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.
More detailed information provided by David Winer, Bethesda, Maryland
For ice to melt into liquid water it must absorb a lot of heat energy. In an ice cream maker this heat needed to melt the ice comes from the mix, and so the mix chills. If the ice were not combined with salt, the mix’s cooling would be slow and would stop when the mix reached the temperature of melting freshwater ice-- at 32 degrees.
But salt makes ice melt faster, and drops the temperature to that of freezing salt water. The heat used to melt the ice is drawn much faster from the surroundings, which in this case consist of the melted ice AND the ice cream mix. Since the surroundings lose large quantities of heat to the ice, they cool rapidly and continue to cool until they reach the freezing temperature of melting saltwater ice. This temperature is below the cream mixture's freezing point.