On Microwaves

Dinner time! From reheating the soup you made from the limp vegetables that were nestled at the back of your fridge to zapping a miserable ready meal, you’ve no shortage of options to choose from. I’m not here to tell you what to do though; I just want to tell you how microwave ovens work. We’ll examine the process through a scientific lens and dispel the many myths about this method of heating food. (e.g., At a glance I’d say there are about thirty false or misleading claims in this article on the topic.)

To cook or not to cook

Let’s back up a bit for a moment. Before addressing how microwaves cook food, we should first establish whether it’s even healthy to cook food at all since the number of people adhering raw diet is on the rise. Proponents of this trend highlight the fact that cooking food denatures enzymes and destroys nutrients within it. For example, an enzyme in broccoli called glucoraphanin, which is being researched for its “anticancer” (excuse the banal buzzword) properties, is damaged during the cooking process.

To pepper this anticancer claim with a little detail: Our bodies convert glucoraphanin into sulforaphane and it was shown that sulforaphane, when applied directly to breast cancer stem cells, hinders cancer growth. This finding was reported on hyperbolised by the Daily Mail. The conclusion? Eating raw broccoli and not reading the Daily Mail helps prevent literal and figurative cancer growth, respectively.

Apparently regularly placed images are required to hold the attention of blog readers so here’s a Pop-Tart magnified by a factor of 450 using a scanning electron microscope. Delicious. (credit: Caren Alpert)

Cooking damages nutrients, it’s true, but it isn’t the full picture. Often we want to inflict damage to increase the bioavailability of the nutrients (i.e., the degree to which nutrients can be absorbed by your body). We need look no further than the humble tomato for evidence of this, where applying heat increases the bioavailability of a tasty antioxidant called lycopene by breaking down the cell walls. Cooking doesn’t just damage the food, it kills bacteria too, as anyone who has endured a bout of BBQ food poisoning will tell you.

In an evolutionary context, cooking played a vital role in giving homo erectus the edge over other apes who stuck with raw foods. Aside from the reduced bioavailability, raw food takes longer to digest and it requires more energy to eat. It also has a higher water content so you get less bang for your buck with respect to calories. Our brain size was limited by the volume of calories we could chew through per day as the brain is a very costly organ to operate in terms of energy. Queue the Michelin Star chefs of the Calabrian age: The additional energy our ancestors started taking in when they began cooking caused our brains to get really big, really fast (a mere 600,000 years) – a process called rapid encephalization.

To say that cooking food is always the better option is an oversimplification of a real-world issue and there’s no shortage of counter-examples but it is, if nothing else, pragmatic.

Let’s heat

Everything is made of atoms and molecules which are in motion. The more the particles move, the hotter something is. Take water: When frozen, the molecules form a neat lattice and are only vibrating a little, forming solid ice.

As you heat the water, you’re supplying energy to the molecules. They begin to rotate and slide past each other, causing the ice to melt. Adding more energy causes the molecules to move even faster still and they end up much further apart – in which case, the water has evaporated to steam.

solid-liquid-gas
Temperature is just the measure of the energy of a particle. When we say something has a higher temperature, we’re really saying the molecules in that substance are moving faster. When you stop the particles moving altogether, you get the lowest possible temperature, -273.15ºC. Nothing in the Universe can ever get colder than that because this is when the molecules are completely still and have no kinetic energy. (credit: Tes.com)

There are three processes of transferring heat to food: conduction, convection, and radiation. Frying something on a pan utilises conduction, where heat is transferred by collisions of the particles in the pan and the food. Convection involves the movement of particles in a substance such as boiling a pot of water. Microwave ovens rely on radiation.

Radiation

Although “microwave radiation” is a bit of a dirty term (a victim in some ways of the appeal to nature fallacy), they are relatively safe. Microwaves are a form of low energy electromagnetic radiation. Visible light, for instance, is another form of electromagnetic radiation except that it’s over a million times more energetic than microwave radiation. Microwaves are also non-ionising meaning that they don’t have enough energy to alter your genetic code. If you were put in a microwave oven on full power, your skin would get really hot and it’d be painful but it wouldn’t cause cancer. The same cannot be said be said for tanning salons, which use UV radiation.

So stuff can be heated by absorbing energy which is carried by electromagnetic radiation. This is the property we use in microwave ovens to transfer energy to food. When you switch on your microwave oven, it produces billions of electromagnetic waves about 12 centimetres in length. These microwaves induce an electric field in the oven. The electric field causes the water and fat molecules in your food to spin faster, becoming hotter.

As it is mainly the water and fat molecules which are being heated, foods which have a high water or fat content tend to heat the quickest. Most microwave meals say that it should be left to stand after cooking. The reason for this isn’t to let the food cool down, it’s to let the heat dissipate evenly throughout the food. You heated the water molecules and they need time to bounce off the surrounding molecules and share the heat.

It is also not true to say that microwaves cook the food from the inside out. The microwaves can penetrate the food so they heat the water molecules wherever they are – not just the surface but not strictly from the centre outwards.

How do you like your eggs in the morning? I like mine with an appreciation of the underlying mechanisms

We know that microwaving is not an inherently unhealthy way to heat food but we can go further than this. It is are actually one of the healthiest. Things cook faster in the microwave so there is less time for the nutrients to become damaged. Some vitamins dissolve in water and this is an issue when boiling vegetables but not microwaving them. Chemicals in charred food have been linked to cancer and, again, this is a concern for other cooking methods like grilling but not using the microwave.

Just because something is quick and easy doesn’t make it bad and the kind of skeptical approach we’ve taken here can easily be generalised to other areas of life. Seeking truth on any issue is highly desirable and often asking “but why?” is usually enough to get you there. Don’t even take this blog post as conclusive evidence that it is safe to microwave food – do some research and evaluate trustworthy sources.

We can’t do much about the fake news phenomena (which has been around for thousands of years and is here to stay) but we can acknowledge our internal preferences, biases, and superstitions. We’re just human – perpetual victims of confirmation bias and cognitive dissonance – but we have to do our best to recognise this and seek the truth, whatever it may be.

“BEEP.” OK, food’s done – gotta go.