Effect of air blast freezing and frozen storage on inactivation of Vibrio vulnificus in Pacific oysters (Crassostrea gigas)

Air-blast freezing followed by frozen storage at different temperatures was assessed for inactivation of Vibrio vulnificus in Pacific oysters (Crassostrea gigas). Pacific oysters were allowed to bioaccumulate a cocktail of six strains of V. vulnificus. They were then air-blast frozen at −55 °C followed by frozen storage at −8 °C, −13 °C, −18 °C, −23 °C and −28 °C. V. vulnificus reductions were enumerated at different sampling times from 0 to 360 days. Air-blast freezing and frozen storage inactivated V. vulnificus in Pacific oysters in a two-phase inactivation pattern, with rapid inactivation during the first 3 days after air-blast freezing (short-term effect) followed by slower log-linear declines during frozen storage (long-term effect). The effects of storage temperature on V. vulnificus inactivation (log10 MPN/day) were assessed using a combined model that includes effects from both short- and long-term storage. The US National Shellfish Sanitation Program requires that postharvest processes to reduce Vibrio concentrations achieve ≥3.52 log reductions. At −18 °C this was achieved after an average of 90 days, but 95% confidence was not achieved until 123 days. Lower frozen storage temperatures caused slower declines of V. vulnificus. As well as artificially bioaccumulated V. vulnificus, we modeled inactivation of naturally occurring V. parahaemolyticus and showed that the results of a previous North American study fitted with our data. Their validated treatment for this organism in half-shell oysters can therefore be applied to New Zealand whole-shell oysters. This study will help guide oyster producers/processors aiming to achieve significant inactivation of Vibrio bacteria in their product, by providing a starting point for developing suitable frozen storage time/temperature conditions for Vibrio inactivation.