Coping with Shelf-Life

A cocktail or mixture of multiple strains of the same organism should be used to challenge a food product in order to simulate the different origins of the strains entering the food products. The challenge organism is grown in media under optimal conditions. In certain studies, the challenge organisms may be adapted to certain conditions for the specific food formulations. For example, E. coli O157:H7 may be acid adapted for low pH products and Listeria monocytogenes may be cold adopted for refrigerated foods. Bacterial spore suspension should be heat shocked prior to inoculation to inactivate vegetative cells.

In microbiological studies, results only apply to those conditions tested. Any changes in product formulation such as an increase in the water activity level, storage temperature, and packaging conditions will invalidate the results of the shelf-life and challenge studies and require new studies. Results of the microbiological shelf-life and challenge studies are interpreted by use of pass or fail criteria. In shelf-life studies, the predetermined limits—10 million bacteria per gram, 100,000 yeast per gram or visible mold—indicate the end of shelf-life. In microbiological challenge studies growth or toxin product is used as determining factor.

A food formulation is considered stable in the absence of toxin formation or less than a 3-log increase over inoculum levels in Bacillus cereus and Staphylococcus aureus challenge studies. The absence of toxin formation is required in Clostridium botulinum studies, while a 3-log or less increase is the preferred criterion in Clostridium perfringens studies. Population growth is limited to less than a 1-log increase in E. coli O157:H7, Listeria monocytogenes, and Salmonella studies.

Proven Proficiency

Shelf-life and challenge studies should be conducted by skilled professionals with demonstrated proficiency using validated methods to ensure the experimental design evaluates the key factors and determined the product’s potential for spoilage and food safety risks.

Dr. Erdogan Ceylan is director of research at Silliker Inc.’s Food Science Center in South Holland, Ill. He can be reached at [email protected].