ReviewBiochemistry of postmortem muscle — Lessons on mechanisms of meat tenderization
Introduction
From one point of view, the question of meat tenderness is quite simple; it is either tender or it is not. However, the explanation of observed variation in meat tenderness often exposes a process with many different pathways that contribute to the development of meat tenderness. From a physical standpoint, the development of tenderness is dependent on the architecture, the integrity of the skeletal muscle cell, the activity of endogenous proteases within the cell and the extracellular matrix (McCormick, 2009). To make matters more complex, the intracellular environment, the availability and the timing of the availability of metabolites are additional controlling factors that must be considered. In fact, it is in actuality, the interplay between these systems that is the critical factor in determining the rate of tenderization. Given the intricacy of the structure of the muscle cell, coupled with the complexity of the regulation of protease activity, and the ever-changing intracellular environment it is not surprising that this area of research is a very dynamic field, and has been for numerous years. While the extracellular matrix is also critical to tenderness, discussion of it is beyond the scope of this review. Readers are encouraged to read these references to gain a greater appreciation of this area of research (Nishimura, 2010, Purslow, 2005).
It is interesting to that in 1948, Bate-Smith (Bate-Smith, 1948) noted “because of the bewildering rate of growth of this fundamental knowledge (of physiological and biochemical properties and behavior of muscle) and the constantly changing conception of muscle which has resulted, there has not been so striking an advance in knowledge of the particular processes involved in the prolonged storage of meat, nor any striking application of the principles of modern biochemistry to the technology of handling of animals and meat” (Bate-Smith, 1948). Even though the scientific community has continued to make great progress in understanding the physiology and biochemistry of muscle, we are still discovering a bewildering array of factors that can influence meat quality, and still are struggling with the ideal predictors of beef tenderness. Lowe (Lowe, 1948) noted that in poultry, microscopic changes were noted that seemed to parallel changes in tenderness. This work has landmark and has been shown numerous times to be the same in multiple species including beef and point out the importance of understanding muscle structure and its relationship to tenderness.
Section snippets
Muscle structure and metabolism
Muscle cells are among the most highly organized cells in the animal body. This is because they perform a diverse array of mechanical functions. They are required for movement of limbs for locomotion and other gross movements, but also, they must perform finer tasks such as maintaining balance and coordination. Muscle movement and metabolism are also associated with other diverse functions such as aiding in maintaining body heat and movement of blood and lymph. Few cells are required to
The calpain proteinase system
The calpain system is composed of several isoforms of the proteolytic enzyme calpain, and an endogenous inhibitor of the calpains, calpastatin. The two best-characterized isoforms are µ-calpain and m-calpain. These isoforms require the presence of calcium to be active and are named in reference to the amount of calcium each requires for activity. In general, µ-calpain requires between 5 and 65 µM Ca2+ for half-maximal activity, while m-calpain requires between 300 and 1000 µM Ca2+ for
Protein oxidation
Another change that occurs in postmortem muscle during aging of whole muscle products is increased oxidation of myofibrillar proteins (Martinaud et al., 1997, Rowe et al., 2004a, Rowe et al., 2004b). This results in the conversion of some amino acid residues, including histidine, to carbonyl derivatives (Levine et al., 1994, Martinaud et al., 1997) and can cause the formation of intra and/or inter protein disulfide cross-links (Martinaud et al., 1997, Stadtman, 1990). In general, both of these
S-Nitrosylation
Nitric oxide is produced in biological systems by a family of enzymes known as nitric oxide synthases (NOS). There are three major isoforms of NOS, neural, inducible and endothelial. Skeletal muscle expresses all three isoforms, however, the neural form, nNOS is thought to be the predominant isoform (Kaminski & Andrade, 2001). Nitric oxide is important in biological systems, particularly because of its role as a second messenger. However, while nitric oxide rapidly diffuses through tissues,
Major postmortem changes in muscle architecture
A great deal of emphasis is placed on enzymes, however, the direct contribution of structural proteins should be considered. Certainly, as muscle goes into rigor, there is a loss of extensibility and along with that, a change in the texture of the meat. During storage, however, that product becomes more tender because of proteolytic changes occurring in the architecture of the myofibril and its associated proteins. There are several key proteins that are modified during postmortem aging.
Summary
Just as the overall integrity and function of muscle cells does not depend on a single protein, the structural weakening of muscle cells postmortem also must not depend on the modification of a single myofibrillar or other cytoskeletal protein. Postmortem degradation and oxidation of proteins can both have significant effects on the integrity of the myofibril and the entire muscle fiber. The proteins discussed in this review are located in different regions of the muscle cell, and most have
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