. In 1906, he left school, and got a job in a mill that made clothes pins and spools, where his brother already worked. Later in life, he would be given an honorary Doctor of Science degree by the University of Massachusetts. His brother, John, also became an inventor. In 1925, he sold the rights for a thermostat he had invented to Westinghouse for 1 million dollars US.
In 1910, aged 16, Percy upgraded his job at the mill to doing electrical wiring for it, learning as he went. Then in 1912, at the age of 18, he joined the US Navy.
After the navy, he went to work for the Wireless Specialty Apparatus Company of Boston, and in 1925, he started work at the Raytheon Corporation in Cambridge, Massachusetts, recommended by his brother who was a friend of the owners.
In 1946, Percy was still an employee at Raytheon. He lived in Newton, Massachusetts, just west of Cambridge and Boston. At Raytheon, he was working on a radar project that involved magnetrons. He noticed that emissions were melting a chocolate bar (a peanut one, for those interested) in his pocket (some say shirt pocket, some say trousers.) He wasn’t the first to notice this phenomenon, but he was the first to twig into the potential. Curious, he sent a boy out to get him a bag of unpopped popcorn kernels. He placed them near the magnetron tube, turned it on, and the popcorn popped.
The next day, he brought a dozen eggs and a metal pot to work. He cut a hole in the side of the pot, put a whole uncooked egg in the pot, put the magnetron up against the hole and turned the magnetron on. These days, of course, anyone could tell him what a bad idea this was: the egg exploded just as a sceptical co-worker peered into the top of the pot to get a closer look.
Still, this is one time that “egg on the face” was a positive thing. Percy began experimenting in earnest. He made a metal box that he directed microwaves from the magnetron into a metal box. The metal box trapped the microwaves, raising the intensity inside, and causing the temperature of the food to increase rapidly. He had, in effect, just invented the microwave oven.
- 1945 — On the 8th of October, Raytheon filed its patent for a microwave oven. The company received US patent number 2,495,429 on 24 January 1950;
- 1947 — Raytheon introduced the first microwave oven for sale, called the Radarange. Selling for $4,000 US, the oven was aimed at the commercial market. It was the size of a North American refrigerator, and used water to cool it. The first one was installed in a restaurant in Boston. The name “Radarange” was the result of an employee naming contest;
- 1952 — Raytheon licenced the technology to the Tappan Stove Company;
- 1952 — Tappan introduced the first microwave oven for homes. It sold for $1,300. It was the size of a stove. It also included the first “mode stirrer”;
- 1964 — Smaller magnetron tubes were developed in Japan, which made more compact models possible;
- 1965 — Raytheon purchased the Amana company, and made plans to sell consumer models through Amana;
- 1967 — In Chicago, Amana introduced a microwave oven small enough to fit on a counter top. It had 650 watts of power, and sold for $495. It was also called “Radarange”. The door opened downwards, rather than sideways. It had two buttons. One to start it, one to turn the light on. It also had two control dials that you turned: one you used to set the time for up to 5 minutes, the other you used for time increments up to 25 minutes. Amana launched a blitz to educate retailers and the press. People in Chicago who bought the first ones got a free visit from an Amana home economist to help them cook their first meal in a microwave, and then had her advice services free by phone for the first year;
- 1971 — The American Food and Drug Administration (FDA) laid down safety standards that microwave oven manufacturers had to meet. This reassured consumers that they could purchase safe ones, so sales took off;
- 1976 — 17% of homes in Japan had a microwave, 4% of American homes;
- 1978 — 10% of homes in America had a microwave;
- 1982 — 27% market penetration in America, 31% in Japan, 10% in Canada;
- 1986 — 60% market penetration in America, 44% in Canada (American figures for this year range from 25% to 40% to 60%, depending on which source you consult);
- 1991 — 55% of households in the UK had a microwave;
- 2002 — 87% of households in the UK had a microwave;
- 2006 — 95% of homes in America had a microwave.
In the mid-1980s, prepared food manufacturers started making food products specifically for the microwave (rather than just sticking microwave instructions on other food.)
By 1993, though, such food products started to be less popular. There was a perception that they weren’t as good or as healthy, and that they used too much packaging. Most of them were frozen as well, requiring premium storage space at the grocery store. They were also seen as very expensive.
Literature & Lore
“It is ludicrous to read the microwave direction on the boxes of food you buy, as each one will have a disclaimer: ‘THIS WILL VARY WITH YOUR MICROWAVE.’ Loosely translated, this means, ‘You’re on your own, Bernice.'” — Erma Bombeck (21 February 1927 – 22 April 1996)
McGee, Harold. On Food and Zapping. New York Times. 2 April 2008.
Murray, Don. Percy Spencer and His Itch to Know. Reader’s Digest. August 1958, page 114.
Schiffman, Robert F. Microwave Technology: A Half-Century of Progress. Northbrook, Illinois. Food Product Design Magazine. May 1997.