Meat Tendernesstoughness

Meat tenderness has been regarded by many (23) as the prime determinant of consumer satisfaction with meat purchases. Although the ultimate measure of tenderness (or its converse, toughness) lies with the consumer, objective assessments can be made with a wide variety of mechanical devices, because these devices can reliably indicate differences attributable to animal and processing factors. Tenderness is the term used to describe the assessment by consumers (high scores being desirable), and the force to shear the meat, that is, shear force, gives an indication of the toughness (low values being desirable). The factors influencing meat quality, in particular tenderness, are covered the article Meat science.

Meat tenderness-toughness depends on both the myofibrillar strength and the connective tissue strength. Electrical stimulation seems mainly to modify the myofibrillar strength, although some work (24) suggests that electrical stimulation could also have an impact on the connective tissue component of meat tenderness. Most studies of electrical stimulation have been concerned with myofibrillar toughness as affected by cold shortening as muscle goes into rigor (termed conditioning). The basic tenets of much of meat science come from work that relates the degree of muscle shortening during the early postmortem period to the tenderness of cooked meat (25). The force required to shear a sample of cooked muscle across the grain rises steeply as the degree of muscle shortening exceeds 20%, reaching a peak at 40% shortening (see Meat science). The shear force of a sample shortened by 40% may be up to four times that of the nonshortened muscles.

If rigor (ie, conditioned) muscle is subsequently aged before cooking, it usually becomes appreciably more tender; however, the degree of tenderization is drastically reduced if the muscle has shortened during the conditioning (prerigor) phase (see Meat science). Even with extensive aging, there seems to be a limit beyond which any given muscle cannot be tenderized without loss of all structure. This background toughness is largely a result of the structure and composition of the connective tissues within the muscle. Aging reduces the myofibrillar strength (26,27) but is generally considered to have little effect on connective tissue, although some work questions this view (24).

Meat aging refers primarily to the tenderness improvement that takes place while postmortem muscle is held above freezing temperatures. Although there is no clear evidence as to when aging commences, much past evidence suggests that most of the aging commences with completion of rigor mortis (28). Other work (29) suggests that aging changes may commence prior to rigor completion, but they are relatively small. The changes in intramuscular connective tissue that have been noted (24) can be considered as aging changes.

The ultrastructural changes that occur during aging consist of a disappearance of Z disks and a disruption of the myofibrils at the junction of the I bands and Z disks. The ultrastructural changes are accompanied by parallel biochemical changes revealed by changes in the extracta-bility of myofibrillar protein (30), changes to myofibrillar ATPase activity, arid breakdown of myofibrillar proteins, for example, loss of troponin T (31,32). Proteinases termed calpains, with the inhibitor calpastatin, are considered to have a role (33). The ability of electrical stimulation to influence aging is disputed. However, it has been recently shown that rigor temperature affects tenderization with the optimum being 15°C (34), and this needs to be considered for optimum tenderization (Fig. 4) (36). Low level exercise affects nitric oxide (NO) in muscle (37), and recent work has shown that NO affects meat tenderizing, suggesting that electrical stimulation can have a similar role (38). Further work is still required to establish the effect of electrical stimulation on aging. The accelerated devel

Figure 3. Diagrams showing electrode configuration used for high-voltage stimulation of sheep carcasses (only one is shown) and a novel chain electrode arrangement for beef sides. Stimulation area is enclosed in both situations and personnel entry controlled by floor switches and light beams. Source: Drawings by P. Hanara.

Ground rail ^

Ground rail ^

Live electrode

Chain electrode

Chain electrode opment of rigor mortis means that even if aging does not start until after rigor is achieved, aging will commence at higher temperatures and, therefore, be more rapid (39).

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