180
keeping at 5 °C showed differences in quality during frozen storage. Frozen oysters,
usually stored at −20 °C, maintained their commercial quality over 1 year, but this
was not achieved with samples frozen after keeping at 5 °C for 3 days (Lee
et al. 2017).
During frozen storage slightly rancid off-flavours can develop and start to acquire
a greenish yellow discoloration. Unacceptable taste and coloration of oysters during
frozen storage has been reported (Hatano et al. 1990). Lipid oxidation was determined in Japanese oyster (C. gigas) during frozen storage. Lipid oxidation was
found to be faster in oysters stored at −20 °C than stored at −35 °C. However, the
use of dibutylhydroxytoluene or natural vitamin E solution was effective in inhibiting oxidation (Jeong et al. 1990).
Thermal Processing of Oysters
Heating
This process was originally developed to facilitate the shucking process by relaxing
the oyster’s adductor muscle. This process also reduces pathogens in the oyster. In
the heat shock post-harvest process, oyster shell-stock is subjected to 65 °C for
5 min, then immediately cooled in an ice slush for 10 min (Manley 2008).
Pasteurization
Pasteurization is usually used for Vibrio elimination in oysters. A heat treatment
above 50 °C has been reported to be sufficient to reduce V. vulnificus to undetectable
levels (Cook and Ruple 1992). Heat treatment of 50 °C for 5 min has been found
effective in reducing the numbers of V. vulnifcus and V. parahaemolyticus by 99.9%.
In case of treatment for 10 min they have been reduced to non-detectable levels.
This pasteurization process also increased the shelf life by reducing the number of
aerobic bacteria by 99.99% (Andrews et al. 2000). Pasteurisation further has the
advantage of increasing the shucking yield of oysters.
Pasteurization further has the advantage of increasing the shucking yield and
shelf life of oysters. The shelf life is extended from 1 week to untreated oysters to
3 weeks under refrigeration. Another advantage of this type of process is its versatility. The process can be adjusted to a small-or large-scale to fit the needs of an individual processing plant. However, the disadvantage of this process is that the
processing temperature and time can change the sensory properties of the oysters
(Espinoza 2013). Andrews et al. (2003) reported that temperatures above 52.5 °C
affect the texture and taste of oyster due to protein degradation. It is reported that
every 10 °C increase in temperature increases protein denaturation by approximately 600-fold, and the cutting strength of oyster meat is significantly reduced
when the temperature rises from 57.5 to 70.0 °C (Lekjing et al. 2017). After thermal
processing, It was reported that the plum and adductor of oyster is shrunken, colour
3 Molluscan Shellfish
keeping at 5 °C showed differences in quality during frozen storage. Frozen oysters,
usually stored at −20 °C, maintained their commercial quality over 1 year, but this
was not achieved with samples frozen after keeping at 5 °C for 3 days (Lee
et al. 2017).
During frozen storage slightly rancid off-flavours can develop and start to acquire
a greenish yellow discoloration. Unacceptable taste and coloration of oysters during
frozen storage has been reported (Hatano et al. 1990). Lipid oxidation was determined in Japanese oyster (C. gigas) during frozen storage. Lipid oxidation was
found to be faster in oysters stored at −20 °C than stored at −35 °C. However, the
use of dibutylhydroxytoluene or natural vitamin E solution was effective in inhibiting oxidation (Jeong et al. 1990).
Thermal Processing of Oysters
Heating
This process was originally developed to facilitate the shucking process by relaxing
the oyster’s adductor muscle. This process also reduces pathogens in the oyster. In
the heat shock post-harvest process, oyster shell-stock is subjected to 65 °C for
5 min, then immediately cooled in an ice slush for 10 min (Manley 2008).
Pasteurization
Pasteurization is usually used for Vibrio elimination in oysters. A heat treatment
above 50 °C has been reported to be sufficient to reduce V. vulnificus to undetectable
levels (Cook and Ruple 1992). Heat treatment of 50 °C for 5 min has been found
effective in reducing the numbers of V. vulnifcus and V. parahaemolyticus by 99.9%.
In case of treatment for 10 min they have been reduced to non-detectable levels.
This pasteurization process also increased the shelf life by reducing the number of
aerobic bacteria by 99.99% (Andrews et al. 2000). Pasteurisation further has the
advantage of increasing the shucking yield of oysters.
Pasteurization further has the advantage of increasing the shucking yield and
shelf life of oysters. The shelf life is extended from 1 week to untreated oysters to
3 weeks under refrigeration. Another advantage of this type of process is its versatility. The process can be adjusted to a small-or large-scale to fit the needs of an individual processing plant. However, the disadvantage of this process is that the
processing temperature and time can change the sensory properties of the oysters
(Espinoza 2013). Andrews et al. (2003) reported that temperatures above 52.5 °C
affect the texture and taste of oyster due to protein degradation. It is reported that
every 10 °C increase in temperature increases protein denaturation by approximately 600-fold, and the cutting strength of oyster meat is significantly reduced
when the temperature rises from 57.5 to 70.0 °C (Lekjing et al. 2017). After thermal
processing, It was reported that the plum and adductor of oyster is shrunken, colour
3 Molluscan Shellfish