The microwave oven is truly one of the greatest inventions of the 20th century. It made life simpler. While for some it’s impact is more about pure convenience – a quick Hot Pocket or fruit pie on the way to work – for those who remember warming baby bottles in hot water on the stove in the middle of the night, it was a godsend! So what happens when the light goes on, the food spins, and a noisy exhaust fan kicks in?
For one thing, we’re not talking typical cookware processes. Here’s the shortest course on microwaves: They (2.5 ghz radio waves) are absorbed by water, fats and sugars and this absorption turns into heat. Luckily, microwaves are not absorbed by most plastics, glass or ceramics. Metal, however, reflects microwaves and doesn’t work work well in a microwave oven. How many of us learned this the hard way? – “Whoa, that really was tin foil!”
Think of it as backwards cooking. Conventional ovens cook by conduction from the outside of the food in, whereas microwaves work just the opposite – from the inside out. Microwaves penetrate food and excite water and fat molecules fairly evenly throughout the food. Heat is generated everywhere all at once because the molecules are all excited together.
It’s not a perfect process, however, because radio waves penetrate unevenly in thick pieces of food and sometimes don’t make it all the way to the middle. Microwave ovens are designed to spin food in an attempt to counteract this tendency. Food ‘hot spots’ are caused by wave interference. The whole heating process is different because you are ‘exciting atoms’ rather than ‘conducting heat.’
And then there’s the lust for crust. Because the air inside microwave ovens is at room temperature, there is no way to form a crust as with a convention oven where the air inside the oven is hot and dry. Not to worry. Manufacturers of microwavable foods cater to our crust craves by adding a little sleeve made out of foil and cardboard.
You put the food in the sleeve and then microwave it. The sleeve reacts to microwave energy by becoming very hot due to ‘resistance,’ or wave interference. This exterior heat lets the crust become crispy as it would in a conventional oven. The same thing happens with microwavable pizza, except that the resistant surface is placed beneath the product to produce ‘acceptable’ crust.
That’s the nature of microwaves – they excite certain molecules in food to produce the heat necessary to cook it. It doesn’t sound like rocket science, but for most of us, no matter how often we read a simple explanation of the microwave principle, it’s still somehow magical when the timer dings.