Vibrio parahaemolyticus is a foodborne pathogen recognized as the leading cause of acute gastroenteritis associated with consumption of raw and undercooked seafood, particularly raw oysters, with major symptoms of nausea, vomiting, abdominal cramps and diarrhea. It is estimated that 45,000 cases of V. parahaemolyticus infection occur each year in the United States. In order to reduce the high risks of V. parahaemolyticus infection associated with raw oyster consumption, post-harvest processes capable of decreasing V. parahaemolyticus levels by >3.52 log MPN/g are recommended by the U.S. Food and Drug Administration for oyster processing upon harvest.
Depuration is a process of holding shellfish in clean seawater allowing shellfish to purge contaminants and may be applied by the shellfish industry as a post-harvest process to reduce contamination of V. parahaemolyticus in oysters. Currently, depuration at controlled temperatures between 7 and 15°C for 5 days has been developed to achieve >3.0 but <3.52 log MPN/g reductions of V. parahaemolyticus in
oysters. The aim of this study was to investigate the factors, including water pH value, water temperature and oyster to water ratio, affecting the efficacy of depuration in decreasing V. parahaemolyticus in oysters and to improve the efficacy of depuration process to achieve >3.52 log MPN/g reductions of V. parahaemolyticus in oysters for application as a post-harvest treatment of oysters by the shellfish industry to produce safe oysters for raw consumption.
Studies of growth of V. parahaemolyticus in trypticase soy broth with 2% salt (TSB-Salt) medium of various pH values (5.5, 7.3 and 9.0) found that growth of five clinical V. parahaemolyticus strains were retarded in TSB-Salt at pH 5.5 compared with at pH 7.3 or 9.0. Investigation of oyster gaping in artificial seawater (ASW) of different pH values ranging from 4.0 to 10.5 unveiled that oysters survived well in ASW of pH between 5.0 and 9.5 at room temperature. Based on these findings, oyster depuration was conducted in a lab-scale depuration system with pH value of ASW being controlled at 5.5 or 7.0 at 12.5°C or 20°C for 5 days with an initial V. parahaemolyticus level in oysters of 10⁴⁻⁵ MPN/g. Depuration with ASW (pH 8.3) without control of pH value was used as a control.
Depuration in ASW of pH 5.5, 7.0 and 8.3 at 20°C for 5 days resulted in 0.7-2.0, 1.7-2.0 and 2.8 log MPN/g reductions of V. parahaemolyticus, respectively. Greater reductions (1.6-2.1 log MPN/g at pH 5.5, 2.9-3.0 log MPN/g at pH 7.0 and 3.5 log MPN/g at pH 8.3) of V. parahaemolyticus in oysters were observed after depuration with ASW at 12.5°C for 5 days. Decreasing pH value of ASW for depuration resulted in decreased efficacy of the process in reducing V. parahaemolyticus contamination in oysters. Study of effects of different oyster to water ratios (number of oyster :
volume of water) of 1:1, 1:1.5, 1:2, and 1:2.5 revealed that depuration with oyster to water ratio of 1:2 could achieve >3.52 log MPN/g reductions after four days.
This study improved the efficacy of depuration with ASW at 12.5°C to deliver >3.52 log MPN/g reductions of V. parahaemolyticus in the Pacific oysters. This controlled depuration may be applied as a post-harvest process to produce safe oysters for raw consumption. Future studies are needed to validate the efficacy of this controlled depuration for commercial application by the shellfish industry.