Food Biochemistry and Food Processing

300 Part III: Muscle Foods

activity and cell respiration (Pearson 1987). Under
normal aerobic values (see an example of resting
muscle in Fig. 13.3), the muscle is able to produce
12 moles of ATP per mole of glucose, and thus the
ATP content is kept around 5–8 mol/g of muscle
(Greaser 1986). ATP constitutes the main source of
energy for the contraction and relaxation of the mus-
cle structures as well as other biochemical reactions
in postmortem muscle. As the redox potential is
reduced towards anaerobic values, ATP generation
is more costly. So only 2 moles of ATP are produced
per mole of glucose under anaerobic conditions (an
example of a stressed muscle is shown in Fig. 13.3).
The extent of anaerobic glycolysis depends on the
reserves of glycogen in the muscle (Greaser 1986).
Glycogen is converted to dextrines, maltose, and
finally, glucose through a phosphorolytic pathway;
glucose is then converted into lactic acid with the

synthesis of 2 moles of ATP (Eskin 1990). Ad-

ditionally, the enzyme creatin kinase may generate

some additional ATP from ADP and creatine phos-

phate at very early postmortem times, but only while

creatin phosphate remains. The main steps in glycol-

ysis are schematized in Figure 13.4.

The generation of ATP is strictly necessary in the

muscle to supply the required energy for muscle

contraction and relaxation and to drive the Na/K

pump of the membranes and the calcium pump in

the sarcoplasmic reticulum. The initial situation in

postmortem muscle is rather similar to that in the

stressed muscle, but with an important change: the

absence of blood circulation. Thus, there is a lack of

nutrient supply and waste removal (see Fig. 13.5).

Initially, the ATP content in postmortem muscle

does not drop substantially because some ATP may

be formed from creatin phosphate through the action

Figure 13.2.Summary of main changes during conversion of muscle to meat.