Growth and No-Growth boundary of Listeria monocytogenes in beef - A logistic modeling

Listeria monocytogenes is a potentially fatal foodborne pathogen. Its growth in ready-to-eat (RTE) foods must be strictly controlled to protect public food safety. This study was conducted to define the growth and no-growth boundary of L. monocytogenes with sodium tripolyphosphate (STPP), sodium lactate (NaL), sodium diacetate (NaDiAc), sodium chloride (NaCl), sodium nitrite (NaNO₂), and pH as control factors. The growth of L. monocytogenes was first examined using a solid medium incubated under 37 °C for 48 h in 96-well microtiter plates. NaNO₂ (1,800 ppm) and NaDiAc (2,500 ppm) were found not effective in preventing the growth when applied alone. STPP was shown highly effective in preventing the growth of L. monocytogenes. Its growth was unhindered at pH 6-7 but was increasingly inhibited beyond the neutral range. High concentrations of NaL and NaCl were needed to inhibit the growth of L. monocytogenes. A multifactor logistic regression model (LRM) was developed to calculate the growth probability (p) and then define the growth boundary using 2 thresholds. With Threshold 1 (p = 0.0104), the Accuracy of classification for growth events is 0.686, with a True positive rate (TPR) of 0.776 (or False negative rate (FNR) of 0.234), True negative rate (TNR) of 0.455 (or False positive rate (FPR) of 0.545), and Precision of 0.803, in PALCAM agar. However, with Threshold 2 (p = 0.04), the Accuracy becomes 0.826, with a TPR of 0.955 (or FNR of 0.045), a TNR of 0.690 (or FPR of 0.310), and Precision of 0.764. For validation in ground beef, the Accuracy of prediction of growth was 0.85, with a TPR of 0.9, TNR of 0.8, and Precision of 0.818 for Threshold 1. With Threshold 2, the Accuracy, TPR, TNR, and Precision are all 0.8, with both FNR and FPR of 0.2. Both thresholds and LRM may be used to formulate RTE products that may prevent the growth of L. monocytogenes even stored under the optimum temperature.