382 Part III: Muscle Foods
(EC 2.4.1.1), producing glucose-1-phosphate. The
highly regulated glycogen phosphorylase is inhibit-
ed by glucose-1-phosphate and glucose and is acti-
vated by AMP. Glucose-1-phosphate is subsequently
transformed into glucose-6-phosphate by phospho-
glucomutase (EC 5.4.2.2). Glucose-6-phosphate is
further degraded by the glycolysis, leading to the
formation of ATP, pyruvate, and NADH. NADH
originates from the reduction of NADduring gly-
colysis. In the absence of oxygen, NADis regener-
ated by lactate dehydrogenase (EC 1.1.1.27), as
pyruvate is reduced to lactate. The formation of lac-
tate forms a metabolic blind end and causes pH to
fall, the final pH level being reached when the for-
mation of lactate has stopped. The postmortem con-
centration of lactate reflects, by and large, the total
degradation of ATP. Since in living fish, however,
removal of lactate from the large white muscle often
proceeds only slowly, most lactate formed just be-
fore death may remain in the muscle postmortem.
Thus, the final pH level in the fish is affected only a
little by the stress to which the fish was exposed and
the struggle it went through during capture (Foe-
geding et al. 1996).
The accumulation of lactate in fish is limited by
the amount of glycogen in the muscle. Since glyco-
gen is present in fish muscle at only low concentra-
tions, the final pH level of the fish meat is high in
comparison to that of other types of meat, typically
6.5–6.7. In species that have more muscular glyco-
gen, this substance rarely limits the formation of lac-
tate, which may continue until the catalysis stops for
other reasons, at a pH of approximately 5.5–5.6 in
beef (Hultin 1984). The final pH depends not only
on the lactate concentration, but also on the pH buf-
fer capacity of the tissue. In the neutral and slightly
acidic pH range, the buffer capacity of the muscle
tissue in most animals, including fish, originates pri-
marily from histidyl amino acid residues and phos-
phate groups (Somero 1981, Okuma and Abe 1992).
Some fish species, in order to decrease muscular aci-
dosis, have evolved particularly high pH buffer
capacities. For example, pelagic fish belonging to
the tuna family (genus Thunnus) possess a very high
glycolytic capacity (Storey 1992, and references
therein) and also accumulate high concentrations of
histidine and anserine (N-(-alanyl)-N-methylhisti-
dine) (van Waarde 1988, Dickson 1995). Since tuna
and a few other fish species can effectively retain
metabolic heat, their core temperature at the time of
capture may significantly exceed the water tempera-
ture. The high muscle temperature of such species
expectedly accelerates autolytic processes, including
proteolysis, and may, in combination with the low-
ering of pH, promote protein denaturation (Haard
2002).
Some species of carp, including goldfish, are able
to survive for several months without oxygen. The
metabolic adaptation this requires includes substi-
tuting ethanol for lactate as the major end product of
glycolysis. Ethanol has the advantage over lactate
that it can be easily excreted by the gills and does
not lead to acidosis (Shoubridge and Hochachka
1980). Under anoxic conditions, NADis thus re-
generated by muscular alcohol dehydrogenase (EC
1.1.1.1).POSTMORTEMDEGRADATION OF
NUCLEOTIDESATP can be metabolized by various ATPases. In
muscle tissues, most of the ATPase activity is re-
presented by Ca^2 -dependent myosin ATPase (EC
3.6.1.32) that is directly involved in contraction of
the myofibrils. Thus, release of Ca^2 from its con-
tainment in the sarcoplasmic reticulum dramatically
increases the rate of ATP breakdown.
The glycogen consumption during and after
slaughter is highly dependent on the fishing tech-
nique and the slaughtering process employed. In
relaxed muscle cells the sarcoplasmic Ca^2 levels
are low, but upon activation by the neuromuscular
junctions, Ca^2 is released from the sarcoplasmic
reticulum, activating myosin ATPase. Since the
muscular work performed during capture is often
intensive, the muscular glycogen stores may be al-
most depleted even before slaughtering takes place.
The activation of myosin ATPase by spinally medi-
ated reflexes continues after slaughter, but the extent
to which it occurs can be reduced by employing
rested harvest techniques such as anesthetization
(Jerrett and Holland 1998, Robb et al. 2000); by
destroying the nervous system by use of appropriate
methods such as iki jime,where the brain is de-
stroyed by a spike (Lowe 1993); or by use of still
other techniques (Chiba et al. 1991, Berg et al.
1997).
ATPases hydrolyze the terminal phosphate ester
bond, forming ADP. ADP is subsequently degraded
by adenylate kinase, forming AMP. The degradation