Research – Initial and Final Cell Concentrations Significantly Influence the Maximum Growth Rate of Listeria monocytogenes in Published Literature Data for Whole Intact Fresh Produce

Journal of Food Protection

Listeria monocytogenes has shown the ability to grow on fresh uncut produce, however the factors that control growth are not well understood. Peer reviewed journal articles (n=29) meeting the inclusion criteria and related to the growth of Listeria monocytogenes of fresh produce were found through university library databases and Google Scholar searches. Growth models were fit to each of the extracted 130 datasets to estimate log CFU/day rates of growth using the DMfit tool. Multiple linear stepwise regression models for factors influencing growth rate were developed using the software R. Factors included were temperature, nutrient level of inoculation buffer, initial cell concentration, final cell concentration, inoculation method, container permeability, and surface characteristics. The full model produced adjusted-R2, AIC and RMSE values of 0.41, 488 and 1.61 respectively. Stepwise regression resulted in a reduced model with parameters for incubation temperature, inoculation buffer type, initial and final cell concentrations, container characteristics and produce surface characteristics. Model fit statistics improved slightly in the reduced model. A further reduced 3-parameter model included storage temperature and initial and final cell concentration with interaction terms. This 3-parameter model had adjusted-R2, AIC and RMSE values of 0.66, 417 and 1.24 respectively. Incubation temperature (p=1.00E-09) initial cell concentration (p=3.05E-12) final cell concentration (p=4.17E-09) all had highly significant effects on maximum growth rate. Our findings show the importance of inoculum concentration and produce microbial carrying capacity on the estimated growth rate and highlight the overall importance that temperature has on growth rate. Future experiments should consider initial inoculum concentration carefully when conducting growth studies for L. monocytogenes on whole produce.

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