No Reverse Microwave

Stupid Question ™
July 5, 2001
By John Ruch
© 2001

Q: Why isn’t there a reverse microwave oven to cool food quickly?
—J.D.


A: Microwave ovens heat food invisibly and quickly, without raising the air temperature in the oven. It seems like magic, so why not just reverse the spell and start flash-freezing?

The problem is that microwaves are energy in electromagnetic form. In the oven, they heat food by adding their energy to it. Cooling food requires the removal of energy, not the addition of it.

Like light and radio waves, microwaves are electromagnetic waves, but they’re of especially high frequency and wavelength. They pass easily through air and bounce harmlessly off metal and most plastics, glass and ceramics. They are extremely useful in radar and telecommunications.

They can also heat food—as long as the food contains water. (Pure oil, for example, won’t heat up in a microwave.) Within a particular frequency range, the cycling of their electromagnetic fields causes water molecules (which are electrically polarized, with a negative charge at one end and a positive charge at the other) to flip back and forth. This constant flipping makes the water molecules rub against each other very rapidly, turning the microwaves’ energy into frictional heat.

The frequency of almost all home microwave ovens is 2.45 gigahertz, which means the water molecules flip at a rate of 2.45 billion times per second. That makes for mighty fast heating. And because the microwaves penetrate all parts of the food item simultaneously, the whole thing is cooked all at once (as opposed to conventional ovens, which use hot air to cook from the outside in).

Microwave ovens are also highly energy-efficient. About 50 percent of the power required to run them goes directly into cooking, compared with only about 10 percent for conventional ovens.

Refrigeration, on the other hand, remains a difficult and relatively inefficient process. Flash-freezers are available, but they use expensive coolants and probably don’t have enough household utility to be worth it.
The problem is that energy can’t be destroyed, only moved to another place or turned into another form. It takes a lot of work to remove that energy and keep it out.

Home refrigerators and freezers work on the principle that fluids (typically Freon or a similar substance) absorb heat as they change state from liquid to gas, thus lowering the temperature of nearby objects. A gas is compressed and forced into a condenser, where it cools and liquefies, then flows into coils inside the walls of the refrigerator. Inside these coils, it vaporizes, which draws heat from the whatever’s inside the fridge. It then rises back to the compressor.

Like the conventional oven, this process ultimately is moving heat through the air. It’s hard, expensive work. Unfortunately, there is no known “cold ray” to replace it, and it’s hard to imagine how there could be one.