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Name:  Emily E.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 9/12/2004


Question:
What keeps ice cream from freezing solid in sub zero temperatures in the freezer... could not get a fix on that one for students!


Replies:
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 crystals.

Vince Calder


Emily,

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?

Regards,
ProfHoff 912


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 freezer (or put it on the door shelf and repeatedly open the freezer compartment) - so what happens is that the ice crystals partially melt and when placed back into 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.

Cordially,

Jim Swenson



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