The charm of the Harold McGee ice cream recipe as it appeared in the New York Times a few weeks ago is that it takes us right back to the earliest days of ice cream making. That is, when frozen cream enthusiasts in Europe discovered that the combination of ice and salt produced a chilling medium sufficiently cold to turn what had heretofore been a slush of ice and milk into a rich and silky wonder.
How does it work? Mr. McGee explains, albeit in a somewhat convoluted way, in the article I linked to above. An easier way to think about it is that pure water, like all substances in nature, can only remain liquid within a certain temperature range. Above 212 degrees Fahrenheit and water turns to gas (steam), below 32 and it turns to a solid (ice). But how cold can that solid get? There’s really no practical limit, but a lot colder than the freezing point of 32, that’s for sure. Home freezers get at least as low as 10 degrees Fahrenheit (good ones go down to zero), so for all intents and purposes, homemade ice is a 10-degree substance.
Put that 10-degree ice into cold, say 45-degree, water and it will cool the water further, but even though the temperature of the ice is well below the freezing point, it can’t cool the water below 32 (otherwise the water itself would freeze). The addition of salt, however, lowers the freezing point of the water, thereby allowing the 10-degree cubes to chill the liquid water to a point much closer to their own temperature. There’s no cold-making “reaction” going on, but it’s still a very neat phenomenon. Well…isn’t it???
Thus explained, it’s easy to see how Mr. McGee’s ice cream procedure works. As I said it’s nothing new, though putting the small amount of ice cream mix in a gallon freezer bag is a neat twist, since it increases the surface area, allowing more mix to come in contact with the brine. That speeds up the freezing process, which is important. For quick freezing, as we will discover as the week progresses, is key to a smooth and delectable finished product. Here’s the recipe:
3 to 4 pounds ice (10 cups or more)
1/2 cup table salt or 1 cup kosher salt, plus a pinch
1 cup milk
1 cup heavy cream
1/2 cup sugar
1/2 teaspoon vanilla extract.
1. Pour all the ice into a large glass or plastic bowl, cover it with salt, and stir.
2. In a bowl, mix milk, heavy cream, sugar, vanilla extract and a pinch of salt together until sugar dissolves. Pour into a sealable 1-gallon freezer bag, push out as much air as possible, and seal.
3. Place a wide plastic bowl with a lid, like a salad spinner bowl, on a kitchen towel. Fill bowl with about half the ice. Lay freezer bag on ice and flatten it with your hand. Dump rest of the ice, along with any melted water, on top of bag, leaving zipper edge exposed. Place lid on the bowl. Let rest for 15 minutes, shaking it once or twice to redistribute ice and brine.
4. Pour about half the ice and brine into another bowl. Lift bag out by the zipper edge and lay it on a towel. (Avoid touching ice or brine, which are cold enough to cause frostbite.) Cover your hands with another towel and gently knead frozen areas for about a minute to mix them with liquid.
5. Return freezer bag to bowl, laying it flat on ice. Cover it with reserved ice and brine. Put lid on bowl and freeze as above for another 15 minutes.
6. Remove bag and carefully towel off the brine. Serve ice cream, or keep bag in freezer until ready to serve.
Yield: One pint.
Note: You can use this method to freeze any ice cream or sorbet mix. If you make ice cream regularly, you can eliminate ice cubes and reuse salt. Make a brine with 3 quarts water and 1 pound salt, divide it between two 1-gallon freezer bags, and store bags flat in freezer. To make ice cream, sandwich the bag of mix between brine bags, enclosing stack in towels.
Joe’s note on the note: Personally, I don’t recommend Mr. McGee’s suggestion of keeping bags of liquid brine permanently in you freezer. Given that even those zip-top style freezer bag are wont to pop open at inopportune times, the result would be a flood of very, very cold liquid in your freezer that would be difficult to mop up.