Ice Cream and Temperature
Name: Emily E.
What keeps ice cream from freezing solid in sub
zero temperatures in the freezer... could not get a fix on that
one for students!
Thanks for your question Emily... The reason for this is known as the
"colligative effect," or what is sometimes referred to as "freezing
point depression." The colligative effect is a phenomenon by which
the freezing point of a solution is lowered when more solute (solid
substance) is dissolved in the solution. As an example, pure water at
a standard pressure of one atmosphere will freeze at 0 degrees
Celcius (32 degrees Fahrenheit). But, adding a solute, such as
salt, to the solution will depress the freezing point in proportion to
how much solute is added. The effect can be quite dramatic.
Incidentally, the freezing point depression induced by the colligative
effect is the reason why salt is added to icy roads in the winter.
The addition of the salt depresses the freezing point of the water/ice
that has accumulated on the road and forces it to revert to the liquid
state (which is typically far less slippery). Water that is heavily
saturated with salt can resist freezing at temperatures which are
several degrees below its normal freezing point.
As far as ice cream is concerned, it is, essentially, a solution of
milk (which contains water, lipids, proteins, and lactose) and refined
sugar. With so many solid substances dissolved in water, we would
expect the freezing point of ice cream to be below that of water.
But, it's slightly more complicated than that. Cow's milk naturally
freezes at a temperature of approximately -.5 degrees centigrade,
which is not much lower than the freezing point of water, so how is it
possible that ice cream can still feel "unfrozen" at temperatures far
below -.5 degrees? The reason is that as the water component of the
ice cream solution begins to freeze, it isolates itself from the rest
of the solution by forming pure ice crystals (which are readily
observable in ice cream). As a consequence, the relative
concentration of the solid substances dissolved in the remaining
liquid solution increases, simply because there is less liquid water
left available for the solutes to dissolve in. The left-over water
can then only freeze at a much lower temperature; when it does get
cold enough to do so, the concentration of the solutes goes up even
higher, again, because there is less liquid water left. You can
imagine that, as the ice cream gets colder and colder, the
concentration of the solutes continues to increase as water is
progressively removed from the liquid solution as it freezes, thereby
greatly depressing the freezing point of whatever amount of liquid is
left. The ice cream eventually becomes a mixture of frozen crystals
and a relatively smaller amount of unfrozen, liquid solution which
gives it a soft feel.
I hope this explanation helps...
Scott J. Badham
Department of Geology and Geophysics
University of Wyoming
The ingredients of an ice cream recipe contain fats, sugars and other
ingredients that cause the viscosity of the mixture to increase as the
temperature is lowered, and of course, the mix is under agitation (at least
initially). When the viscosity increases sufficiently fast as the
temperature is decreased the water molecules can't "find one another" to
form ice and a "creamy" mixture results. If not the product will contain ice
crystals that are immediately evident from the texture of the product. It is
for this reason that when ice cream is allowed to melt and then is
re-frozen, it often has a "grainy" texture due to the presence of ice
Given a freezer that's cold enough, how "solid" ice cream gets depends on
its water content, fat content, and the presence of anything that could
act as an antifreeze in water. The "mouth-feel of the dessert depends on
the fat content and size of the ice particles that comprise the finished
product. Fat makes it feel smooth, small ice particles make it melt at a
desirable rate. Additives, such as vanilla extract, that may contain
alcohol serve as an antifreeze to the water phase.
I suggest you make a few different batches if ice cream of varying water
content by using dry milk, skim milk, 1% butterfat, 2% butterfat, and
whole milk -- make a control mix by following a good ice-cream recipe.
Place all formulations in the same freezer for the same length of time
until the control mix feels just right. Check them for "solidity." Which
one ends up the hardest?
The quick answer is that all the water that can be frozen in ice cream has
already been frozen into very small ice crystals. So placing ice cream in the
freezer will not induce more water crystallization.
However, these crystals do melt when you take the ice cream out of the
(or put it on the door shelf and repeatedly open the freezer compartment)
what happens is that the ice crystals partially melt and when placed back
the freezer reform into bigger crystals. This is why one should minimize the
time that ice cream is outside the freezer - to prevent the production of
large ice crystals from the melted ones.
Greg (Roberto Gregorius)
Ice-cream has a large percentage of fat, which remains soft until
substantially below the freezing point of water.
Also this fat tends to keep the first water crystals small and separated,
so the ice cream can't take on the hardness of a dense re-frozen snow-ball.
However, even fat has a glass-transition temperature, so I guess that
after soaking a scoop of ice-cream in LN2 for a few minutes it would be a rock.
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Update: June 2012