Science Questions

Why is it that if you want to cook two items in the microwave, you’ve got to cook them for twice as long when you don’t have to do the same with a normal cooker? And do microwaves kill bacteria in food? Dave & Kevin

Chris: When you put something in the microwave it’s getting hot because it’s soaking up energy from the microwaves which are flowing from one side of the microwave to the other. The microwaves make the molecules in the food vibrate. That soaks up energy and makes them hotter. If you put twice as much stuff in the microwave it’s going to soak up twice as much of the energy from the microwave, therefore you’ll have to cook them for longer because you’ve not got as much energy to share just in once piece of food.

In terms of microwaves killing bacteria, if you don’t heat food to the right extent there’s a danger of getting hotspots and colds spots. That’s why there’s a turntable in a microwave. Without a hot enough area in the food any bugs could persist so you could end up getting food poisoning. Microwaves are just radio waves and the reason they make food cook is that it absorbs the energy of the microwave, the molecules vibrate and this raises the temperature. They’re not doing anything special or zapping bugs by gamma irradiation or blasting DNA. They’re just heating them up. If you don’t make the food hot enough then you’re not going to kill anything. It’s all down to heat and nothing else.

November 2007