Temperature Fluctuations and Ice Recrystallization
Ice crystals are relatively unstable, and during frozen storage, they undergo changes in number, size, and shape, known collectively as recrystallization. This is probably the most important reaction leading to quality losses in all frozen foods. Some recrystallization occurs naturally at constant temperatures, but by far the majority of problems are created as a result of temperature fluctuations. If the temperature during the frozen storage of ice cream increases, some of the ice crystals, particularly the smaller ones, melt and consequently the amount of unfrozen water in the serum phase increases. Conversely, as temperatures decrease, water will refreeze but does not renucleate. Rather, it is deposited on the surface of larger crystals, so the net result is that the total number of crystals diminish and the mean crystal size increases. Temperature fluctuations are common in frozen storage as a result of the cyclic nature of refrigeration systems and the need for automatic defrost. However, mishandling of product is probably the biggest culprit. The sight of ice cream sitting unrefrigerated on a loading dock, in the supermarket aisle, in a shopping cart, or in someone's grocery bag is too common. If one were to track the temperature history of ice cream during distribution, retailing, and finally consumption, one would find a great number of temperature fluctuations. Each time the temperature changes, the ice to serum content changes, and the smaller ice crystals disappear while the larger ones grow even larger. Recrystallization is minimized by maintaining low and constant storage temperatures.
The graph below provides data to show the increase in size of ice crystals that occurs with temperature cycles (from the work of A. Flores and H. D. Goff).
Below are several cryo-scanning electron micrographic images of ice cream after temperature fluctuations. In the first composite, all pictures are at the same magnification, the top two are fresh, the bottom two are heat-shocked. You can see the tremendous increase in crystals size that has occurred. The next image shows an example of accretion, where crystals fuse as they grow.