384 Part III: Muscle Foods
hypoxanthine is catalyzed by one and the same
enzymes, xanthine oxidase (EC 1.1.3.22). The oxi-
dation requires a reintroduction of oxygen in the
otherwise anaerobic pathway, which often limits the
extent of the xanthine oxidase reaction and causes
inosine and hypoxanthine to accumulate. Hypox-
anthine is an indicator of spoilage and may con-
tribute to bitter taste (Hughes and Jones 1966). The
hydrogen peroxide produced by the oxidation of
hypoxanthine can be expected to cause oxidation,
but the technological implications of this are minor,
since it is only after the quality of the fish has
declined beyond the level of acceptability that these
reactions take place on a large scale.
The relation between the accumulation of break-
down products of ATP and the postmortem storage
period of seafood products has resulted in various
freshness indicators being defined. Saito et al. (1959)
defined a freshness indicator Kas a simplified way
of expressing the state of nucleotide degradation by
a single number:
Higher Kvalues correspond to a lower quality of
fish. The Kvalue often increases linearly during the
first period of storage on ice, but the Kvalue at sen-
sory rejection differs between species. An alterna-
tive and simpler Kivalue correlates well with the K
value of wild stock fish landed by traditional meth-
ods (Karube et al. 1984).
BothKvalues reflect fish quality well, provided the
numerical values involved are not compared directly
with the values from other species. StillKvalues nec-
essarily remain less descriptive than the concentra-
tions of the degradation products themselves.
ATP is an important regulator of biochemical pro-
cesses in all animals and continues to be so during
the postmortem processes. One of the most striking
postmortem changes is that of rigor mortis. When
the concentration of ATP is low, the contractile fila-
ments lock into each other and cause the otherwise
soft and elastic muscle tissue to stiffen. Rigor mortis
is thus a direct consequence of ATP depletion. The
biochemical basis and physical aspects of rigor mor-
tis are reviewed by Hultin (1984) and Foegeding et
al. (1996).
Ki[Inosine] + [Hypoxanthine]
[IMP] +=× 100
[[Inosine] + [Hypoxanthine]K=×+
100[][ ]Inosine Hypoxanthine
[ATP] + [ADP] + [AMP] + [IMP]
+ [Inosine] + [Hypoxanthinee]⎛
⎝⎜⎞
⎠⎟ATP depletion and the onset of rigor mortis in fish
are highly correlated with the residual glycogen con-
tent at the time of death and with the rate of ATP
depletion. The onset of rigor mortis may occur only
a few minutes after death or may be postponed for up
to several days when rested harvest techniques, gen-
tle handling, and optimal storage conditions are em-
ployed (Azam et al. 1990, Lowe et al. 1993,
Skjervold et al. 2001). The strength of rigor mortis is
greater when it starts soon after death (Berg et al.
1997). Although the recommended temperature for
chilled storage is generally close to 0°C, in some fish
from tropical and subtropical waters, the degrada-
tion of nucleotides has been found to be slower, and
the onset of rigor mortis to be further delayed when
the storage temperature is elevated to 5, 10, and even
20°C (Saito et al. 1959; Iwamoto et al. 1987, 1991).
Rigor mortis impedes filleting and processing of
fish. In the traditional fishing industry, therefore, fil-
leting is often postponed until rigor mortis has been
resolved. In aquaculture, however, it has been shown
that rested harvesting techniques can postpone rigor
mortis sufficiently to allow prerigor filleting to be
carried out (Skjervold et al. 2001). Fillets from pre-
rigor filleted fish shorten, in accordance with the rig-
or contraction of the muscle fibers, by as much as
8%. Prerigor filleting has few problems with gaping,
since the rigor contraction of the fillet is not hin-
dered by the backbone (Skjervold et al. 2001). Gap-
ing, which represents the formation of fractures be-
tween segments of the fillets, is described further in
the section Postmortem Proteolysis in Fresh Fish.
All enzyme reactions are virtually stopped during
freezing at low temperatures. The ATP content of
frozen prerigor fish can thus be stabilized. During
the subsequent thawing of prerigor fish muscle, the
leakage of Ca^2 from the organelles results in a very
high level of myosin Ca^2 –ATPase activity and a
rapid consumption of ATP. This leads to a strong
form of rigor mortis called thaw rigor. Thaw rigor
results in an increase in drip loss, in flavor changes,
and in a dry and tough texture (Jones 1965) and gap-
ing (Jones 1969). Thaw rigor can be avoided by con-
trolled thawing, holding the frozen products at inter-
mediate freezing temperatures above 20°C for a
period of time (Mcdonald and Jones 1976, Cappeln
et al. 1999). During this holding period, ATP is
degraded at moderate rates, allowing a slow onset of
rigor mortis in the partially frozen state.
Rigor mortis is a temporary condition, even
though in the absence of ATP, the actin-myosin com-