For dessert, we mixed up some blackberry sorbet with our new ice cream maker, using a recipe book from William Sonoma. We got to talking about the old fashioned way of making ice-cream (our machine has a space-age cooling liquid canister), and the question came up of why you always had to add rock salt to the ice. Laura found a great reference at MakeIceCream.com.
Rock salt forces the ice surrounding the can of ice cream mix to melt. The "brine solution" or liquid that forms in the wooden bucket absorbs heat from the mix and gradually lowers the temperature of the mix until it begins to freeze. If there were no salt added to the ice, it would melt at 32 degrees Fahrenheit and eventually the ice water and mix would come to equilibrium at 32 degrees. The ice cream mix, however, does not begin to freeze until its temperature falls below 27 degrees. Therefore, in order to freeze the mix, we need a salt concentration, or a ratio of 5 cups of ice to 1 cups of salt. At this concentration, our brine temperature should remain constant at 8 to 12 degrees F. This will give the rapid cooling and freezing that is essential to making smooth creamy ice cream.
That's cool!. For more info, check out this after-school science experiment. I like the final comment in the instructions for adult helpers:
If any of the student pairs do not get solid ice cream, take their ziplock bag and place in the Dry Ice container for a few minutes.
So even if science fails, those kids are getting ice cream. :)
