Milk is a complicated mixture of lots of different kinds of molecules, including fats, proteins, and water. Normally, freezing would cause milk to separate into its various parts. The trick to making ice cream is to get the cream cold enough to freeze without the parts separating. The success of this recipe comes down to keeping it all moving. Get ready to shake your way to dessert!
Put the cream, milk, vanilla, and sugar in one qt.-size plastic bag. Remove as much air as possible and seal the bag well. Put this bag inside the second qt.-size bag, again removing as much air as possible and sealing it well. (Double- bagging the ice cream mixture will help to prevent leakage.)
Put the double-bagged mixture into the gallon-size plastic bag and add the ice and salt. Once again, remove as much air as possible from the bag and seal it well.
Wrap the bag in the dish towel. Shake and massage the bag, ensuring that the inner bag is surrounded by the ice. Keep massaging it for 5–8 minutes. Open the bag. Inside, the mixture will have frozen into ice cream!
THE SCIENCE SCOOP
When water molecules freeze, they line up in a neat, orderly, and tight formation, so there’s not much room for other molecules to mix in. If you just put your ingredients in the freezer, the water molecules would separate out as large chunks of ice. But when you toss and shake the molecules in the bag, they can’t connect or easily get a grip on one another. Only microscopic chunks of ice are able to form, keeping the water mixed in with everything else. It’s hard for water to freeze when there are other molecules in the way, so the temperature has to be colder than that for making regular ice for it to happen. That’s where your outer bag of ice and salt comes in. Salt lowers the temperature of the ice so that your ice cream mix is cold enough to freeze.