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Q & A: evaporative cooling of food

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Most recent answer: 11/20/2012
When do french fries and pizza cool off to a temperature colder than room temperature ? If left out on the table, french fries and pizza (originally very hot) will be ice cold. Why do they just not cool off to room temperature ?
- Alan Tischler (age 60)
Baltimore, Maryland, USA
The food typically starts out out of equilibrium with the room in two major ways:
1. It's hotter than the room
2. It's got more moisture (higher chemical potential of water) than the atmosphere.

If there was only effect (1), the food would gradually cool to the room temperature. It might still feel cold to the touch, since it may conduct heat better than other things you touch at the same temperature. However, effect (2) causes evaporative cooling. There's a net flow of water from inside the food to vapor in the air. (This is assuming that the air in the room is fairly far from saturated with water vapor.)

Evaporation cools things, because it takes energy to pull a water molecule out from a liquid and leave it isolated in a gas. That's why you cool as sweat evaporates. This can cool the food down to a lower temperature than the surroundings, temporarily. Once the food has dried out, the extra cooling stops and it should come back to room temperature.

A particularly dramatic example of evaporative cooling can be obtained by pumping on a container of liquid water with a vacuum pump. Often you can get some of the remaining water to freeze, even though it's only in contact with room-temperature surroundings. If the room has a high concentration of water vapor, or if you dry out the food first, I bet you don't get this extra cooling.

Mike W.

(published on 11/20/2012)

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