Rigor mortis is the process of cell death.3,4When an animal dies, its individual cells remain alive, continuing their metabolism by using energy stored in them. With the loss of blood as an oxygen supply, the cells gradually shift from aerobic (oxygen-dependent) metabolism to anaerobic (oxygen-independent) metabolism. They continue to use energy but make it more slowly because anaerobic metabolism is less efficient than aerobic. This imbalance causes the cell’s supply of the primary energy compound, adenosine triphosphate (ATP),
to decrease. The production of lactic acid, the end product of this form of metabolism also occurs with the increased anaerobic activity. While lactic acid would be removed by the blood in the living animal, this compound accumulates in the muscle cells of dead animals and causes the cell pH to decrease from near neutrality (7) to a more acidic pH of about 5.7. This decline in pH reduces the activity of some of the ATP-producing enzymes, further reducing the production of ATP (Figure 4.2). The pH reduction during rigor mortis deve- lopment affects protein functionality and further processed products, as described in sub- sequent chapters.
Adenosine triphosphate is an important compound in the function of a muscle cell because it not only provides energy for many reactions, but also helps regulate the interac- tions of the protein fibers involved in contraction. A muscle consists of overlapping protein filaments, thick filaments made of a protein called myosin, and thinner filaments made of a protein called actin (Figure 4.3). These filaments are part of a repeating structure called a sarcomere, which serves as the basic contractile unit of the muscle. One end of each thin filament is anchored in a structure called a “Z-disc” or “Z-line” at one end of the sarcomere, and the other partially overlaps one end of some thick filaments in the middle of the sar- comere. The other end of each thick filament overlaps the thin filaments at the other end of the sarcomere.
When a nerve signal reaches the muscle, it signals the release of calcium from storage vesicles into the fluid surrounding the filaments (Figure 4.3). In the presence of ATP, these calcium ions trigger the ATP to form a bridge between the thin and thick filaments. The ATP molecule then releases its energy, providing the fuel to pull the thin filaments and the ends of the sarcomere (to which they are attached) together. A new ATP molecule is then
needed to break the bond between the filaments and allow the muscle to relax to its origi- nal length. So, ATP causes contraction by providing energy and relaxation by breaking the bond between contracted thick and thin filaments. The minimum concentration for ATP to function in these roles is about 1 M ATP/g muscle (Figure 4.2).3Therefore, when a mus- cle cell’s ATP concentration falls below this level, it is no longer responsive to nervous or other stimuli and is in rigor mortis.
Cutting and deboning the muscle before rigor mortis is developed will cause a nervous signal response in the muscle and cause it to contract. Furthermore, the extent of the mus- cle’s contraction is no longer limited by skeletal restraints, so the degree of shortening is greater for the free muscle. Additionally, when the muscle is removed from the carcass, it cools more rapidly because it no longer has the insulating skin cover and surrounding mus- cles. When muscles chill rapidly, the calcium storage vesicles leak. If this happens early enough after death, there can be sufficient ATP still present to initiate contraction and sar- comere shortening, a process called “cold shortening.”4Overlap of the contractile filaments is important to toughness because meat with more overlap (shorter sarcomeres) is more dense and has more filaments per cross-sectional area for teeth to cut through during bit- ing. Also, shorter sarcomeres have less fluid space in them, and therefore less fluid.5Less fluid means less juiciness, a characteristic that contributes to the toughness sensation. Deboning is not the only stimulus that can induce shortening and toughness in pre-rigor muscle. Cooking meat before rigor mortis is developed will also induce toughness.
The toughness due to the overlapping contractile filaments (“contractile toughness”) is not to be confused with toughness of meat from older animals. This other toughness is pri- marily due to cross-linking of the connective tissue protein, collagen. In young animals, the Figure 4.3 Muscle diagram showing sarcomere, filaments, and roles of calcium and ATP in cross bridge formation between the filaments.
collagen is not cross-linked and therefore is not stable at heat, and melts during cooking. Collagen in meat from younger animals provides very little contribution to toughness. However, as an animal gets older, the collagen forms heat-stable crosslinks with itself and other collagen molecules, forming a heat resistant network that does not melt during cook- ing.4,6,7This network makes the meat from older animals tough, regardless of its state of rigor mortis. This collagen network only breaks down with prolonged cooking in a moist heat, the reason stewing hens need just such a cooking method to produce acceptable meat.