Refrigerated seawater depuration for reducing Vibrio parahaemolyticus contamination in raw Pacific oysters (Crassostrea gigas) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8p58ph22v

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  • Vibrio parahaemolyticus is a seafood-borne pathogen that can cause gastroenteritis in humans. This study investigated the effectiveness of refrigerated seawater (5°C) depuration on reducing V. parahaemolyticus in raw Pacific oysters (Crassostrea gigas). Raw Pacific oysters were inoculated with a mixed culture of five clinical strains of V. parahaemolyticus and depurated with cold seawater (5°C) in a pilot scale recirculating system. The refrigerated seawater depuration was more efficient in reducing V. parahaemolyticus contamination in oysters harvested in winter than in summer. Populations of V. parahaemolyticus in oysters harvested in winter were reduced by >1.2 log MPN/g after 24 h of depuration in refrigerated seawater. Reductions of V. parahaemolyticus in the oysters increased to about 2.3 log MPN/g after 48 h and reached 3.1 log after 96 h of the process. However, it required 144 h of depuration in the refrigerated seawater to achieve a 3-log (MPN/g) reduction of V. parahaemolyticus in oysters harvested in summer. The efficacies of refrigerated seawater depuration in reducing V. parahaemolyticus were determined at a rate of 0.0211-log/h in oysters harvested in the summer and 0.0362-log/h in oysters harvested in the winter. This is probably due to the differences between water temperatures of the oyster harvest site (7-9ºC in winter, 16-17ºC in summer) and the refrigerated seawater (5ºC). Because of the increased temperature difference in the summer, it would require a longer time for oysters to adjust their biological activity to the new environment. Depuration of raw oysters in recirculated refrigerated seawater (5ºC) for up to 144 h did not cause a noticeable fatality of oysters, but increased their ability to survive in subsequent cold storage. The process also reduced fecal coliform contamination in oysters from 10³ MPN/g to less than 20 MPN/g. When oysters were stored in a refrigerator, 90% of oysters with or without depuration treatment survived after 7 days. However, the survival rate of oysters that had not been depurated in refrigerated seawater dropped sharply from 90 % to 44% after 9 days while 87% of depurated oysters remained alive after the same period of storage. No viable but nonculturable (VBNC) cells of V. parahaemolyticus were detected in the oysters depurated in refrigerated seawater for 144 h by reverse transcriptase polymerase chain reaction (RT-PCR) and multiplex PCR. Refrigerated seawater (5ºC) depuration can be used as a simple and economical post-harvest treatment for reducing V. parahaemolyticus contamination in oysters. This process can easily be adopted by the shellfish industry for producing safe oysters for consumers and to reduce V. parahaemolyticus infection associated with raw oyster consumption.
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