Thermal Inactivation Kinetics of Murine Norovirus and Feline Calicivirus

Introduction: Due to the recent rise in the number of foodborne norovirus outbreaks, novel preservation methods have increasingly focused on their control. Determination of required thermal treatment parameters to inactivate microorganisms is based on precise understanding of their inactivation kinetics. In the absence of human norovirus infectivity assays, cultivable surrogates such as feline calicivirus (FCV-F9) and murine norovirus (MNV-1) have been used to begin to understand thermal inactivation behavior.

Purpose: This study aimed to characterize the thermal inactivation kinetics of MNV-1 and FCV-F9 at 55, 65 and 72 °C for different treatment times (0-3 min).

Methods: Thermal inactivation was performed using the capillary tube method on the surrogate viruses at titers of 4.0x10⁷ (MNV-1) and 5.8x10⁸ (FCV-F9) plaque forming units (PFU)/ml, followed by standard plaque assays in duplicate. Each treatment was replicated thrice and data analysis was performed. Inactivation data were fitted to the Gompertz, logistic, Weibull and first-order models and compared statistically using regression coefficients (R²), chi square (χ²), and root mean square error (RMSE) values.

Results: Complete inactivation was achieved for both norovirus surrogates at 65 and 72 °C (> 6.68 log PFU/ml after 2 and 1 min, respectively); however, the total mean reduction at 55 °C was 0.83 log PFU/ml and 0.48 log PFU/ml for MNV-1 and FCV-F9 after 3 min, respectively. To determine thermal inactivation kinetics for the norovirus surrogates, all models were compared at each temperature. Results indicated that the model appropriateness was virus strain and temperature dependent.

Significance: Thus, use of multiple models is recommended and necessary for the precise determination of virus inactivation characteristics. The logistic model was found appropriate for both norovirus surrogates at 55 °C; however, at 65 and 72 °C the appropriate model was the Weibull model for MNV-1 and Gompertz model for FCV-F9. These results provide the parameters and data/thermal process calculations to accomplish thermal inactivation of norovirus surrogates.