Survival and Heat Resistance of Freeze-dried Pathogens on Various Carriers

Introduction: Dry inocula are often used in challenge or validation studies on low water activity foods. Ingredients like wheat flour, wheat starch, or whey can be used to create dry inocula that mimic a product’s formula. The survival of Salmonella at low water activities has been well documented for specific species and environments using dry inoculation techniques. However, there are gaps in the literature concerning other foodborne pathogens’ survival using dry inocula. In addition, the heat resistance of foodborne pathogens in dry inoculation studies is not characterized.

Purpose: The objectives of this investigation were to evaluate the stability (population and heat resistance changes during storage) of dry inocula by monitoring: (1) survival of Escherichia coli O157:H7, Listeria monocytogenes, Salmonella mons, and Staphylococcus aureus in calcium carbonate, wheat starch, wheat flour, and whey, (2) the change in heat resistance with respect to storage time.

Methods: Dry inocula were prepared by harvesting lawns of E. coli O157:H7, L. monocytogenes Scott A, S. mons, and S. aureus grown on tryptic soy agar with 0.1% peptone. Pathogen slurries were frozen overnight at -70°C with one gram of each carrier in separate vials and transferred to a freeze dryer. Inocula were diluted with uninoculated carrier and then stored at 3°C for the duration of this study. Survival and heat resistance were assayed every ten days for 100 days.

Results: Not all carriers are suitable for the four pathogens. For example, the population of E. coli O157:H7 freeze-dried on whey powder decreased by three orders of magnitude within the first week of the study. Trends in heat resistance of the pathogens were similar throughout storage. The influence of the carrier on stability and heat resistance was varied.

Significance: Use of carriers for dry inocula in challenge studies must be validated as part of the experimental design.