This study investigated the effects of flash freezing, followed by frozen storage, on reducing Vibrio parahaemolyticus in Pacific raw oysters. Raw Pacific oysters were inoculated with a five-strain cocktail of V. parahaemolyticus at a total level of approximately 3.5 × 105 most probable number (MPN) per gram. Inoculated oysters were subjected to an ultralow flash-freezing process (−95.5°C for 12 min) and stored at −10, −20, and −30°C for 6 months. Populations of V. parahaemolyticus in the oysters declined slightly by 0.22 log MPN/g after the freezing process. Subsequent storage of frozen oysters at −10, −20, and −30°C resulted in considerable reductions of V. parahaemolyticus in the oysters. Storing oysters at −10°C was more effective in inactivating V. parahaemolyticus than was storage at −20 or −30°C. Populations of V. parahaemolyticus in the oysters declined by 2.45, 1.71, and 1.45 log MPN/g after 1 month of storage at −10, −20, and −30°C, respectively, and continued to decline during the storage. The levels of V. parahaemolyticus in oysters were reduced by 4.55, 4.13, and 2.53 log MPN/g after 6 months of storage at −10, −20, and −30°C, respectively. Three process validations, each separated by 1 week and conducted according to the National Shellfish Sanitation Program's postharvest processing validation–verification interim guidance for Vibrio vulnificus and Vibrio parahaemolyticus, confirmed that a process of flash freezing, followed by storage at −21 ± 2°C for 5 months, was capable of achieving greater than 3.52-log (MPN/g) reductions of V. parahaemolyticus in half-shell Pacific oysters.
Associations of Environmental Conditions and Vibrio parahaemolyticus Genetic Markers in Washington State Pacific Oysters