386 Part III: Muscle Foods
The enzyme is stable and tolerates both high salt
concentrations and freezing. The accumulation of
DMA and formaldehyde progresses only slowly,
and most of the accumulation is produced during
prolonged frozen storage or cold storage of the salt-
ed fish (e.g., salted cod and baccalao). During freez-
ing, the enzymatic reaction proceeds as long as liq-
uid water and substrate are available. In practice, the
formation of DMA and formaldehyde is found at
temperatures down to approximately 30°C (Sotelo
and Rehbein 2000). At higher temperatures the rate
of formation of formaldehyde is also higher; thus,
freezing of gadiform fish at insufficiently low tem-
peratures may result in dramatic changes in the sol-
ubility of myofibrillar proteins within a few weeks
(Nielsen and Jørgensen 2004).
Despite the TMAOase concentration of the white
muscle being low, it is the enzyme activity of the
white muscle that is responsible for the accumula-
tion of formaldehyde in whole fish and in fillets
(Nielsen and Jørgensen 2004). In the case of minced
products, even minor contamination by TMAOase-
rich tissues leads to a marked rise in the rate of accu-
mulation (Dingle et al. 1977, Lundstrøm et al. 1982,
Rehbein 1988, Rehbein et al. 1997). Despite its de-
pendency upon other factors, such as cofactors, the
rate of accumulation of formaldehyde can to a large
extent be predicted from the TMAOase enzyme ac-
tivity of fish meat alone (Nielsen and Jørgensen
2004).
The physiological function of TMAOase remains
unknown. The formation of formaldehyde could
have a digestive function, yet this would not explain
the high TMAOase content of the kidney and spleen.
Accordingly, it has been speculated that TMAO may
not even be its primary natural substrate (Sotelo and
Rehbein 2000).
The in vitro reaction rate is low without the pres-
ence of a number of redox-active cofactors. Al-
though the overall TMAOase reaction is not a redox
reaction, its dependency on redox agents shows that
the reaction mechanism must include redox steps.
Little is known about the in vivo regulation of
TMAOase, but studies suggest the importance of
nucleotide coenzymes and iron (Hultin 1992).
TMAOase appears to be membrane bound and is
only partly soluble in aqueous solutions. This has
complications for its purification and is one of the
main reasons why the complete characterization of
the enzyme remains yet to be done.
POSTMORTEM PROTEOLYSIS IN
FRESH FISHPostmortem proteolysis is an important factor in
many changes in seafood quality. During cold stor-
age, the postmortem proteolysis of myofibrillar and
connective tissue proteins contributes to deteriora-
tion in texture. Despite the natural tenderness of
seafood, texture is an important quality parameter,
in fish and shellfish alike. Deterioration in seafood
quality through proteolysis involves a softening of
the muscle tissue. This reduces the cohesiveness of
the muscle segments in fillets, which promotes gap-
ing, a formation of gaps and slits between muscle
segments. The negative character of this effect con-
trasts with the effect of postmortem proteolysis on
the meat of cattle and 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 terrestial animals. 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 adjacent myotomes by collagenous sheets termed
the myocommata (Bremner and Hallett 1985). See
Figure 17.2.
A myotome resembles a mammalian skeletal mus-
cle in terms of its intracellular structure (see Fig.
17.3) and the composition of the extracellular ma-
trix. Each muscle fiber is surrounded by fine colla-
gen fibers, the endomysium, joined with a larger net-
work of collagen fibers, the perimysium, which is
contiguous with the myocommata (Bremner and
Hallet 1985). Although the endomysium, perimysi-
um, 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 extracellu-
lar matrix proteins. Initially the disintegration of the
attachment between the myocommata and muscle
fibers leads to the resolution of rigor (Taylor et al.