179
Freezing of Oysters
The oysters for freezing should be alive. Freezing causes the shells of oysters to
open and makes subsequent shucking easier. Oysters can also be frozen in the half
shell. Oysters should never be put into a cold store to freeze. Cold stores are designed
only to hold already frozen products at the required low temperature. Frozen whole
oysters packed in polyethylene bags can be kept in good condition for 6 months in
a cold store at −30 °C. The liquor within the shell acts as a glaze to protect the meat
from dehydration (Stroud 1981).
Unless there is a market for frozen whole oysters, or freezing is to be used as
means of opening the shell, it is more economical to freeze the shucked meats.
Misshapen whole oysters that are unacceptable for the live trade can also be utilized
in this way. Meats frozen either individually or in blocks can yield an excellent
thawed product. The shucked meats are thoroughly washed to remove sand, grit,
and other shell debris, inspected to remove any ragged or broken meats, drained to
remove excess water, and frozen individually in an air blast freezer before packing.
Typically, the frozen meats are glazed and packed under vacuum in bags made of a
flexible film that has a high resistance to the passage of water vapour and oxygen.
An alternative procedure is to pack the meats under vacuum in a rigid container
before freezing them; containers of metal, plastics or waxed cardboard with inner
liners have been used, sometimes with shaped pockets for individual meats. The
strained shell liquor is also sometimes added to the packed meats before freezing
them; this has the added advantage that any remaining air spaces in the pack are
filled (Stroud 1981).
Freezing temperature is important in frozen storage (Hatano et al. 1990; Wang
et al. 2007). Faster ATP degradation was reported in oysters frozen at −20 °C than
those frozen at −30 °C (Wang et al. 2007).
It is reported that frozen storage inhibits growth of bacteria and capable of
achieving certain degrees of reductions of V. parahaemolyticus in oyster meat.
Muntada-Garriga et al. (1995) determined that the viable cells of V. parahaemolyticus in oyster meat frozen and stored at different temperatures (−18 and −24 °C) can
be inactivated and stated that the time of total inactivation depends on the initial
number of micro-organisms and incubation temperature. Other researchers have
also reported considerable reductions of V. parahaemolyticus in Pacific oysters
(C. gigas) homogenates during frozen storage at −10, −20 and −30 °C and inactivation of V. parahaemolyticus is more effective in oysters stored at −10 °C. Similarly,
Chae (2007) reported V. parahaemoliticus reductions and more effective reductions
at −10 °C in frozen stored oysters at −10, −23 and −30 °C (Liu et al. 2009).
Moreover, it was reported that blast freezing or heat shocking could be used as an
effective post-harvest process to reduce V. parahaemolyticus in oysters (C. virginica) which exposure to cold air at −29 °C and stored at −20 °C (Manley 2008).
The quality of frozen oysters is influenced by many factors, such as storage temperature and time, packaging, and thawing conditions. Freshness of raw oysters is
another factor affecting the quality of frozen oyster. Shucked and then frozen oysters (C. gigas) immediately after harvest and periodically frozen oysters after
3.1 Bivalves
Freezing of Oysters
The oysters for freezing should be alive. Freezing causes the shells of oysters to
open and makes subsequent shucking easier. Oysters can also be frozen in the half
shell. Oysters should never be put into a cold store to freeze. Cold stores are designed
only to hold already frozen products at the required low temperature. Frozen whole
oysters packed in polyethylene bags can be kept in good condition for 6 months in
a cold store at −30 °C. The liquor within the shell acts as a glaze to protect the meat
from dehydration (Stroud 1981).
Unless there is a market for frozen whole oysters, or freezing is to be used as
means of opening the shell, it is more economical to freeze the shucked meats.
Misshapen whole oysters that are unacceptable for the live trade can also be utilized
in this way. Meats frozen either individually or in blocks can yield an excellent
thawed product. The shucked meats are thoroughly washed to remove sand, grit,
and other shell debris, inspected to remove any ragged or broken meats, drained to
remove excess water, and frozen individually in an air blast freezer before packing.
Typically, the frozen meats are glazed and packed under vacuum in bags made of a
flexible film that has a high resistance to the passage of water vapour and oxygen.
An alternative procedure is to pack the meats under vacuum in a rigid container
before freezing them; containers of metal, plastics or waxed cardboard with inner
liners have been used, sometimes with shaped pockets for individual meats. The
strained shell liquor is also sometimes added to the packed meats before freezing
them; this has the added advantage that any remaining air spaces in the pack are
filled (Stroud 1981).
Freezing temperature is important in frozen storage (Hatano et al. 1990; Wang
et al. 2007). Faster ATP degradation was reported in oysters frozen at −20 °C than
those frozen at −30 °C (Wang et al. 2007).
It is reported that frozen storage inhibits growth of bacteria and capable of
achieving certain degrees of reductions of V. parahaemolyticus in oyster meat.
Muntada-Garriga et al. (1995) determined that the viable cells of V. parahaemolyticus in oyster meat frozen and stored at different temperatures (−18 and −24 °C) can
be inactivated and stated that the time of total inactivation depends on the initial
number of micro-organisms and incubation temperature. Other researchers have
also reported considerable reductions of V. parahaemolyticus in Pacific oysters
(C. gigas) homogenates during frozen storage at −10, −20 and −30 °C and inactivation of V. parahaemolyticus is more effective in oysters stored at −10 °C. Similarly,
Chae (2007) reported V. parahaemoliticus reductions and more effective reductions
at −10 °C in frozen stored oysters at −10, −23 and −30 °C (Liu et al. 2009).
Moreover, it was reported that blast freezing or heat shocking could be used as an
effective post-harvest process to reduce V. parahaemolyticus in oysters (C. virginica) which exposure to cold air at −29 °C and stored at −20 °C (Manley 2008).
The quality of frozen oysters is influenced by many factors, such as storage temperature and time, packaging, and thawing conditions. Freshness of raw oysters is
another factor affecting the quality of frozen oyster. Shucked and then frozen oysters (C. gigas) immediately after harvest and periodically frozen oysters after
3.1 Bivalves