Strategies to enhance high pressure inactivation of murine norovirus in strawberry puree and on strawberries

Due to the increasing concern of viral infection related to berries, this study investigated strategies to enhance high hydrostatic pressure (HHP) inactivation of murine norovirus 1 (MNV-1), a human norovirus (HuNoV) surrogate, on strawberries and in strawberry puree. Strawberry puree was inoculated with ~ 106 PFU/g of MNV-1 and treated at 350 MPa for 2 min at initial sample temperatures of 0, 5, 10 and 20 °C. MNV-1 became more sensitive to HHP as initial sample temperature decreased from 20 to 0 °C. To determine the effect of pressure cycling on MNV-1 inactivation, inoculated puree samples were treated at 300 MPa and 0 °C with 1, 2 and 4 cycles. Pressure cycling offered no distinct advantage over continuous HHP treatment. To determine the effect of presence of water during HHP on MNV-1 inactivation, strawberries inoculated with ~ 4 × 105 PFU/g of MNV-1 were either pressure-treated directly (dry state) or immersed in water during pressure treatment. MNV-1 was very resistant to pressure under the dry state condition, but became sensitive to pressure under the wet state condition. The inactivation curves of MNV-1 in strawberry puree and on strawberries were obtained at 300 and 350 MPa and initial sample temperature of 0 °C. Except for the curve of strawberries treated at 350 MPa which had a concave downward shape, the other three curves were almost linear with R2 value of 0.99. The fate of MNV-1 in the un-treated and pressure-treated strawberries and strawberry puree during frozen storage was determined. The virus was relatively stable and only reduced by < 1.2 log during the 28-day frozen storage. In all, this study provides practical insights of designing strategies using HHP to inactivate HuNoV on strawberries and in strawberry puree assuming that HuNoV behaved similarly to MNV-1 when treated by HHP.