388 Part III: Muscle Foods
storage (Shigemura et al. 2004). This indicates that
the degradation of collagen and the resulting weaken-
ing of the intramuscular pericellular connective tis-
sue also play a role in the textural changes in fish
muscle that occur early postmortem.
A number of studies have revealed structural
changes in the myofibrillar proteins in fish muscle
during cold storage. Analysis of myofibrillar pro-
teins from the muscle of salmon stored at 0°C for as
long as 23 days has shown that several new protein
fragments form (Lund and Nielsen 2001). Other
studies suggest that predominantly proteins of the
cytoskeletal network, such as the high molecular
weight proteins titin and nebulin, are degraded (Bus-
coni et al. 1989, Astier et al. 1991). The extent of
degradation of the muscle myofibrillar proteins var-
ies among species. It has been found, for example,
that the intermediate filament protein desmin is
clearly degraded during the cold storage of turbot
and sardines but that no degradation occurs during
the cold storage of sea bass and brown trout (Verrez-
Bagnis et al. 1999).
As research shows, both myofibrillar and extra-
cellular matrix proteins in the muscle of many fish
are degraded during storage, and textural changes
can be expected to result from degradation of pro-
teins from both structures.
PROTEASES INFISHMUSCLE
The structural and biochemical changes just de-
scribed can be considered to largely represent the
concerted action of different endogenous proteolytic
enzymes. Proteolytic enzymes of all major classes
have been documented in the muscle of various fish
species. An overview of the different proteases be-
lieved to play a role in the postmortem proteolysis of
seafood is presented below. Further information on
the proteases found in fish and marine invertebrates
can be found in reviews by Kolodziejska and Si-
korski (1995, 1996).
Matrix Metalloproteinases
Matrix metalloproteinases are extracellular enzymes
involved in the in vivo catabolism (degradation) of
the extracellular matrix (degradation of the helical
regions of the collagens). They have been isolated
from rainbow trout (Saito et al. 2000), Japanese
flounder (Kinoshita et al. 2002) and Pacific rockfish
(Bracho and Haard 1995). Collagenolytic and gel-
atinolytic activities have also been detected in the
muscle of winter flounder (Teruel and Simpson
1995), yellowtail (Kubota et al. 1998), ayu (Kubota
et al. 2000), salmon, and cod (Lødemel and Olsen
2003). Type I collagen is solubilized and degraded
by matrix metalloproteinases in rainbow trout (Saito
et al. 2000), Japanese flounder (Kubota et al. 2003),
and Pacific rockfish (Bracho and Haard 1995), sug-
gesting that these proteases can participate in post-
mortem textural changes.CathepsinsLysosomal proteinases such as cathepsins B, D, and
L have been isolated from a number of fish species,
including herring (Nielsen and Nielsen 2001), mack-
erel (Aoki and Ueno 1997), and tilapia (Jiang et al.
1991). In vitro studies show that certain cathepsins
are capable of cleaving myofibrillar proteins (Ogata
et al. 1998, Nielsen and Nielsen 2001) and may also
participate in degradation of the extracellular matrix
since they can cleave both nonhelical regions of the
collagen (Yamashita and Konagaya 1991) and colla-
gen that has already been partly degraded by matrix
metalloproteinases. Since cathepsins in the muscle
of living fish are located in the lysosomes, they are
not originally in direct contact with either the myo-
fibrils or the extracellular matrix, although it has
been shown that the enzymes leak from the lyso-
somes in fish muscle postmortem, and from lyso-
somes in bovine muscles postmortem as well (Gero-
mel and Montgomery 1980, Ertbjerg et al. 1999).CalpainsCalpains have been reported in the muscle of various
fish species (Wang and Jiang 1991, Watson et al.
1992). They are only active in the neutral pH range,
although research shows that they also remain active
at the slightly acidic postmortem pH (Wang and
Jiang 1991) and are capable of degrading myofibril-
lar proteins in vitro (Verrez-Bagnis et al. 2002, Gees-
ink et al. 2000). This suggests a participation in the
postmortem hydrolysis of fish muscle. Watson et al.
(1992) found evidence for calpains being involved in
the degradation of myofibrillar proteins in tuna, lead-
ing to the muscle becoming pale and grainy, which is
referred to as “burnt tuna.”