Any food manufacturer would prefer that the product leaving his shop will survive both the retail and the customer's storage environments until it is consumed. Unfortunately, that hope is often frustrated. Even canned foods develop problems. Shelf life limiters may be chemical, physical or biological. Shelf life may also be determined by some combination of all three.
The shelf life of different foods will be quite variable. The problem is to get a handle on that variability. For example, variations in the chemistry of a food product may occur because slow oxidation/reduction processes react with the components of food modify their chemical makeup. This implies that, upon storage, the overall chemical bond structure of the food slowly changes. To check for that, a food product may be scanned soon after manufacture with a technique (like Fourier Transform - Infrared Spectroscopy (FT-IR) able to give one at least a general idea of the different types and relative amounts of bonds a food contains. As a food ages within its packaging, rescanning the aged product may be able to determine if chemical changes have taken place. If it has, the line describing the spectrograph will have a different shape. The problem then will be to repeat these measurements frequently enough over the aging cycle to define (within the sampling error observed) when the food product is still essentially unchanged from what it was when it was made. Other techniques may be more appropriate for different foods. The chemical shelf life will be the time period when the chemical composition remained unchanged.
Physical changes are also worth looking at. Water content and distribution in the food may change upon storage. Color may change. Density may change. If a food package contains a headspace, the pressure of that headspace gas may change. Indeed, pressure changes may be due to changes in composition. This change would be worth noting because it may relate to both chemical and biological changes. For example, if the volatile compounds present in the headspace were to be analyzed, say by gas/liquid chromatography, the presence of new compounds may indicate an ongoing redox reaction in the food. Conversely, certain volatile compounds are known to be either intermediates or end products of microbial growth. Consequently, finding an increase of these compounds in the food might well be an indication of microbial spoilage.
Microbial spoilage (or stability) may well be the most significant determinant of shelf life simply because this limiting factor may be the most variable. Microbes have been living on this planet for billions of years. In all that time they have gotten downright clever, metabolically. A given food may contain more than one different environments, which will then harbor different microbes. Even microbial cells rendered unable to reproduce (i.e., "killed" by treatments such as pasteurization) may still contribute enzymatic activity that may compromise the shelf life of food. This possibility, for example, is one reason it is useful to analyze the headspace gas in a food package and track changes, if any occur. The appearance, upon storage, of metabolic intermediates or end products in the absence of any viable microbes may give a clue to how to extend shelf life.
Shelf life extension may sometimes be a simple process and other times become quite involved. Before one can intervene in this process, one must come to understand it.
Copyright © 2012 by M. Mychajlonka, Ph. D.