Coping with Shelf-Life

Each microorganism has a minimum, optimum and maximum growth temperature. As the temperature increases from the minimum toward the optimum, the microbiological growth rate increases. The microbiological growth is fastest at the optimum temperature. As the temperature increases from the optimum toward the maximum, the microbiological growth rate decreases. As the storage temperature changes, not only the growth rate of the organisms but also the type of the spoilage organisms changes.

It is important to determine the shelf-life of the product at its intended shelf-life storage temperature. A few degree changes in storage temperature may have a significant effect on the length of the shelf-life. In the real world conditions during distribution and storage, the product temperature often fluctuates between low and high temperatures. It is, therefore, important to test the product under both commonly used target storage temperatures (e.g. 40°F for refrigerated storage and 75°F for ambient temperature storage) and abuse temperatures (e.g., 45° to 50° F for refrigerated storage and 85° to 90° F for ambient temperature storage).

Duration of the shelf-life and challenge studies should match the target shelf-life of the product. It is desirable to test the product formulation beyond its intended shelf-life. Some regulatory agencies require testing the product formulation over product shelf-life storage plus one-third of the target shelf-life. The food product is sampled periodically to determine microbiological, chemical and organoleptic changes. The number of time points is decided based on reference studies.

The intended storage time is often divided into five to 12 time points. An excessive length of time between samplings may over- or under-estimate the changes in the product during storage. Significant changes in the product will be determined more accurately with high number of analyses. Triplicate samples from three different lots of product produced on different manufacturing dates should be used at each time point to improve the precision of testing.

In shelf-life studies, the type of the microbiological analyses and the plating procedures are selected based on the products’ characteristics or the types of the microorganisms known to be present in the product. In challenge studies, samples are analyzed for the counts of the challenge organism or its toxins. Background microflora may affect the survival of the inoculated challenge organisms. Thus, it is important to analyze the uninoculated control sample at each time point during its intended shelf-life.

Validated Methods

Reliability of the testing is directly related to the accuracy of the methods used. The use of validated methods from sampling to final data analyses is the key element for the precision of testing. Analyses of the samples should be performed using the methods described in the FDA Bacteriological Analytical Manual, AOAC, Compendium of Methods for the Microbiological Examinations of Foods, USDA Microbiology Laboratory Guidebook, or other validated methods.

The number of organisms inoculated to the food in a challenge study is normally higher than what would normally be present in the product as results of chance contamination. Die-off might occur after inoculation due to the abrupt changes (e.g. a low pH level) in the environment. If the initial inoculation level is too low, the incorrect conclusion can be made that the product formulation is stable. If the initial inoculation level is too high, the challenge organisms may overcome the inhibitory effect preservatives or hurdles. This may lead to the incorrect conclusion that the product formulation is stable. Typically, an inoculum level in the range of 102 to 104 cells per gram is used to observe either increases or decreases.

A number of strains of bacteria, yeast and mold are used in challenge studies as spoilage organisms. The ideal spoilage organisms are strains that have been isolated from similar products. The ideal foodborne pathogens are the strains that have been isolated from known foodborne outbreaks. Well-characterized strains of challenge organism from a reference culture collection such as the American Type of Culture Collection (ATCC) should be used. Strains of Bacillus cereus, Clostridium botulinum, Clostridium perfringens E. coli O157:H7, Listeria monocytogenes, Salmonella spp., and Staphylococcus aureus are the commonly used pathogens. A summary of foodborne pathogens that may be used in challenge studies for different types of foods is given in Table 1.