Muscle To Meat

The changes that occur when muscle in a living animal becomes meat are initiated from the moment the circulation stops, that is, slaughter. This stoppage of the circulation generally is preceded by preslaughter stunning and sticking. These events can affect meat quality in many different ways because they are extraordinarily variable. Slaughter by throat cutting can occur without a prior stun, can take place after a shooting with a penetrative captive bolt or nonpenetrative percussion head, or can take place after electrical stunning and be followed by electrical immobilization with important implications that are covered later. From the moment the blood supply stops, nutrients and oxygen are no longer available to the muscle from outside sources, so the energy stores present within the muscle start to be used up. Glycogen particles (see following) lying between the myofilaments and at various locations beneath the cell membrane are slowly depleted as the mus cle's energy requirements are maintained, which may continue for a considerable period after slaughter. The depletion of muscle energy stores leads eventually to rigor mortis and a change in status; that is, the muscle is now meat. The pattern and extent of these changes are not the same for every muscle or species of animal and are influenced by a variety of physiological and physical interventions that have a major bearing on the ultimate quality of the meat.

The sliding of the myofilaments relative to each other can occur in muscle from living animals with the contraction initiated by the nervous system, as well as in muscle from animals that have been slaughtered, with the contraction initiated by physical and chemical changes. For example, if the carcasses of freshly slaughtered animals, particularly cattle and sheep, are exposed to cold, and as the normal metabolic arrangements fall down, the leakage of calcium into the cytoplasm (from mitochondria and sarcoplasmic reticulum) can cause irreversible cold contracture (a shortening with no relaxation). This ability to contract can last for many hours after death, depending on postslaughter conditions such as temperature and whether processes such as postmortem electrical stimulation were used.

The theory of muscle contraction based on a sliding of filaments with respect to each other is pivotal to the understanding of changes in muscle and the resulting effects on meat quality. Not only are the contractile filaments important, so too are a recently discovered set of filaments, originally termed gap filaments. These filaments, which contain a protein initially named connectin but now called titin, appear to contribute to the integrity of the myofibril (4,5,8-10) as well as play an important part in the postmortem changes that increase meat tenderness. Other minor proteins such as nebulin have an as-yet-unknown effect on meat tenderness, although desmin is susceptible to proteolysis and is likely to be involved in meat tenderness.

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