Impact of pH and protein hydrophobicity on norovirus inactivation by heat-denatured lysozyme

Norovirus, the leading cause of non-bacterial food poisoning, is responsible for several outbreaks associated with bivalves and ready-to-eat food products worldwide. As norovirus is resistant to alcohol, which is commonly used in food manufacturing processes, sodium hypochlorite is used for its inactivation. However, sodium hypochlorite has two disadvantages: it cannot be added to foods, and its effect is significantly reduced in the presence of organic compounds. Thus, a novel disinfectant against norovirus is urgently required for food hygiene. Thermally denatured egg white lysozyme inactivates norovirus; however, the optimal inactivating conditions and the underlying mechanism are unclear. In the present study, the inactivating mechanism of heat-denatured lysozyme against norovirus was analyzed using murine norovirus strain 1 (MNV-1). We found that the inactivating effect was enhanced by adjusting the pH of the lysozyme solution before thermal denaturation to 6.5 or higher. The reaction of heat-denatured lysozyme and MNV-1 was irreversible, and norovirus was completely inactivated after exposure to heat-denatured lysozyme. Furthermore, it was found that lysozyme residues 5-39 contributed to the norovirus-inactivating effect. Notably, the hydrophobicity and positive charges in this region contributed to the norovirus-inactivating effect, as evidenced by the norovirus inactivation test using mutated residues 5-39. These findings are novel and highlight the possible application of heat-denatured lysozyme as a disinfectant against norovirus in a wide range of food processes.