Thermal inactivation of human norovirus surrogates in oyster homogenate

Human norovirus (HNV) is the most frequent causative agent of foodborne diseases in the US. Raw and undercooked oysters are commonly associated with outbreaks caused by HNV. Many guidelines recommend that shucked oysters be boiled for at least 3 min, but it is not clear this thermal treatment can inactivate HNV. The objective of this research was to evaluate whether this recommendation was sufficient to inactivate two HNV surrogates, murine norovirus (MNV-1) and Tulane virus (TV) in oyster homogenate as well as to determine their thermal inactivation kinetics. Inoculated oyster homogenate was heated in boiling water and circulating water bath at 49 to 67 °C for different time durations. After 3 min of boiling, both MNV-1 and TV titers decreased to below the detection limits. First-order model and Weibull model were used to describe thermal inactivation kinetics. TD = 1 values from Weibull mode are used as an analog to D values in first-order model. The D values of MNV-1 and TD = 1 values ranged from 28.17 to 0.88 min and 26.64 to 0.78 min at 49 to 67 °C, respectively. The D values of TV and TD = 1 values ranged from 18.18 to 1.56 min and 19.35 to 1.56 min at 49 to 63 °C, respectively. The kinetics demonstrated that at temperatures > 58 °C, TV was much more heat sensitive than MNV-1. As the temperature increased over 58 °C, the inactivation of both viruses occurred at a faster rate. Boiling treatment for 3 min as recommended by FDA for cooking shucked oysters, inactivated MNV-1 and TV in oyster homogenate below detection limit. One minute heating of TV at 63 °C or MNV-1 at 67 °C in contaminated oyster homogenate reduced the viral titers below the detection limits. Our research identified effective combinations of time and temperature to inactivate two HNV surrogate viruses, and thus provides insights on thermal processing to reduce the risk of foodborne viral illness outbreaks associated with consumption of oysters.