Conversion of muscle to meat | rigor mortis, cold and rigor shortening

Rigor is a major post-mortem event that significantly influences the quality of meat. Rigor mortis refers to the irreversible formation of cross-bridged actomyosin complexes in muscle. The muscle structure is composed of muscle fibers, connective tissue and intramuscular fat. In a living muscle, the relaxation and contraction of the myofibrillar proteins myosin and actin are powered by adenosine triphosphate (ATP) and these two proteins are kept separate from each other in the presence of the ATP. ATP continues to be generated immediately post-slaughter via anaerobic metabolism via glycolysis. The production of ATP ceases when the glycogen stores in each muscle of a carcass are exhausted or the enzymes involved in the glycolysis are inhibited. Ca2+ is released from the sarcoplasmic reticulum and rigor occurs sequentially in each fiber, with ATP bound to myosin resulting in a rigor contracture that is greatest at high temperatures—termed rigor shortening (or heat shortening). When the temperature falls below 12–15 °C, calcium pumps on the sarcoplasmic reticulum become less effective so that Ca2+ ions leak causing a contracture—termed cold shortening. The rate of ATP production, pH fall, and time of rigor are dependent on various factors, including species, muscle type, pre-slaughter feeding and stress, and carcass chilling. Meat from a shortened muscle is tough. The sensory quality of meat from cold- or heat-shortened muscles can be improved with various post-mortem interventions, including efficient electrical stimulation, stretching and aging.