Inactivation of Murine Norovirus and Feline Calicivirus during Oyster Fermentation

Introduction: Fermented oysters, called ‘eoriguljeot’, are popular representative fermented seafood in Korea. While reduction of foodborne pathogens in fermented foods has been well studied, the survival of foodborne viruses in fermented foods is not well understood. As many norovirus (NoV) outbreaks were associated with the consumption of raw oysters, it is important to ensure the microbial safety of fermented oysters.

Purpose: The aims of this study were to examine the survival of norovirus surrogates including feline calicivirus (FCV) and murine norovirus (MNV) and to investigate the change of pH and enzyme production during oyster fermentation.

Methods: Oysters spiked with FCV or MNV were fermented with 5% or 10% salt at 18°C for 15 days. The titers of MNV and FCV, populations of lactic acid bacteria (LAB), pH, and enzymatic activity were measured at 0, 1, 3, 5, 7, 10, and 15 days post-fermentation (DPF).

Results: Oysters supplemented with 5% NaCl showed more rapid fermentation than those supplemented with 10% NaCl. In 5% NaCl-supplemented oysters at 15 DPF, MNV and FCV titers were significantly decreased by 1.60 log and 3.01 log, respectively. As the populations of LAB increased up to 8.77 log CFU/g during oyster fermentation, pH gradually decreased to 4.17. The production of α-amylase, protease, and lipase were significantly higher in 5% salted oysters than in 10% salted oysters.

Significance: MNV and FCV were significantly reduced during oyster fermentation. The combination of LAB, low pH, and digestive enzymes produced in fermented oysters could contribute to reduce norovirus surrogates.