A hit not to be missed: Ice cream in a bag

A bowl of apple and pear sorbets.
A bowl of apple and pear sorbets. (Mette Randem for The Washington Post)
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By Andreas Viestad
Special to The Washington Post
Wednesday, August 4, 2010

It seems as if the entire world is melting. Still, there is one summer activity that can cool things down a bit and even tease the brain into a little bit of learning, with school still weeks away.

In my family, making ice cream and sorbet in a bag is a favorite pastime. Not much can go wrong: There are no machines that need to be assembled or cleaned or that can otherwise conspire against you, no advanced technical skills to learn or preparation times to keep track of.

Fill a large sealable plastic bag with ice and salt. Fill a smaller, food-safe plastic bag with an ice cream mixture or sweetened juice. Cradle the smaller bag inside the larger one and seal. Shake for a few minutes. Eureka! You have the freshest, softest, most perfect ice cream or sorbet. And, unlike kitchen-based ice cream making, this technique is best when performed outdoors: under a tree, on the beach or while hiking far from civilization.

There is nothing novel about the method. Apart from the use of plastic, the technique has been used since at least the 13th century, when there were reports of ice cream being savored by European visitors to the Arab world, and possibly all the way back to 10th-century China, from whence a poem titled "Ode to the Ice Cheese" has led food historians to speculate on the method's earlier origins.

I like making ice cream this way because of the sheer joy of watching children control the process, from the actual freezing to the happy jostle for the last spoonful. (There is always one more spoonful to be found inside the bag.) I also like the way the treat introduces an element of learning into summertime laziness.

What are the principles at work? That is what I show my kids as they stir together ice and salt, and while we all watch the magic that ensues. I know the basics, but when I have to explain it, I reconnect to a bit of everyday knowledge.

When you have a mixture of ice and water, such as a glass of water with lots of ice cubes, the two find equilibrium and the temperature stabilizes at the freezing point, or melting point, of 32 degrees Fahrenheit. There will be some melting of the ice, and at the same time there will be some freezing of the water. But it does not all suddenly freeze or melt. If the surrounding area is warm, there will be some net melting. But the process is relatively slow.

Water needs a lot of persuading to pass from one phase to another. When water reaches the boiling point, it starts to turn into gas. But as we all know after having boiled potatoes and pasta innumerable times, the vaporization takes time and lots of energy; after 10 or 20 minutes of vigorous boiling, water has evaporated noticeably but not dramatically.

The same applies to the processes of freezing and melting. The ice takes energy from its surroundings in order to melt. But if you place your bag of ice cream mixture inside a bag of ice, the only thing you will achieve is the same sort of equilibrium as with a glass of water and ice cubes: The ice remains as melting ice-water slush at 32 degrees, and the ice cream mixture is cooled to 32 degrees but remains in its liquid state.

Add salt to the ice, and the equilibrium is disrupted. The melting process speeds up. At the same time, salt lowers the freezing point of the water, or brine, surrounding the ice.

After a few minutes, the temperature drops 10, 20 or even as much as 32 degrees, to zero degrees (or below), depending on the amount of salt added, a phenomenon so fascinating that Daniel Fahrenheit used it as the starting point when making his temperature scale in the early 1700s.

A mixture of about 10 percent salt in water will take the freezing point to 21 degrees. The lowest possible temperature that can be achieved by this method is with 23.3 percent salt, which can lower the temperature to 6 degrees below zero; there is no need to add more than that. (Because we are not doing this in a lab, it is doubtful that we will achieve temperatures quite as low. Without making any special effort to dissolve all of the salt, I have achieved 2 degrees, and that is more than cold enough to make ice cream.)

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