175
Pacific oysters (Crassostrea gigas) were stored in air (out of water) for 20 weeks,
and the survivors re-immersed in the sea water and investigated for a few months.
Survival rate was found to be 52–80% for oysters sprinkled with water during storage at 7 °C. Re-immersion affected survival positively (Seaman 1991).
Depuration of Oyster
Depuration is an effective post-harvest treatment for the removal of most bacterial
species from oysters. Depuration has been used for reducing sewage-associated
bacteria, such as coliforms and E. coli in shellfish. The efficacy of a depuration
process is largely dependent on the biological activities of oysters and the nature of
the microorganism. Temperature is the most critical factor affecting the pumping
rate. The volume of water pumped by oysters is regarded as a predictor of biological
activity of oysters (Shen et al. 2019).
Shen et al. (2019) reported that the rate of water to oyster is effective in V. parahaemolyticus depuration in oysters. They achieved a reduction of >3.00 log with
depuration with a water to oyster (Crassostrea gigas) ratio of 2:1 for 2 days for V.
parahaemolyticus.
The combination of chlorine and UV radiation has been reported to increase
disinfection efficiency and reduce the time required for depuration. Two disinfection methods are thought to have a synergistic effect (Koivunen and HeinonenTanski 2004). Although chlorine is used as a disinfectant in depuration tanks, it can
be toxic to bivalves. On the other hand, UV radiation can eliminate bacteria and
viruses without toxic effects (Correa et al. 2012). Chlorine has also been suggested
to cause organoleptic changes in shellfish meat (Lee et al. 2008).
Viruses are mainly concentrated in the digestive gland of the bivalves. It has been
reported that if depuration of shellfish is carried out appropriately, bacteria can often
be reduced to undetectable levels, but viruses can be reduced at lower rates because
they are more resistant and persistent than bacteria (Dore and Lees 1995). The oysters were contaminated with hepatitis A virus (HAV) or human adenovirus type 5
(HAdV5) by Correa et al. (2012). Oysters which are harvested after 48, 72 and 96 h
and infected with viruses, were placed into a closed system depuration tank recirculating seawater. A sterilising system consisting one 18 W ultraviolet (UV) lowpressure tube, a sand filter and a refrigeration system was used. The temperature of
the depuration system was maintained at 19–20 °C during all of the experiments to
prevent the molluscs from spawning. In conclusion, after 96 h of UV treatment, the
depuration system studied in this work purified oysters that were artificially contaminated with HAdV5 and HAV.
Populations of fungi and bacteria showed marked reduction after depuration of
Mangrove oysters collected from Benya lagoon, located at Elmina in the Central
Region of Ghana (Obodai et al. 2010).
It has been reported that the presence of V. parahaemolyticus in marine environments is associated with water temperature and that V. parahaemolyticus can rarely
be detected in oysters or environmental samples until water temperatures rise to
3.1 Bivalves
Pacific oysters (Crassostrea gigas) were stored in air (out of water) for 20 weeks,
and the survivors re-immersed in the sea water and investigated for a few months.
Survival rate was found to be 52–80% for oysters sprinkled with water during storage at 7 °C. Re-immersion affected survival positively (Seaman 1991).
Depuration of Oyster
Depuration is an effective post-harvest treatment for the removal of most bacterial
species from oysters. Depuration has been used for reducing sewage-associated
bacteria, such as coliforms and E. coli in shellfish. The efficacy of a depuration
process is largely dependent on the biological activities of oysters and the nature of
the microorganism. Temperature is the most critical factor affecting the pumping
rate. The volume of water pumped by oysters is regarded as a predictor of biological
activity of oysters (Shen et al. 2019).
Shen et al. (2019) reported that the rate of water to oyster is effective in V. parahaemolyticus depuration in oysters. They achieved a reduction of >3.00 log with
depuration with a water to oyster (Crassostrea gigas) ratio of 2:1 for 2 days for V.
parahaemolyticus.
The combination of chlorine and UV radiation has been reported to increase
disinfection efficiency and reduce the time required for depuration. Two disinfection methods are thought to have a synergistic effect (Koivunen and HeinonenTanski 2004). Although chlorine is used as a disinfectant in depuration tanks, it can
be toxic to bivalves. On the other hand, UV radiation can eliminate bacteria and
viruses without toxic effects (Correa et al. 2012). Chlorine has also been suggested
to cause organoleptic changes in shellfish meat (Lee et al. 2008).
Viruses are mainly concentrated in the digestive gland of the bivalves. It has been
reported that if depuration of shellfish is carried out appropriately, bacteria can often
be reduced to undetectable levels, but viruses can be reduced at lower rates because
they are more resistant and persistent than bacteria (Dore and Lees 1995). The oysters were contaminated with hepatitis A virus (HAV) or human adenovirus type 5
(HAdV5) by Correa et al. (2012). Oysters which are harvested after 48, 72 and 96 h
and infected with viruses, were placed into a closed system depuration tank recirculating seawater. A sterilising system consisting one 18 W ultraviolet (UV) lowpressure tube, a sand filter and a refrigeration system was used. The temperature of
the depuration system was maintained at 19–20 °C during all of the experiments to
prevent the molluscs from spawning. In conclusion, after 96 h of UV treatment, the
depuration system studied in this work purified oysters that were artificially contaminated with HAdV5 and HAV.
Populations of fungi and bacteria showed marked reduction after depuration of
Mangrove oysters collected from Benya lagoon, located at Elmina in the Central
Region of Ghana (Obodai et al. 2010).
It has been reported that the presence of V. parahaemolyticus in marine environments is associated with water temperature and that V. parahaemolyticus can rarely
be detected in oysters or environmental samples until water temperatures rise to
3.1 Bivalves