176
15 °C. Therefore, depuration at ambient temperatures has been reported to be ineffective in reducing oyster Vibrio contamination. For this purpose, the efficacy of
refrigerated-seawater depuration for reducing Vibrio parahaemolyticus levels in
Pacific oyster (Crassostrea gigas) was investigated. Raw Pacific oysters were inoculated with a mixed culture of five clinical strains of V. parahaemolyticus and depurated with refrigerated seawater (5 °C) in a laboratory-scale recirculation system
equipped with a 15-W gamma UV sterilizer. It was concluded that the depuration of
refrigerated sea water for 96 h showed a reduction in V. parahaemolyticus populations of >3.0 log MPN/g in oysters collected in winter, however, it was concluded
that 144 h of depuration at 5 °C was required to achieve 3-log reduction in summer
collected oysters (Su et al. 2010).
Depuration was also applied to heavy metals. Oysters (Crassostrea gigas) contaminated with heavy metals were transplanted to cleaner waters. The oysters kept
in the new environment were sampled at regular intervals and heavy metal levels
were examined. According to the results of the study, Cd, Cu, Hg, Ag and Zn were
eliminated by C. gigas, while As, Cr, Pb, MN, Ni and Se showed no significant
changes. It has been reported that the difference in heavy metal depuration is related
to the bonding and deposition characteristics of each metal (Okazaki and
Panietz 1981).
Shucking of Oysters
Oysters are composed of two calcareous valves or shells wherein the soft body of
the oyster lies. The left valve is usually cupped and has a projection at the anterior
region that curves upward called the umbo. The oyster normally rests on the left
valve. The right valve is usually less cupped and flatter in appearance. The valves
are joined by a resilient lamellar ligament at the anterior margin of the oyster. This
ligament is biased in the open position and counteracts the force of the adductor
muscle which closes the valves. The ligament goes into tension when the oyster is
closed and into compression when the oyster is in the relaxed state and open.
Adductor muscle is fast acting and provides protection for the oyster. Sessile intertidal bivalves such as oysters frequently remain closed for long periods of time.
Thus, to the side of the translucent muscle, there is a smaller, crescent-shaped muscle with smooth muscle fibres and opaque in colour that holds the valves closed for
extended periods of time. Both muscle types are attached to the inside of the shell
on both the left and right valves. The strength of this opaque muscle is what makes
it difficult to open an oyster. In the mid-1800s, the oyster knife was invented. The
oyster knife is still the standard today as it allows for inexpensive access to the oyster. It is good for prying the shells loose at the hinge and severing the adductor
muscle. Some shuckers use a hammer in conjunction with the knife to crack the bill
of the oyster so the knife can be inserted more easily. As there is no heat involved in
the process, the final product also remains in a raw state. Shucking by knife is labour
3 Molluscan Shellfish
15 °C. Therefore, depuration at ambient temperatures has been reported to be ineffective in reducing oyster Vibrio contamination. For this purpose, the efficacy of
refrigerated-seawater depuration for reducing Vibrio parahaemolyticus levels in
Pacific oyster (Crassostrea gigas) was investigated. Raw Pacific oysters were inoculated with a mixed culture of five clinical strains of V. parahaemolyticus and depurated with refrigerated seawater (5 °C) in a laboratory-scale recirculation system
equipped with a 15-W gamma UV sterilizer. It was concluded that the depuration of
refrigerated sea water for 96 h showed a reduction in V. parahaemolyticus populations of >3.0 log MPN/g in oysters collected in winter, however, it was concluded
that 144 h of depuration at 5 °C was required to achieve 3-log reduction in summer
collected oysters (Su et al. 2010).
Depuration was also applied to heavy metals. Oysters (Crassostrea gigas) contaminated with heavy metals were transplanted to cleaner waters. The oysters kept
in the new environment were sampled at regular intervals and heavy metal levels
were examined. According to the results of the study, Cd, Cu, Hg, Ag and Zn were
eliminated by C. gigas, while As, Cr, Pb, MN, Ni and Se showed no significant
changes. It has been reported that the difference in heavy metal depuration is related
to the bonding and deposition characteristics of each metal (Okazaki and
Panietz 1981).
Shucking of Oysters
Oysters are composed of two calcareous valves or shells wherein the soft body of
the oyster lies. The left valve is usually cupped and has a projection at the anterior
region that curves upward called the umbo. The oyster normally rests on the left
valve. The right valve is usually less cupped and flatter in appearance. The valves
are joined by a resilient lamellar ligament at the anterior margin of the oyster. This
ligament is biased in the open position and counteracts the force of the adductor
muscle which closes the valves. The ligament goes into tension when the oyster is
closed and into compression when the oyster is in the relaxed state and open.
Adductor muscle is fast acting and provides protection for the oyster. Sessile intertidal bivalves such as oysters frequently remain closed for long periods of time.
Thus, to the side of the translucent muscle, there is a smaller, crescent-shaped muscle with smooth muscle fibres and opaque in colour that holds the valves closed for
extended periods of time. Both muscle types are attached to the inside of the shell
on both the left and right valves. The strength of this opaque muscle is what makes
it difficult to open an oyster. In the mid-1800s, the oyster knife was invented. The
oyster knife is still the standard today as it allows for inexpensive access to the oyster. It is good for prying the shells loose at the hinge and severing the adductor
muscle. Some shuckers use a hammer in conjunction with the knife to crack the bill
of the oyster so the knife can be inserted more easily. As there is no heat involved in
the process, the final product also remains in a raw state. Shucking by knife is labour
3 Molluscan Shellfish