- Vibrio parahaemolyticus infections in the United Stated have been linked to consumption of raw shellfish, particularly oysters, with symptoms of headache, abdominal pain, nausea, diarrhea, vomiting. Depuration, as a post-harvest process, has a long history of being applied in shellfish industry to reduce sewage bacteria. In order to reduce the risks of V. parahaemolyticus infection associated with raw oyster consumption, FDA (2015) requires that post-harvest processes reduce V. parahaemolyticus and V. vulnificus to non-detectable levels (<30 MPN/g) and achieve a 3.52 log reduction.
Previous investigations have indicated that a refrigerated seawater depuration process at 12.5°C could significantly reduce V. parahaemolyticus contamination in Pacific oysters; however, further optimization is necessary to achieve the regulatory target of >3.52 log MPN/g. The aim of this study was to investigate several factors, including flow rate and feeding status, to improve the efficacy of depuration in decreasing V. parahaemolyticus in oysters. The long-term goal for this research is adoption of depuration by the shellfish industry to produce safe oysters for raw consumption. Pacific oysters (Crassostrea gigas; n = 35 per trial) were inoculated with a cocktail of V. parahaemolyticus (10290, 10292,10293, BE 98-2029, and 027-1c1) in freshly prepared artificial seawater (70 L). Depuration was conducted on inoculated oysters with flow rates of 15, 20, 25 and 35 L/min at 12.5°C for up to 5 days. V. parahaemolyticus contamination was determined using a three-tube most probable number (MPN) method. The efficacy of microbial reductions of V. parahaemolyticus was significantly enhanced when the flow rate increased from 15 to 35 L/min. Depuration with a lower flow rate (15 L/min) for 5 days resulted in 2.39 log (MPN/g) reduction of V. parahaemolyticus in oysters, while depuration with higher flow rates of 20 and 25 L/min reached 2.73 and 2.80 log (MPN/g) reductions, respectively. Further increase in flow rate to 35 L/min resulted in an average reduction of 3.39 log MPN/g of V. parahaemolyticus in oysters after 5 days. These results suggest that depuration efficacy can be enhanced by further increasing the flow rate of the system.
Studies of application of algae treatment in oyster depuration revealed that addition of algae mixture in artificial seawater (ASW) for depuration did not significantly impact the efficacy of depuration for reducing V. parahaemolyticus populations. Oysters inoculated with the V. parahaemolyticus cocktail were subjected to depuration with and without feeding (algae=0.036 ml/gram of oyster) for 6 days at 12.5°C with water ratio of 1:2. Oysters (n = 5) were analyzed for V. parahaemolyticus using a three-tube most probable number (MPN) method after 0, 1, 3, 5, and 6 days of depuration. Depuration over 6 days achieved average V. parahaemolyticus reductions of 2.75 log MPN/g and 3.03 log MPN/g in the fed and unfed systems, respectively. The lack of impact of feeding status on the efficacy of depuration provides the oyster industry with the flexibility to utilize fed or unfed conditions to apply in depuration process.
In these studies, only the factor of flow rate had a positive impact on reduction of V. parahaemolyticus. However, neither flow rate nor feeding treatments were able to achieve 3.52 MPN/g reduction of V. parahaemolyticus. Further optimization of depuration is necessary to achieve the regulatory target for V. parahaemolyticus decontamination in raw oysters.