16 Biochemistry of Seafood Processing 367time because microbial growth and enzymatic and
biochemical reaction kinetics are reduced at low
temperatures. However, ice crystals formed as a re-
sult of freezing may damage cells and disrupt the
texture of food products, and the concentrated un-
frozen matrix may result in changes in pH, osmotic
pressure, and ionic strength. These changes can af-
fect biochemical and physicochemical reactions
such as protein denaturation, lipid oxidation, and
enzymatic degradation of trimethylamine oxide
(TMAO) in frozen seafood. It is therefore essential
to understand these reactions in order to extend the
shelf life and improve the quality attributes of sea-
food.
PROTEINDENATURATION
During freezing or frozen storage, changes in the
physical state of water and the presence of lipids
create an environment that induces protein denatura-
tion. This denaturation can be caused by one or
more of the following factors: (1) ice crystal forma-
tion, (2) dehydration effect, (3) increase in solute
concentration, (4) interaction of protein with intact
lipids and free fatty acids (FFA), and (5) interaction
of protein with oxidized lipids (Shenouda 1980).
Denaturation of protein changes the texture and
functional properties of protein. The texture of fish
may become more fibrous and tough as a result of
the loss of protein solubility and water-holding ca-
pacity. These textural changes give rise to undesir-
able sensory attributes, which are often described as
sponginess, dryness, rubbery texture, and loss of
juiciness (Haard 1992a). Tseng et al. (2003) sug-
gested that to retain good eating qualities, that is, to
maintain tenderness and cooking yield, and reduce
lipid oxidation, red claw crayfish (Cherax quadri-
carinatus)should not be subjected to more than
three freeze-thaw cycles.
Ice Crystal Effect
Ice may form inter- and intracellularly during freez-
ing, which ruptures membranes and changes the
structure of the muscle cells (Mazur 1970, 1984;
Friedler et al. 1988). At a slow freezing rate, fluids
in the extracellular spaces freeze first, thus increas-
ing the concentration of extracellular solutes and
drawing water osmotically from the unfrozen cell
through the semipermeable cellular membrane (Ma-
zur 1970, 1984). The diffusion of water from the
internal cellular spaces to the extracellular spaces
results in drip, collected from frozen muscle tissues
when thawed (Jiang and Lee 2004). Drip contains
proteins, peptides, amino acids, lactic acid, purines,
vitamin B complex, and various salts (Sulzbacher
and Gaddis 1968), and their concentration in drip
increases with storage time (Einen et al. 2002). On
the other hand, when the tissue is frozen rapidly, the
cellular fluids do not have enough time to migrate
out to the extracellular spaces and will freeze as
small crystals uniformly distributed throughout the
tissue (Mazur 1984). Thus, rapid freezing has a less
detrimental effect on the cell or tissue (Coggins and
Chamul 2004). However, if the storage temperature
fluctuates, the intracellular ice recrystallizes, form-
ing large ice crystals and causing cell disruption,
which leads to drip loss as well. Mechanical damage
from ice crystals on the texture of food during freez-
ing and frozen storage has been studied in kiwi fruit
(Fuster et al. 1994), lamb meat (Payne and Young
1995), squid mantle muscle (Ueng and Chow 1998),
pork (Ngapo et al. 1999), and beef (Farouk et al.
2003).
During storage, if the product is not wrapped or
is improperly packaged, ice may sublime into the
headspace and produce a product defect called
freezer burn (Blond and Le Meste 2004, Coggins
and Chamul 2004). The sublimation of ice crystals
leaves behind small cavities and causes the surface
of fish to appear greyish. It is more pronounced
when the storage temperature is high (Blond and Le
Meste 2004). Freezer burn increases the rate of ran-
cidity and discoloration because of the greater
exposed surface, resulting in a “woody” (Blond and
Le Meste 2004), tough, and dry texture in fish, mak-
ing and leaving the fish less acceptable (Coggins and
Chamul 2004).Dehydration EffectNative protein is stabilized by the folding of hy-
drophobic chains into the protein molecules and is
held together by many other forces, including hyd-
rogen bonding, dipole-dipole interactions, electro-
static interactions, and disulphite linkages (Benjakul
and Bauer 2000). The formation of ice during freez-
ing removes water from the protein molecules, thus
disrupting the hydrogen bonding network between
the native proteins and water molecules and expos-
ing the hydrophobic or hydrophilic sites of the pro-
tein molecules (Shenouda 1980). The exposed