To construct a predictive growth model of Listeria monocytogenes in ground pork, growth data were collected by real-time PCR and Listeria selective agar according to Ottaviani and Agosti (ALOA). Most probable number (MPN) was used to estimate viable numbers of L. monocytogenes at the beginning, middle, and end of the incubation period. Growth curves obtained from this study were fitted to the Baranyi and Roberts model to obtain growth parameters. Furthermore, the theoretical minimum temperature of growth was estimated by Ratkowsky's model. L. monocytogenes growth rates estimated from ALOA data were lower than those estimated by real-time PCR. Moreover, cell concentrations at all incubation temperatures were underestimated and lag phase duration at refrigeration temperature (4 °C) was overestimated by ALOA. However, the estimation from MPN more closely resembled the real-time PCR quantification results. Thus, the direct plate count tends to result in fail-dangerous prediction for bacterial risk from the model.

The goals of this study were to monitor the growth kinetics of Salmonella Enteritidis in chicken juice using real-time polymerase chain reaction (PCR) and to evaluate its efficacy by comparing the results with an experimental database. Salmonella Enteritidis was inoculated in chicken juice samples at an initial inoculum of 10 CFU/mL with inoculated samples incubated at six different temperatures (10, 15, 20, 25, 30, and 35°C). Sampling was carried out for 36 h to observe the growth of Salmonella Enteritidis. The total DNA was extracted from the samples, and the copy number of the Salmonella invasion gene (invA) was quantified by real-time PCR and converted to Salmonella Enteritidis cell concentration. Growth kinetics data were analyzed by the Baranyi and Roberts model to obtain growth parameters, whereas the Ratkowsky's square-root model was used to describe the effect of the interactions between growth parameters and temperature on the growth of Salmonella Enteritidis. The growth parameters of Salmonella Enteritidis obtained from an experiment conducted at a constant temperature were validated with growth data from chicken juice samples that were incubated under fluctuating temperature conditions between 5°C and 30°C for 30-min periods. A high correlation was observed between maximum growth rate (μ) and storage temperature, indicating that the real-time PCR-monitoring method provides a precise estimation of Salmonella Enteritidis growth in food material with a microbial flora. Moreover, the μ data reflected data from microbial responses viewer database and ComBase. The results of this study suggested that real-time PCR monitoring provides a precise estimation of Salmonella Enteritidis growth in food materials with a background microbial flora.

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