Development Of Predictive Models And Control Of Listeria Monocytogenes In Salmonroe

Salmon caviar (roe) is one kind of lightly preserved ready-to-eat seafoods with extremely high nutritional value and economic importance. In retail stores, large amount of salmon caviar are stored at refrigerated temperature and sold without pasteurization, only small part of them were pasteurized. The ability to grow at low temperature and the relatively higher fatality rate of Listeria monocytogenes (L. monocytogenes) pose a serious threat to the ready-to-eat salmon caviar. Growth kinetics of L. monocytogenes in salmon roe was investigated and the primary and secondary models were established. Nisin and potassium sorbate were used to prevent the growth of L. monocytogenes in salmon roe (3% salt). The effect of temperature, Nisin and potassium sorbate on the growth of L. monocytogenes was examined, and the response surface model was developed. Thermal inactivation kinetics of L. monocytogenes in salmon roe was studied. The canned salmon roe can be pasteurized by hot water immersion, and 2D heat transfer model of the pasteurization were developed. Combined with the thermal inactivation kinetics, the lethality rate of L. monocytogenes can be deduced. The key result is as follow.The inoculated samples were incubated isothermally at emperatures between 5 and 30℃ to develop growth curves. All growth curves were analyzed using the Huang and Baranyi models. The experimental results showed that L. monocytogenes in salted samples exhibited pproximately 40% longer lag times than the cells under the same temperature condition in unsalted samples, while the rates of bacterial growth were not affected by the addition of salt. The Ratkowsky square-root (RSR) model, Huang square-root (HSR) model, and an Arrhenius-type model were shown suitable for evaluating the effect of temperature on specific growth rates. The nominal temperature in the RSR model was -0.5℃. The minimum growth temperature estimated by the HSR model was 2.57℃. The HSR models may be suitable for describing the temperature effect in salted salmon roe. The lag time of L. monocytogenes was found to change log-linearly with the specific growth rates. In theBaranyi model, the mean ho was 0.742 in unsalted samples and 1.193 in salted samples. The ho values did not appear to change with temperature in a systematic manner.Nisin and potassium sorbate can be used to control L. monocytogenes in ready-to-eat foods. The objective of this study was to examine and model the effect of Nisin (0 to 500ppm) and potassium sorbate (0 to 3000ppm) in salmon roe on the lag phase duration (LPD, h) and growth rate (GR, Log CFU/h) of L. monocytogenes at a range of temperatures (0 to 45℃). Resulting LPD and GR equations for storage temperatures within the range of 0 to 37℃ significantly represented the experimental data with a regression coefficient of 0.99 and 0.96, respectively. Significant factors (P<0.05) that affected the LPD and GR were temperature, Nisin, potassium sorbate, and the interactions between temperature and Nisin and potassium sorbate. At suboptimal growth temperature (≤25℃) the increase of Nisin and potassium sorbate concentrations, individually or in combination, extend the LPD. The effect of higher concentrations of both Nisin and potassium sorbate on reducing the GR was observed only at temperatures that were more favorable for growth of L. monocytogenes, that is 25 to 37℃.Salmon roe containing 0.0%,1.5%,3.0%, and 4.5% salt, were first inoculated with L. monocytogenes F2365, F4260, and V7 separately, then they were heated at 57.5,60.0,62.5, and 65.0℃ to investigate the thermal inactivation kinetics. The D-values of L. monocytogenes F2365, F4260, and V7 ranged from 12.09,19.67, and 17.13 min (4.5% salt salmon roe) at 57.5℃ to 0.14,0.10, and 0.11 min (0.0% salt salmon roe) at 65.0℃, respectively. The Z-values of L. monocytogenes F2365, F4260, and V7 in different salt content salmon roe ranged from 5.37 to 6.91℃,4.09 to 4.67℃, and 4.42 to 4.90℃, respectively. While linearly decreasing with temperature, the log D-values of L. monocytogenes increased linearly with salt content increase. These results suggested that salt provide protective effect to L. monocytogenes in salmon roe during heating, and L. monocytogenes F2365 exhibited less sensitive to temperature change than those of L. monocytogenes F4260 and V7.Salmon roe containing 0,250,500 ppm Nisin, were first inoculated with three strains cocktail of L. monocytogenes, then they were heated at 57.5,60.0,62.5, and 65.0℃ to investigate the thermal inactivation kinetics. The D value decreased with both the increase of temperature and Nisin content, which shows the synergistic effect between temperature and Nisin.A 2D unsteady heat transfer differential equation was developed to simulate the temperature distributions within the canned salmon roe. Thermal diffusivity of the salmon roe was determined by using analytic solution. Then the heat transfer differential equation was then solved by a numerical method. The results of numerical simulation indicated that the simulated temperature histories were in close agreement with the experimentally observed curves. The heat transfer differential equation was then integrated with thermal inactivation kinetics to calculate the theoretical lethality of L. monocytogenes during hot water pasteurization of canned salmon roe. To obtain an equal theoretical lethality, thermal treatment intensity increased as the salt content increase, but decreased as the Nisin content increase. In general, the result of numerical simulation for calculating the log reduction was more conservative, therefore suitable for designing processes to ensure the safety of products and to reduce the incidence of foodborne listeriosis associated with salmon roe.