Food Biochemistry and Food Processing

17 Seafood Enzymes 387

2002). Endogenous proteolytic enzymes that cleave
muscle proteins under physiological conditions and
at neutral pH can be a factor in the resolution of rig-
or mortis. The continuation of this process is regard-
ed as one of the main causes of gaping (Taylor et al.
2002, Fletcher et al. 1997).
The softening of fish muscle appears to be the
result of multiple changes in muscle structure. His-
tological studies have shown that the attachment
between muscle fibers in fish muscle is broken dur-
ing ice storage and has been associated with a loss of
“hardness” as measured by instrumental texture an-
alysis (Taylor et al. 2002). During cold storage, the
junction between the myofibrils and connective tis-
sue of the myocommata is hydrolyzed (Bremner
1999, Fletcher et al. 1997, Taylor et al. 2002), and
the collagen fibers of the perimysium surrounding
the bundles of muscle fibers are degraded (Ando et
al. 1995, Sato et al. 2002).

It appears that cleavage of the costameric proteins

that link the myofibrils (the sarcomere) with the sar-

colemma (the cell membrane), and of the basement

membrane, which is attached to the fine collagen

fibers of the endomysium (see Fig. 17.3), leads to

muscle fibers being detached, which results in soft-

ening. Studies have shown that the costameric pro-

teins found in fish muscle are already degraded 24

hours postmortem (Papa et al. 1997). This empha-

sizes the importance of early postmortem changes.

It has been shown that changes in the collagen

fraction of the extracellular matrix and the softening

of fish are related. Collagen V, a minor constituent of

the pericellular connective tissue of fish muscle,

becomes soluble when rapid softening takes place,

whereas no solubilization is observed in fish that do

not soften (Sato et al. 2002). Recent studies also

indicate a degradation of the most abundant collagen

in fish muscle, collagen I, during 24 hours of cold

Figure 17.2.Gross anatomy of fish muscle. (Drawing courtesy of A. S. Matforsk, Norwegian Food Research
Institute.)

Figure 17.3.A longitudinal view of the intracellular structure of a skeletal muscle fiber showing the contractile ele-
ments (actin and myosin), the cytoskeleton, and the attachment to the extracellular matrix (endomysium and perimy-
sium). (Adapted from Lødemel 2004.)