Muscle Energy Supply and Changes During Rigor

In a living muscle, or a muscle just at animal death, the immediate source of energy for contraction comes from ATP. Muscle ATP is quickly used up in two to five contractions, whereupon creatine phosphate immediately becomes dominant. Upon depletion of this source, glycogen takes over. Depending on the intensity and duration of muscle activity, and on the presence of blood circulation, either anaerobic or aerobic glycolysis occurs to sustain activity. Although protein or fat can be useful energy sources in sustained aerobic activity, glycogen is the only fuel that can be used for anaerobic activity. Therefore, glycogen is the only available energy source in postmortem muscle. It is metabolized when the muscle is triggered to contract via physical effects such as cold, and it is also metabolized slowly in noncontracting muscle. Without an intact circulation, lactic acid, a by-product of anaerobic glycolysis, accumulates in the tissue, causing the pH to fall (for reviews, see Ref. 6). At death, muscle tissue from rested, unstressed well-fed animals tends to contain 80 to 90 /┬┐mol of glycogen per gram of muscle tissue, but this is highly variable depending on muscle type and animal species. This glycogen allows the muscle to "survive" and contract until the pH falls from around 7.1 to below 6.0 to 6.2 (depending on animal species) and will still provide energy until the muscle goes into rigor at a pH of about 5.5 (Fig. 3) (11).

Immediately after slaughter, the muscle has an initial pH of 7.1 and is floppy and extensible. Upon attainment of rigor, the muscle is inextensible, and the pH has fallen to approximately 5.5. These changes have been explored in detail elsewhere (6,12). If the glycogen stores are less than optimum, the muscle pH cannot fall to the same degree. The consequences of this on meat quality are discussed in the section "Animal Factors Affecting Meat Quality."

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