Food Biochemistry and Food Processing (2 edition)

BLBS102-c13 BLBS102-Simpson March 21, 2012 13:15 Trim: 276mm X 219mm Printer Name: Yet to Come

13 Seafood Enzymes 253

Figure 13.2.Gross anatomy of fish muscle. (Drawing courtesy of A. S. Matforsk, Norwegian Food Research Institute.)

pigs, in which the degradation of myofibrillar proteins produces

a highly desired tenderization in the conversion of muscle

to meat.

The proteolytic enzymes that cause a softening of seafood are

of basically the same classes as those found in terrestrial ani-

mals. The special effects of proteolysis on seafood result from

the combined action of the homologous proteolytic enzymes on

the characteristic muscle structures of seafood. The muscle of

fish differs on a macrostructural level from that of mammals, fish

muscle being segmented into muscle blocks called myotomes.

A myotome consists of a single layer of muscle fibers arranged

side by side and separated from the muscle fibers of the adja-

cent myotomes by collagenous sheets termed the myocommata

(Bremner and Hallett 1985). See Figure 13.2.

A myotome resembles a mammalian skeletal muscle in terms

of its intracellular structure (see Fig. 13.3) and the composition

of the extracellular matrix. Each muscle fiber is surrounded

by fine collagen fibers, the endomysium, joined with a larger

network of collagen fibers, the perimysium, which is contiguous

with the myocommata (Bremner and Hallet 1985). Although the

endomysium, perimysium, and myocommata are considered to

be discrete areas of the extracellular matrix, they join to form a

single weave.

Such phenomena in seafood as softening, gaping, and the

resolution of rigor are believed to be caused by hydrolysis of the

myofibrillar and the extracellular matrix proteins. Initially, the

disintegration of the attachment between the myocommata and

muscle fibers leads to the resolution of rigor (Taylor et al. 2002).

Endogenous proteolytic enzymes that cleave muscle proteins

under physiological conditions and at neutral pH can be a factor

in the resolution of rigor mortis. The continuation of this process

is regarded 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 multi-

ple changes in muscle structure. Histological studies have shown

that the attachment between muscle fibers in fish muscle is bro-

ken during ice storage and has been associated with a loss of

“hardness” as measured by instrumental texture analysis (Taylor

et al. 2002). During cold storage, the junction between the my-

ofibrils and connective tissue 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 sarcolemma (the cell

membrane), and of the basement membrane, which is attached

Figure 13.3.A longitudinal view of the intracellular structure of a skeletal muscle fiber showing the contractile elements (actin and myosin),

the cytoskeleton, and the attachment to the extracellular matrix (endomysium and perimysium). (Adapted from Lødemel 2004.)