How safe is the microwave oven?

What you need to know:

  • Some say this modern kitchen appliance pumps cancer-causing waves into food during cooking, while others say it is as safe as they come. Who’s telling the truth, and who’s baking fibs?

A decade ago, the microwave oven was a key indicator of the standards of living in a country. Now you can get one for as little as Sh5,000.

Yet the fact that it can be found everywhere, including backstreet food kiosks, hasn’t helped to dispel the myths and the fears, real or imagined, about it.

The fantastic heating speeds of a microwave oven make it a very convenient appliance. The device uses relatively less-understood heating technology, fuelling suspicion that it might release radiation that is harmful to health.

In 1946, Dr Percy Spencer, a curious engineer, was developing a tube called magnetron for Raytheon Corporation in the US when a candy bar in his pocket melted. Voila! He realised that the magnetron could be used to heat food.

Magnetron, the main component in the oven, generates the microwaves used to heat food. Following the melting of the candy, Dr Spencer placed popcorn kernels near the tube, and they popped. Curiosity mounting, he placed an egg near the magnetron, and it exploded. The rest is history.

Before Spencer’s discovery, during World War II, magnetron was used by the British to detect German warplanes as microwave radar. It is after World War II, while Spencer was working on a new type of magnetron for American radar system, that he bumped onto the food heating effect.

From that coincidence, the microwave is today found in almost every kitchen in the developed world and most middle class homes in developing countries­.

Food regulating organisations, such as the Food and Drug Administration (FDA) and the World Health Organisation (WHO), have approved the use of the microwave oven. However, some believe that microwaved food is unsafe for human consumption.

Understanding the risks associated with the oven, or the lack of such risks, requires thorough familiarity with the mechanism the appliance employs to heat food.

Is microwaved food safe for human consumption?

Yes. According to the World Health organisation (WHO), if the microwave oven is used according to the manufacturer’s instructions, the food is safe and of the same nutrient value to that cooked by conventional methods.

The term ‘nuke the food’ is frequently used when warming food in a microwave, suggesting that food becomes radioactive because the term “nuke” refers to radioactive material. Using this reference in association to warming food in a microwave oven is misleading. Microwaves are not radioactive and do not cause the microwave oven or the food to become radioactive.

Is it safe to use containers in a microwave oven?

Always use containers labeled “microwave safe”. Using unsuitable containers to warm food in a microwave might cause toxic compounds to migrate from those containers to the food. Those compounds might be extremely toxic, and may actually be cancer-causing.

Fats and foods high in fat content can hit very high temperatures in a microwave oven, making them attract compounds from containers.

Most plastic containers found in the kitchen — water bottles, plastic tubs or containers made to hold margarine, cooking fat, yoghurt and so on, can be used in a microwave. However, this doesn’t mean they are microwave-safe. Don’t use them. They are only ideal for holding cold foods and not suitable for reheating.

Also, never use metallic utensils inside a microwave oven. Metals, including aluminium foil, reflect micro-waves, leading to generation of sparks that can lead to fire.

Microwaved food cools faster than food cooked over open fire, gas or electric cooker. Why?

Microwave ovens heat food by heating the water in it. Water, mark you, has a boiling point of 100 degrees Celsius. Once it reaches boiling point, it starts to evaporate, and there is minimal increase in temperature above 100 degrees Celsius. After all the water evaporates, it is ideally difficult to heat the food further in a microwave (sometimes it burns). Due to this phenomenon, foods heated in a microwave achieve lower maximum temperatures compared to open fire, hot plate or gas-cooked foods.

In contrast, food cooked over hot plates, gas cookers or electric cookers continues to heat up even after all the water has evaporated.

Can I warm my baby’s food in the microwave?

Yes. However, great care is necessary to ensure that the hot food does not scald the baby’s mouth. A baby bottle warmed in a microwave might seem just warm on the surface, while the contents might be extremely hot. Mixing the warmed food or milk thoroughly minimises the chances of hot spots.

A baby’s raw food, particularly raw meat, might not be cooked in the microwave oven because the heating might not guarantee complete microbial inactivation in such food.

Is there harm in defrosting my food more than once?

Thawing or defrosting food such as meat more than once creates a conducive environment for growth of harmful bacteria.

Ideally, frozen food suppresses the growth of bacteria, but it doesn’t kill them. Microwave defrosting is very fast, allowing food to reach ambient temperatures that allow bacteria to multiply. Re-freezing the defrosted food suppresses a higher number of bacteria than the initial freezing. A follow up thawing would allow that high number of bacteria to multiply.

This applies to any method of thawing food, such as use of warm water. Defrosted food has to be consumed once it has been thawed, otherwise it should be disposed.

The best method to defrost food is slowly in the refrigerator because its temperatures are not conducive to the proliferation of bacteria.

Do the waves remain in the food after it’s warmed?

According to FDA, WHO and respected published scientific data, micro-waves are not embedded in food or in the cavity of the oven after warming the food. Microwaves are like a conventional light bulb. After switching off, no light is left in the room.

Why I should cover the food?

Covering food while in a microwave oven prevents soiling the microwave cavity with spattering. During microwaving, food generates steam, which creates local pressure that causes the food to be splashed around. Manufacturers of microwavable food advise on creating holes on the covering lid to allow steam to vent to avoid explosions.

Remember, the partial covering of food in the microwave helps to retain the steam, aiding in uniform heating.

My food turns into missiles

Micro-waves heat food from inside out. The water at the centre of the food heats faster to very high temperatures than water in the outer parts. This creates steam at very high pressures. The steam tries to escape, and the lack of an exit point causes the food to explode.
There is a risk of getting burnt by the exploding food. Therefore, it is not advisable to heat eggs or completely covered food (in air-tight containers) in a microwave oven.

The ability of foods to explode in a microwave oven is being productively utilised to make popcorns and other expanded snack foods.

Some cups and plates heat up in a microwave. Is this risky?

There are types of plates and cups that absorb microwave energy and heat up. Such utensils can heat up to high temperatures and pose a risk of burning the holder.

It is always advisable to use microwave-safe containers which are transparent to micro-waves.

Some parts of warmed food remain cold. Are they safe?

The answer is Yes and No. One disadvantage of heating food in a microwave oven is non-uniform heating; some parts of the food heat up very well, while others remain unheated. The heated parts are called hot spots, while the unheated ones are called cold spots. Harmful bacteria may grow on the cold spots, while the hot spots can cause scalding. This is why the food is rotated to guarantee the uniform distribution of heat. It is also a good practice to stop microwave mid-way heating and stir the food.

After heating, the food should be left in the microwave oven for a few minutes to allow the heat to distribute evenly.

Can the waves leak through the appliance’s transparent glass door?

No. A respected pioneer of microwave heating of food, Prof Buffler, writes that the glass door helps the user to see inside the oven, but it has a metallic mesh that prevents micro-waves from passing through. To increase the level of safety, the door has a mechanism to ensure that the oven doesn’t work when the door is open.

Attempts to repair broken microwave ovens, or tamper with parts of a functioning oven, particularly the outer covering metal, can lead to serious consequences. A broken microwave oven should be disposed safely. Leaking micro-waves are highly harmful.

Dr Arimi filed this piece from the Food Science Centre at the School of Agriculture, Food Science and Veterinary Medicine, University College Dublin, Ireland. Send your comments and suggestions to [email protected]