The Pros and Cons of a Reverse Shelf Life Study

A Reverse Shelf Life Study can be utilized in some, but not all situations, to determine the shelf life of food and nutraceutical products.  The basic concept and process is to store shelf life samples in a stable environment, such as frozen storage, and remove samples to refrigerated or room temperatures according to the testing protocol time periods.  Samples are pulled from the freezer in reverse order.  For example, if a shelf life study calls for product storage at ambient conditions and testing every 10 days for 50 days, the first samples would be placed in ambient conditions and be identified as the day 50 testing point.  After 10 days of frozen storage, the next samples would be placed in ambient conditions and labeled as the day 40 samples.  After 20 days of storage, the next samples would be placed at ambient conditions and so on until all samples are removed from frozen storage.  The process is shown below:

The biggest win from this experimental design is reduced costs associated with batch testing.  Reverse shelf life studies provide the ability to test all samples the same day in one batch.  From a lab operations stance, the cost associated with testing is reduced by using a single set of controls in one test set up vs. multiple days of testing with assay controls for each day.  In addition to material saving, there is also a cost savings on the labor.

Shelf life studies are a three prong approach involving evaluation microbial load, chemical attributes, and sensory characteristics.  The impact of frozen storage and slacking on the accuracy of results must be assessed prior to implementing a reverse shelf life study. 

For microbiology, most organisms present in a food matrix can withstand frozen storage, but for some, frozen storage can be lethal.  Changes in water activity, solute concentration, the formation of ice crystals, and thermal shock are all possible causes of cell death when freezing microbes.¹  Not only is cell death possible, sublethally injured organisms may not repair to healthier states during subsequent plating on selective media for colony counts.  Stressed cells would go undetected when plating the sample on agar due to the environmental stress of the selective agaents.²  The use of frozen storage, therefore, can in some cases favor cold tolerant organisms, such as psychrotrophs and spore forming bacteria.

The effect of frozen storage on the chemical composition of food is complex and requires an in depth review of the literature for a specific food matrix and analyte of interest.  In general, mineral and salt content is relatively unaffected with frozen storage, but the stability of vitamins can vary depending of the food matrix, packaging, and freezing processes.  Similarly, proximate results (ash, protein, moisture, pH, fat) can vary greatly depending on matrix with decomposition of proteins and fat during storage.³  

Packaging is a factor in maintaining sensory characteristics and the desired refrigerated or room temperature packaging may not sufficient to maintain food integrity during frozen storage.  Barriers to prevent moisture loss, minimization of air, and the ability to withstand the freezing and thaw process should be taken into consideration.  In addition to packaging, the matrix composition may be sensitive to freezing resulting in sensory changes such as texture and color during and after freezing.

Reverse shelf life studies are similar in many ways to accelerated shelf life studies.  The storage temperature in these study designs can impact testing results.  Some, but not all matrices, are good candidates for these study designs.  A comprehensive evaluation of the food matrix, testing considerations, and goal of the study should be evaluated prior to initiating this type of shelf life study.

https://link.springer.com/chapter/10.1007/978-1-4615-5975-7_10

https://www.sciencedirect.com/science/article/abs/pii/S0963996923000819

https://www.davidpublisher.com/Public/uploads/Contribute/5f50b234b7b16.pdf