a technique in which electromagnetic radiation is used to heat food. All radiation is a form of energy, and energy can be converted from one form to another. In this case the radiation absorbed by the food is transformed into heat. Microwaves are similar to radio waves but of higher frequency: in a domestic microwave oven this is 2,450 megahertz (MHz, millions of cycles a second), compared to around 100 megahertz for VHF radio. Radar also uses microwaves, so that in theory it would be possible to cook food by putting it at the focus of a radar dish.
Microwave cooking is quick, because the radiation penetrates the food more readily than the ‘heat radiation’ given off by the heating element in a conventional oven. (Heat is propagated by infrared radiation, similar in nature to microwaves but of much lower frequency.) It is also efficient, because radiation of a frequency around 2.5 GHz is strongly absorbed by the large organic molecules in food. This is apparent from the fact that the container in which the food is placed is hardly warmed at all, because its molecules are much smaller—they would need a higher frequency to be heated effectively.
Thanks to the efficiency of the process, microwave ovens use much less power than ordinary ones; typically they consume 600 to 800 watts, against 3,000 or more for an electric oven, and only for a fraction of the time needed for cooking by conventional methods.
A common misconception about microwave ovens is that they heat food ‘from the inside’. This is not so: all the radiation is beamed into the food from a waveguide (a type of antenna) in the roof of the oven, and has to penetrate it like any other radiation. When using the oven to thaw a large piece of meat, it is possible to leave the centre frozen if the heating time is too short. Nor is the radiation evenly distributed throughout the cabinet: there are ‘hot’ and ‘cold’ spots. Most ovens have either an electrically driven turntable to revolve the food, or rotating ‘stirrers’ to deflect the radiation, so that heating is as even as possible.
A drawback of microwave cooking is that the food is not browned on the outside. This can be overcome to some extent by putting the cooked food briefly under an ordinary grill. Some microwave ovens have been made with a small conventional heating element inside the cabinet, which can be switched on to brown the food.
Some cooks scorn microwave ovens because they do not cook food ‘properly’. But only a diehard traditionalist would deny that they are very useful for thawing frozen foods and reheating dishes made in advance and frozen.
Two spectacular accidents can occur. If food is put inside the oven in a metal dish, even a thin one made of foil, it will produce electrical discharges like a small thunderstorm in the cabinet. It is tempting to make a single cup of tea by using the oven to boil a cup of water and then putting in a teabag. The oven can ‘superheat’ the water above its normal boiling point, without the water actually boiling—there has to be some kind of ‘nucleus’ for the first bubble to form around, such as a speck of solid matter in the water. When the teabag is dropped into the superheated water it provides plenty of nuclei; the water boils suddenly and the teabag explodes with great force.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
