Pasteurizing Cold Smoked Salmon (Oncorhynchus nerka): Thermal Inactivation Kinetics ofListeria monocytogenesandListeria innocua

Fate of osmotically adapted and biofilm Listeria monocytogenes cells after exposure to salt, heat, and liquid smoke, mimicking the stresses induced during the processing of hot smoked fish

The study aimed to investigate the response of osmotically adapted and detached biofilm Listeria monocytogenes cells following sequential stresses that occur during the processing of hot smoking, such as heating and smoke application. Thermal resistance of L. monocytogenes was significantly affected by previous osmotic adaptation of the cells. D60oC-values of osmotically adapted L. monocytogenes cells were significantly higher than control cells. The osmotically adapted and subsequently heat-injured cells were more resistant to PALCAM and less resistant to TSAYE with 5.00% NaCl (TSAYE/NaCl) than control cells. Detached biofilm cells were more thermotolerant and less resistant to PALCAM and TSAYE/NaCl than control cells. The sequential effect of smoking against heat-treated (60 °C, 20 min) and osmotically adapted or detached L. monocytogenes biofilm cells was investigated using two liquid smoke extracts (L9 and G6). L9 led to significantly higher reductions (>3.00-Log CFU) compared to G6. The heat-treated, detached biofilm cells revealed resistance to L9, presumably due to metabolic downregulation and physical protection by the extracellular polymeric substances (EPS). These data highlight the potential of the food industry to make informed decisions for using safe heat treatments during hot smoking to effectively inactivate L. monocytogenes and maintain rigorous environmental sanitation practices to control biofilm cells.

Increased thermal sensitivity of Listeria monocytogenes in sous-vide salmon by oregano essential oil and citric acid

Inadequate cooking during sous-vide processing may cause foodborne diseases in case the food is contaminated with pathogens such as Listeria monocytogenes. In this study, thermal inactivation of L. monocytogenes in sous-vide processed salmon was investigated. Oregano oil and citric acid were used alone or in combination to determine the probability of increasing the efficiency of heat treatment. Control (C); 0.5% citric acid added (S); 1% oregano essential oil added (O); and citric acid and oregano essential oil combined (OS) groups were prepared. Samples were inoculated with L. monocytogenes, vacuum packed, then sous-vide cooked at 55, 57.5, 60, or 62.5 °C for predetermined times. The D-values of all treated samples were significantly lower than control. The use of oregano oil (O), citric acid (S) and their combination (OS) significantly reduced the time required to inactivate L. monocytogenes. The z-values of L. monocytogenes in C, O, S and OS groups were 5.50, 5.62, 6.54, and 6.92 °C, respectively. It was determined that effective results could be achieved by adding natural antimicrobials to provide safety in sous-vide fish.

Reconciliation of the D/z model and the Arrhenius model: The effect of temperature on inactivation rates of chemical compounds and microorganisms

The effect of temperature on thermal inactivation of microorganisms and thermal degradation of certain chemical compounds can be either described by the Arrhenius model for the rate constant (k) or by the D/z model for the decimal reduction time (D). Although equivalent, it is difficult to directly reconcile these two models which have different model structures. This study hypothesized that the two models can be reconciled by linearization of the inverse of absolute temperature (1/T) and heating temperature (T), and the activation energy (E_a) can be directly calculated from z and their relationship is independent of T. The hypothesis was validated using the z and E_a of various chemical compounds, enzymes, and vegetative cells and spores of microorganisms in various substrates published in the literature. The results of this study show that the empirical models are practically reconciled.