Preparation and Advantages in the Use of a Dry Inoculum for Challenge Testing

Introduction: At times it is not practical to use a wet inoculum due to time constraints in growing and preparing the inoculum as well as variability culture levels.  The freeze drying protocol was developed to provide a dry stable bacterial inoculum on an inert material (talc) for use on dry products and/or materials that cannot be mixed with water.

Purpose: The objectives of this protocol were to evaluate the stability of a dry inoculum of pathogens such as Salmonella spp. and provide a means to prepare consistent cultures for inoculating dry products where it was important to not alter the physical conditions of the material.

Methods: Talc has demonstrated superior characteristics as a carrier due to its fine uniform particle structure.  The culture is grown on agar plates and the growth is harvested during the stationary phase via washing the surface of the agar plates.  The collected organisms are combined with a small amount (<1.0 grams) of the dry carrier (talc) and frozen at -76°F/-60°C.  After freezing the cultures are freeze dried, pulverized with a mortar and pestle and brought to desired volume in talc.  The inoculum is then counted and held for use under vacuum in a desiccator at 39°F/4°C.

Results: Results indicated that this freeze dried bacterial culture is stable for an extended period of time, greater than 3 months at 39°F/4°C.

Significance: The use of a dry culture can mimic the mode of contamination of food in dry manufacturing and storage.  A dry inoculum can be used with materials that are not compatible with water (triacetin, chocolate, medium chain triglycerides, dried fruit, etc.).