What temperature does a mixture of rock salt and ice reach?
Rock salt forces the ice surrounding the container of ice cream mix to melt. The “brine solution” or liquid that forms in the outer container 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. The length of time it will remain this cold will depend on many factors, including the amount of ice and water used, and the ambient temperature of the environment.
