PMP FAQs - Interpreting Model Output

1.  How do I interpret the PMP model predictions if the model was developed in a different food matrix than the one I'm interested in?

Without experience in the use of models, it is difficult to know if the model you use is over- or under-predicting bacterial growth or inactivation when applied to another food matrix. As such, it is best to use models to understand potential trends in bacterial behavior as the environmental conditions change. Only through validation studies (e.g. inoculated pack studies) would you be able to have confidence in model interpretation for your food of interest.

2.  How can you interpret a model that was developed in a sterile system to a situation where the food contains spoilage flora?

In general, this situation is most relevant to growth models. Depending on the pathogen, spoilage flora (e.g., bacteria, fungi) can markedly inhibit the growth of pathogenic bacteria. This is especially apparent at refrigerated temperatures where the growth rates of psychrotrophic (cold-loving) organisms may be greater than that of the pathogen. Therefore, in these situations, the maximum density (level at stationary phase) of a pathogen may be 3 to 5 log₁₀ levels less that observed in a pure culture. Also, the growth rate may be inhibited. Therefore, in general, Generation Time will be shorter and growth rates and maximum population densities will be higher in sterile culture systems compared to systems containing spoilage flora.