Application of High Pressure Processing on Frozen Strawberries to Inactivate Murine Norovirus

Introduction: Frozen strawberries are often used as ingredients in food products such as salads, desserts and yogurt, or packaged with other kinds of frozen berries in wholesale markets. Human norovirus (HuNoV) outbreaks have occurred in recent years over the world due to the consumption of contaminated frozen strawberry products.  High pressure processing (HPP) has been recognized as a novel processing technology to preserve the natural flavor and raw character of fresh produce.

Purpose: The objective of this study was to apply HPP to inactivate murine norovirus (MNV-1), a surrogate for the uncultivable HuNoV, on whole frozen strawberries.

Methods: Fresh strawberries were spot-inoculated with approximately PFU/g of MNV-1 on the surface and frozen at -20℃ for one day. The frozen strawberries were then treated at 250, 300, 350 and 400 MPa for 3 minutes with the initial start temperature of 4℃. Plaque assay analysis was performed to assess the viral inactivation results.

Results: The initial inoculum level was 4.0±0.1 log PFU/g of strawberry samples. Frozen storage did not have significant reductions of MNV-1 whereas HPP effectively inactivated the viability of MNV-1.  The viral reduction level for whole frozen strawberries increased as treatment pressure increases. After high pressure treatments at 250, 300 and 350 MPa, the reduction of MNV-1 was 1.2±0.5, 2.2±0.3 and 2.4±0.4 log PFU/g, respectively. At 400 MPa for 3 minutes, MNV-1 was inactivated to a level beyond the detection limits of the plaque assay and the level of inactivation was estimated to be more than 3.6 log PFU/g.

Significance: By using the surrogate virus (MNV-1) to test the feasibility of inactivating HuNoV, our study provided insight for fresh and frozen produce industry to apply high pressure on strawberries as an effective processing tool to mitigate the risk of HuNoV.