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- Photo: Alan_D / Flickr
Ever wondered how the sun’s heat reaches us across such a huge expanse of empty space? It’s an interesting question since there’s nothing there to conduct the solar energy. The answer is: thermal radiation. Thermal radiation requires no direct physical contact between an object and the source of the heat. We owe our ability to cook food in a microwave oven to the process of radiation. Heat radiation makes standard cooking possible. Everything emits radiation all of the time.
Different radiation carries different energies
Harold McGee explains that when you feel heat from sunlight it comes from thermal radiation. ‘It’s emitted by atoms and molecules which, having absorbed energy, release it again not in the form of faster movement but as waves of pure energy.’
Different energies are carried by different kinds of radiation. The associated energy will influence the effect that particular radiation will have.
Moving up the scale to microwaves
Starting at the bottom of the scale, we have radio waves. These are weak waves, which is why antennas are required to receive and transmit radiation. Radio waves are only able to cause an increase in movement in free electrons.
Microwaves are next. These waves are shorter than radio waves which makes them more energetic. They have the ability to get polar molecules like water to move faster. Microwave ovens use radiation which is effective because most foods are made up of water.
Moving further up the scale – and into the pot
Heat radiation comes next with its ability to increase the movement of polar water and non-polar molecules. These include fats, proteins and carbohydrates. Heat radiation is the energy source for ordinary cooking.
Going too far – as far as food is concerned
Next up the scale are visible light and ultraviolet light. In this spectrum, orbits of electrons bound in molecules are affected. This is when chemical reactions can occur, as witnessed by damage to fats and pigments, leading to rancid flavours.
End of the spectrum
X rays and gamma rays have the ability to penetrate matter and ionize it. They are used to sterilise and ‘cold-pasteurise’ food because they’re able to kill microbes.
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