Questions & Answers
from "On Ice Cream" featured in Dairy Foods magazine
and sourced from "On Ice Cream" technical short courses.

Accelerated Shelf-life Testing:

Question: Can accelerated shelf-life testing methods be used with ice cream?

Answer: In considering ice cream's accelerated shelf life testing, temperature is the condition with the biggest potential to cause loss of quality. That involves the development of an undesirable texture – loss of smoothness - as a result of the growth of crystals during temperature fluctuation, known as heat shock. Therefore, achieving accelerated shelf life testing of textural changes involves exposing ice cream to repeated and extreme changes in its storage temperature.

At appropriate intervals during this cycling, sensory evaluation is applied to assess the effects of the treatment, and the extent of treatment needed for the development of detectable crystals is recorded. The conditions of this exposure - temperature and time – should be as consistent between tests as possible.

A basic approach used is to cycle product between frozen storage and ambient room temperature. This method has several shortcomings. It produces a gradient of temperature increase from the outside of the product to its core that results in the development of a similar gradient of ice crystal growth. This can produce a lack of agreement between the observations of the members of the evaluation panel depending upon the location of the source of the portion which each has evaluated.

A more uniform increase in temperature can be achieved by momentarily warming the product by a few seconds of exposure in a microwave oven. This approach is most useful for small portions – pint-sized or smaller – and best suited for application using an industrial microwave oven, in which the double magnetron accomplishes a more uniform distribution of energy.

A more effective, but slower, method is to store the samples for a week or two in a horizontal, "coffin" type display freezer set to apply frequent defrost cycles, in which the samples are arrayed with enough space between them to allow uniform exposure to the atmosphere and thereby undergo a more uniform temperature change. A more sophisticated version of this involves the use of a freezing chamber in which the temperature is closely controlled by a programmable microprocessor. With this freezer, it is possible to program specific conditions of heat shock over a wide range of conditions. A common set of conditions involves programming the unit to produce conditions in which the temperature fluctuates from 0 F to +20 F twice in a 24-hour period.

Because the conditions of storage and distribution of ice cream vary over a virtually infinite number of conditions, only a general correlation of any accelerated test method with shelf life is possible. The most specifically useful application of accelerated heat shock testing involves relative comparisons. For example, if a cost-saving compositional change or a new product composition is considered, it is important to determine the relative shelf life of that composition as reflected by the results of accelerated testing.

Changes in atmospheric pressure can affect shelf life through an influence on ice cream structure by expansion and/or shrinkage of air cells. To evaluate that, ice cream packages are placed into a hermetically sealable chamber in which the pressure is then reduced to an appropriate level. The samples in the chamber are then placed into frozen storage involving whatever temperature fluctuation is relevant.

The only flavor change that can occur during storage – the development of oxidized flavor – is caused by fat oxidation catalyzed by ultraviolet light, such as that generated by fluorescent lights. It is limited to products packaged in containers through which that light can pass (including "windows" in carton lids). Milk fat varies in its sensitivity to oxidation for factors related to milk production itself through the nature of the flavoring used. (For reasons not understood, strawberry ice cream has been observed to be particularly sensitive to oxidation).

For ice cream packaged in ultra-violet transparent packages, consideration should be given to evaluating sensitivity to oxidation by deliberate exposure to ultra-violet light. This can be in the packaged ice cream itself or could involve exposing mix in glass bottles to sunlight for a few hours, then observing any relative differences in the intensity of oxidized flavor developed.

It is important to include some form of accelerated shelf life testing in any ice cream quality assurance program. It is hoped the guidelines provided here will be helpful in establishing such a program or evaluating the nature of one that already exists.