L

-100 -80 -60 -40 -20 0 20 40 60 Stretch Shortening

Percentage shortening

Figure 5. Influence of prerigor muscle shortening on the force score values of unaged and aged beef m. sternomandibularis. rigor muscle; O, muscle aged for three days at 15°C after setting in rigor at different degrees of shortening.7 At shortenings close to rest, lengths the meat can tenderize, but as the shortenings increase, tenderization becomes less. The tenderization that occurs after the peak is as a result of shortenings being so large, the muscle tears itself apart. Source: Ref. 27, used with permission.

temperature on shortening and ATP utilization has been made (28).

Although shortening at temperatures higher than 15°C occurs, it is not strong and can be counteracted at these temperatures by postural alteration (29). With processing temperatures normally experienced, extreme toughness of the cooked muscle does not seem to be a problem except in poultry. Therefore, for most meat species, it seems likely that enzymes involved in postrigor proteolytic breakdown are more effective at higher temperatures and counteract any toughness due to shortening, although there are some problems with rigor at high temperatures where tenderizing enzymes are reduced (see "Variability in Tenderness"). The apparent anomalous shortening at temperatures below 15°C, which increases with colder temperatures, applies to most muscles and is termed cold shortening. Cold shortening is not a theoretical issue; it is a practical problem that results in tough meat, especially when carcasses are subjected to the rapid rates of chilling or freezing that currently take place in meat processing operations. When carcasses are treated in this way, and muscle temperatures fall below 10°C before the pH falls below pH 6.0, the muscles can shorten markedly. With extremes of shortening (arising from cold shortening, thaw shortening, or heat from prerigor cooking) the muscle fibers will supercontract in some regions with a consequent disruption in adjacent regions, leading to a tenderization (30) that corresponds to the tenderness beyond the peak of toughness in Figure 5.

Shortening can be partially prevented by hanging carcasses by the aitch bone (31), termed tenderstretch, or by other maneuvers of the carcass (29), or by electrical stimulation to accelerate rigor and to shorten possible exposure times to low temperatures while shortening can occur. If meat is frozen before rigor mortis is complete, the energy stores ensure that contraction will take place as the meat thaws, that is, thaw shortening (6,32). There are likely to be other as-yet-unknown effects that take place as muscles go into rigor. For example, even cold-shortened muscle becomes somewhat tender after being held at 37°C (33), and meat subjected to extremes of pressure will become tender (34). Even explosives will cause this (35).

The effects of cold shortening are greater when the low temperatures are attained soon after slaughter and seem to progressively disappear at a muscle pH between 6.2 and 6.0. Animals with a high ultimate pH generally reach rigor before shortening is manifest.

Cold shortening does not occur in all muscles of all species under the same conditions of temperature, and perhaps this is the underlying reason for its relatively late discovery. With the masseter, which consists of red (slow-twitch Type I) fibers, cold shortening readily occurs, yet in the cutaneous trunci (fast-twitch Type II A) cold shortening does not occur until muscle temperature reaches 0°C (22). Cold shortening is a problem with sheep and cattle carcasses, but not so much with pigs and poultry. With pig muscle, the rate of rigor is sufficiently fast and the tendency to shortening appears to be less in any event, so cold shortening does not appear to be a problem unless extremely high rates of cooling are involved. With poultry, cold shortening is also not a problem, as maximum shortening occurs at around 0°C and rigor occurs in approximately five hours. However, heat shortening at temperatures above 20°C can occur (25), and meat toughening occurs with stressed birds going into rigor early at high temperatures.

Prerigor muscle excised from the bone is free to shorten large amounts, the amount depending on the restraint of adjoining muscle and connective tissue and its temperature, and electrical stimulation is beneficial. If there is enough shortening, the meat will become tough. The practice of hot boning, therefore, has to be examined with respect to the end use of the meat. If the end use is for manufacturing processed meats, muscle shortening may not be a problem. Cold shortening does not cause changes in water-holding capacity (13), but thaw shortening changes the water-holding capacity, with some disadvantages in further processing. Muscle shortening, and hence toughening of the meat, cannot be predicted by meat appearance. Electrical stimulation can reduce the toughening of meat associated with hot boning for certain cuts (36).

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