Field of Science

The Scoop on Thawing

My mom made delicious home-made ice cream. When I was a kid, she made it in a turquoise electric crank machine, the ice had to be chopped up with an ice pick and added, along with layers of rock salt to the bucket. The machine needed careful tending as the motor whirred away, a hot job on what was sure to be a hot day, but the reward for layering in the salt and ice was to lick the paddle.

Stephen Metcalf has a review of the next generation of ice cream makers up at Slate magazine: The Inside Scoop The next generation includes those with built-in freezers and and some with a gel filled canister you pre-freeze -- they still make the ones like my mother used and ones you can use at a picnic to soak up excess kid energy. Metcalf notes that

"More expensive machines contain built-in freezers, with obvious advantages: A built-in compressor maintains a constantly low temperature throughout the freezing process (whereas a gel canister starts to thaw the instant you remove it from your freezer), and consecutive batches can be made without waiting 24 hours for the canister to re-freeze."

There is physical chemistry here -- even though the gel canister does begin to thaw as soon as you take it out of the freezer, it doesn't actually get warmer immediately. As the gel thaws, the temperature remains constant until the thawing is complete. Why? A very simple-minded way of thinking about it, is that all the energy going into the canister goes into melting the gel, so there isn't any "left-over" to raise the temperature. Don't believe me? Take a glass of water and ice and stick a thermometer in it. Wait a while, stir (the stirring is important), and take it again. If all the ice hasn't melted yet, the temp should be unchanged.

A wonderful recipe for ice cream made with liquid nitrogen.

Another post on the physical chemistry of ice cream.

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