High pressure inactivation of murine norovirus and human noroviruses on strawberries, blueberries or raspberries or in their purees
University of Delaware
The consumption of fresh and frozen strawberries, blueberries, raspberries and their purees is continuously growing. However, these berries and berry puree have been associated with human norovirus outbreaks. Therefore, interventions are urgently needed. High hydrostatic pressure (HHP) has been successfully used to process various foods since it has less detrimental effects on nutrients and sensory qualities compared with thermal processing. The overall goal of this study was to investigate high hydrostatic pressure inactivation effect of human norovirus on strawberries, blueberries, raspberries and in their purees. The study consisted of two parts: 1. Test influence of different parameters on HHP inactivation of murine norovirus 1 (MNV-1) on strawberries and in strawberry puree; 2. Determine the efficacy of HHP inactivation of human norovirus GI.1 and GII.4 on strawberries, blueberries, raspberries and in their puree. MNV- 1, a common surrogate for human norovirus, was tested on strawberries and in strawberry puree to determine the inactivation effect of different parameters during high hydrostatic pressure processing. Strawberry puree inoculated with ~106 PFU/g of MNV-1 was 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 at lower initial sample temperature. 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. 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 0 °C. Most curves were characterized by rapid reduction of titers at the beginning followed by tailing, indicating that increasing pressure level is a better way to enhance HHP inactivation of MNV-1 than increasing treatment time. The fate of MNV-1 in the un-treated and pressure-treated strawberries and strawberry puree during frozen storage was determined. MNV-1 was relatively stable during the 28-day frozen storage. Human norovirus (HuNoV) GI.1 and GII.4 were tested on strawberries, blueberries, raspberries and in their puree to determine the HHP inactivation effect. Strawberry puree inoculated with HuNoV GI.1 strain was treated at 450, 500 and 550 MPa for 2 min at initial sample temperatures of 0, 4 and 20 °C. HuNoV GI.1 strain showed less sensitiveness to HHP at higher temperature at all three pressure levels. As for GI.1 strain, 2 min HHP treatment at 550 MPa and 0 °C achieved > 2.9 log reduction in strawberry puree, blueberry puree and raspberry puree. As for GII.4 strain, 2 min HHP treatment at 0 °C achieved > 4 log reduction at 500, 550 and 400 MPa in strawberry puree, blueberry puree and raspberry puree, respectively. HuNoV GI.1 strain showed more resistance to HHP treatment than HuNoV GII.4 strain. Strawberry quarters, blueberries and raspberries were spot-inoculated with HuNoV and HHP-treated with water. Two min HHP treatments of 650 MPa at 0 °C could reduce HuNoV GI.1 and GII.4 by 1.7 and 3.1 log respectively on strawberry quarter. As on blueberry, 2 min HHP treatments at 0 °C achieved > 3.2 log reduction of GI.1 strain at 550 MPa and > 4 log reduction of GII.4 strain at 300 MPa. As for raspberry, 2 min HHP treatments at 0 °C achieved 2.48 log reduction of GI.1 strain at 650 MPa and 4 log reduction of GII.4 strain at 600 MPa. pH of surrounding water of blueberries was higher than strawberry quarters and raspberries and HHP treatment turned out to have better HuNoV inactivation effect on blueberries than strawberry quarters and raspberries.