Crystallization a Concern? Effects in Ice Cream During -35 degrees C Shipping

Dr. Douglas Goff is a professor at the University of Guelph in Ontario, Canada, and author of the Dairy Science and Technology Education Series and Ice Cream on the Internet. His research interests are focused on the physical chemistry, formation, and structure of complex food systems, particularly dairy products and frozen food. His primary area of study is looking at stability and ice recrystallization in frozen systems, and the action of polysaccharide stabilizers in slowing down deteriorative reactions associated with growth of the ice phase and mobility in the unfrozen phase. His secondary research area is related to partial coalescence and structure formation in emulsions during aeration and whipping. Ice cream is a food product that overlaps both areas, and, as a result, Dr. Goff has conducted considerable research with ice cream, earning him an international reputation.

Due to the fragility in the structure of ice cream and the dollars at risk with each ice cream container shipment, understanding the effects various transport temperatures have on the commodity is essential. Dr. Goff's web site (www.foodsci.uoguelph.ca/dairyedu/icecream.html) includes most anything a person would like to learn about ice cream – from its history, to its composition, ingredients and structure, to the manufacturing process and more. For our purposes, we will focus on temperature fluctuations and ice recrystallization as that has been a common question when shipping ice cream at -35 degrees C with the Thermo King MAGNUM® reefer unit.

Dr. Goff discusses this issue within his Ice Cream Shelf-Life section of the web site (www.foodsci.uoguelph.ca/dairyedu/icshelflife.html). Excerpts follow:

"The development of a course, icy texture within ice cream is the most common occurring textural defect. It is the primary limitation to the shelf life of ice cream and probably accounts for countless lost sales through customer dissatisfaction with quality. The question of shelf life longevity depends primarily on storage and distribution temperature conditions.

"Processor's have known for a long time how to prevent iciness and the answer is still the same: formulate the ice cream properly to begin with, freeze the ice cream quickly in a well-maintained barrel freezer, harden the ice cream rapidly, and avoid as much as possible temperature fluctuations during storage and distribution. Ice crystals need to be numerous and of small, uniform size so they are not detected when eaten. It is heat shock, large temperature fluctuations, which is the greatest culprit to the loss of these small, uniform ice crystal size distributions and resulting coarse, icy texture.

"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 re-nucleate. Rather, it is deposited on the surface of larger crystals, so the net result is that the total number of crystals diminishes 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 un-refrigerated 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. Maintaining low and constant storage temperatures minimizes recrystallization."

These are cryo-scanning electron micrographic images of ice cream after temperature fluctuations (as seen on Dr. Goff's web site). The photos at left are of the same magnification; the top two photos are fresh, the bottom two are heat-shocked. You can see the tremendous increase in crystal size that has occurred. The photo at right shows an example of accretion, where crystals fuse as they grow.

What about shipping ice cream at -35 degrees C? How will this affect ice crystal growth after unstuffing the container into an average temperature of -22 degrees C (rather than transporting at that temperature in the first place)?

"Storage at -35 degrees C for a few weeks followed by raising the temperature to -22 degrees C (-7 degrees F) would produce better product than maintaining at -22 degrees C for the entire time.

"In processing, ice cream is hardened to approximately -30 degrees C (-22 degrees F). This establishes the ice crystal distribution in the package. Once it is warmed up to the range of -22 degrees C or warmer, ice recrystallization will occur, and the rate is a function of the temperature. At -15 degrees C (5 degrees F), it is quite rapid, and at -22 degrees C, it is rather slow but still significant. Recrystallization causes coarsening of the texture, and it is cumulative over the life of the product. For example, if ice cream has been stored at -18 degrees C (0 degrees F) for a period of time, then reduced back to -35 degrees C, the quality will not improve, but it will not deteriorate further than what it was either. Temperature fluctuations do exacerbate recrystallization, but only when the temperature has been brought up into the critical range of greater than about -15 degrees C."