302 Part III: Muscle Foods
of the enzyme creatine kinase and through anaerobic
glycolysis. As mentioned above, once creatine phos-
phate and glycogen are exhausted, ATP drops within
a few hours to negligible values by conversion into
ADP, AMP, and other derived compounds such
as 5-inosine-monophosphate (IMP), 5-guanosine-
monophosphate (GMP), and inosine (see Fig. 13.6).
An example of the typical content of ATP break-
down products in pork at 2 and 24 hours postmortem
is shown in Table 13.6. The reaction rates depend on
the metabolic status of the animal prior to slaughter.
For instance, reactions proceed very quickly in pale,
soft, exudative (PSE) muscle, where ATP can be
almost fully depleted within a few minutes. The rate
is also affected by the pH and temperature of the
meat (Batlle et al. 2000, 2001). For instance, the
ATP content in beef Sternomandibulariskept at 10–
15°C is around 5 mol/g at 1.5 hours postmortem
and decreases to 3.5mol/g at 8–9 hours postmor-
tem. However, when that muscle is kept at 38°C, ATP
content is below 0.5mol/g at 6–7 hours postmortem.
Once the ATP concentration is exhausted, the
muscle remains contracted, as no more energy is
available for relaxation. The muscle develops a rigid
condition known as rigor mortis, in which the cross-
bridge of myosin and actin remains locked, forming
actomyosin (Greaser 1986). The postmortem time
necessary for the development of rigor mortis is
variable, depending on the animal species, size of
carcass, amount of fat cover, and environmental
conditions such as the temperature of the chilling
tunnel and the air velocity (see pork pieces after cut-
ting in a slaughterhouse in Fig. 13.7). The rates of
enzymatic reactions are strongly affected by temper-
ature. In this sense, the carcass cooling rate will
affect glycolysis rate, pH drop rate, and the time
course of rigor onset (Faustman 1994). The animal
species and size of carcass have a great influence on
the cooling rate of the carcass. Furthermore, the
location in the carcass is also important because sur-
face muscles cool more rapidly than deep muscles
(Greaser 2001). So, when carcasses are kept at
15°C, the time required for rigor mortis develop-
ment may be about 2–4 hours in poultry, 4–18 hours
in pork, and 10–24 hours in beef.
Muscle glycolytic enzymes hydrolyze the glucose
to lactic acid, which is accumulated in the muscle
because muscle waste substances cannot be elimi-
nated due to the absence of blood circulation. ThisFigure 13.5.Scheme of energy generation in post-
mortem muscle.
Table 13.6.Example of Nucleotide and Nucleoside Content in Pork
Postmortem Muscle at 2 and 24 Hours2 h Postmortem 24 h Postmortem
Compound (mol/g muscle) (mol/g muscle)
ATP 4.39 —
ADP 1.08 0.25
AMP 0.14 0.20
ITPGTP 0.18 —
IMP 0.62 6.80
Inosine 0.15 1.30
Hypoxanthine 0.05 0.32
Source:From Batlle et al. 2001.