Temperature-assisted pressure inactivation of Vibrio parahaemolyticus and Vibrio vulnificus in oysters

Vibrio parahaemolyticus and Vibrio vulnificus are frequently associated with oysters, and since oysters are typically consumed raw on a half shell, they can pose a threat to public health due to ingestion of these two pathogenic marine microorganisms. V. parahaemolyticus is the most frequent cause of seafood-associated gastroenteritis and V. vulnificus is the cause of 95% of the all seafood related deaths in the United States. Oysters should be processed to reduce the number of these pathogens; however, traditional thermal processing adversely affects the sensory qualities of oysters. High pressure processing is gaining more and more acceptance among oyster processors due to its ability to inactivate V. parahaemolyticus and V. vulnificus while keeping the fresh-like characteristics of oysters. Ten strains of V. parahaemolyticus and nine strains of V. vulnificus were tested for their sensitivities to high hydrostatic pressure. The two most pressure-resistant strains of V. parahaemolyticus, ATCC 43996 and DIE12-052499, and the most pressure-resistant strain of V. vulnificus, MLT 403, were used in this study to represent a worst case scenario for evaluation of the processing parameters for inactivation. To evaluate the effect of temperature on pressure inactivation of these two pathogens, oyster meats were inoculated with V. parahaemolyticus and V. vulnificus and incubated at room temperature for 24 h. Oyster meats were then blended and treated at 250 MPa for 5 min, 300 MPa for 2 min, and 350 MPa for 1 min for V. parahaemolyticus, and 150 MPa for 4 min, and 200 MPa for 1 min for V. vulnificus. Pressure treatments were carried out at -2, 1, 5, 10, 20, 30, 40, and 45°C. Slightly elevated temperatures (>30°C) considerably enhanced pressure inactivation of V. parahaemolyticus in oysters. For temperature levels of ≤30°C, the effect of temperature on pressure inactivation of V. parahaemolyticus depended on the pressure level. Cold temperatures (<20°C) and slightly elevated temperatures (>30°C) substantially increased pressure inactivation of V. vulnificus. For example, a 4-min treatment of 150 MPa at -2 and 40°C reduced the counts of V. vulnificus by 4.7 and 2.8 log, respectively, while at 20°C the same treatment only reduced counts by 0.5 log. Temperatures of -2 and 1°C were used to determine the effect of pressure level, temperature, and treatment time on the inactivation of V. vulnificus in oysters which were infected with V. vulnificus through feeding. To achieve a >5-log reduction in the counts of V. vulnificus in a relatively short treatment time (≤4 min), pressure treatment needs to be conducted at pressure levels of 250 MPa at -2 or 1°C.