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Freezing
point is the temperature at which a liquid changes to
a solid. As soon as you add a substance (or solute)
that dissolves in the liquid portion (or solvent) of
the food, the solute physically interrupts the structure
and changes the freezing point. Note that in order to
change freezing point, the solute must become evenly
distributed throughout the liquid. It doesn't precipitate
out, nor can it be filtered out. The two most common
ingredients in cooking that affect freezing point are
salt and sugar.
Sugar lowers the freezing point of water, which makes
frozen desserts fair game for changes in freezing point.
Most desserts freeze between 29.5 to 26.6 degrees F
(-1.4 to -3.0 C) depending on the concentration of sugar.
To balance their higher proportion of acids (which give
flavors such as lemon, raspberry, etc. their tang) sorbets
and ices usually contain a higher proportion of sugar
than ice cream and ice milk. And of course to remain
frozen they must remain at these colder temperatures.
Not a problem though, as the recommended temperature
for home freezers is 0 F (-18C), below the freezing
point of both. Assuming that all parts of the freezer
are at the same temperature, both ice cream and the
sorbet would eventually reach the same temperature as
the freezer.
As they warm up outside the freezer, it makes good sense
that on a very hot day, a scoop of sorbet is more refreshing
than a scoop of ice cream, because it remains frozen
longer.
Salt has an even more powerful effect on freezing point,
than sugar. In order for this to happen to a significant
degree however, the concentration of salt must be so
great that the food is unpalatable. So while salt is
unlikely to have an affect on the freezing temperature
of foods in our freezers, it does explain why, old fashioned
ice cream makers used a salt brine to cool a freezing
ice cream mixture.
Regards,
Anne and Sue
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