Note: One side of each animal was chilled in 2°C air at 0.3 m/s, the other in 7°C air at 0.3 m/s. The lower the shear force valuee, the more tender the meat. Values above 11 kg F indicate unacceptable toughness. These results are for a MIRINZ tenderometer and correspond to a Warner Bratzler ten-derometer if multiplied by 0.7 (67).

Note: One side of each animal was chilled in 2°C air at 0.3 m/s, the other in 7°C air at 0.3 m/s. The lower the shear force valuee, the more tender the meat. Values above 11 kg F indicate unacceptable toughness. These results are for a MIRINZ tenderometer and correspond to a Warner Bratzler ten-derometer if multiplied by 0.7 (67).

carcasses in a chiller for long durations to age to achieve the same desired tenderness of stimulated carcasses that occurs much earlier; thus, stimulation can be a cost-effective process.

The degree of tenderness improvement that researchers attribute to stimulation varies because of the widely different methods of assessing tenderness and the innumerable processing variations; comparison of results is, therefore, difficult.

Stimulation does not always result in tenderization, but neither does it in itself toughen meat. Studies showed that electrical stimulation of stressed animals caused a slight toughening of the m. longissimus in beef and sheep (68,69), although it was hypothesized (68,69) that the toughening was a consequence of muscles going into rigor during stimulation and remaining contracted. It has been suggested (70) that under the conditions present, the rapid reduction of the time that muscle pH is high reduces the aging that occurs, but additionally the lower tenderizing rate of intermediate pH meat may be important (71).

Electrical stimulation is also claimed to improve the color of lean and influence beef grading under the U.S. system (20,67,72). One study found no effect of low-voltage electrical stimulation on meat color as measured by reflectance but did find that postmortem aging significantly reduced reflectance values (73).

Comparisons of low- and high-voltage stimulation under comparable conditions have shown that if environmental conditions are such that cold shortening is unlikely, low- and high-voltage stimulation both result in an increase in tenderness (74), and both processes improve the youthful appearance and marbling scores of beef (75). Under conditions where chilling rates are more rapid, low-voltage stimulation does not give the same degree of protection against cold shortening as high-voltage stimulation (67). The interaction between three moderate chilling conditions and two electrical stimulation treatments were examined (76); it was shown that the temperature regime to which the muscles were exposed became more critical when low-voltage stimulation was used. When all things are considered, the use of electrical stimulation must be seen as part of a total process. The tuning of the amount of stimulation with chilling rate to reach rigor at 15°C results in optimum tenderization (Fig. 7) (36).

Not all muscles of a carcass are equally affected by stimulation. One study (11) showed that although the beef striploin and rump improved in tenderness, there was no change in the topside. Similarly a dramatic improvement in the tenderness of beef m. longissimus and m. semimembranosus was shown but not of the m. triceps brachii and serrates ventralis after high- or low-voltage stimulation. Some of the difference has been attributed to uneven distribution of the current during stimulation (59), but obviously some of the difference must be biochemically and physiologically based.

Because of the rapid fall in pH of pork and the tendency for PSE conditions of high temperatures and low pH values to be present, it was considered that there would be no advantage in stimulation. However, it has been shown that if electrical stimulation is applied at 20 min postslaughter and the chilling rate is sufficiently fast under ideal commercial conditions (eg, deep leg to 10°C in 5-10 h), then the meat in those animals susceptible to PSE does not show it and the remaining meat avoids cold shortening (15). There is an improvement in tenderness and drip loss with pelvic suspension (77). Further work has shown that there appears to be no advantages with electrical stimulation and slow chilling (77).

Cold shortening in poultry does not seem to cause the same toughness problems as in other species, as the maximum shortening occurs around 2°C (79), which is generally lower than the temperatures the birds reach following chilling by ice water and air chilling. However, rapid processing would be advantageous because the toughness that arises from portioning the birds early and removing the breast muscles in particular could be avoided. A stimulation system, part of the minimum time process system, was developed and patented by Campbell Institute for Research and Technology (80). The process proved ideal for further process applications with the meat at 3 h being the same as that of normal processing at 24 h, although meat was slightly tougher mainly because longer aging still would be beneficial. There are problems if the birds are either not chilled rapidly enough (81,82), or chilled rapidly and held at low temperatures (83), when it appears that there is no advantage in stimulation. If early boning is desirable, then stimulation is necessary. In a system developed and now in use in New Zealand with spin-chilling, when rigor is achieved at approximately 6°C, the breast muscles of such birds were able to be boned out in just over an hour poststun without any loss of tenderness (84). (D. J. C. Wild, C. E. Devine, and H. Reed, unpublished observations, 1988).

It is important to consider all aspects of stimulation and processing comparing results. It is almost impossible to compare results from one laboratory with those from another, because of the many variables introduced during processing. Not only are stimulation parameters different but also the processing conditions and the methods of evaluation can sometimes introduce artifacts; for example, freeing a normally restrained muscle can allow shortening, which can mask any other changes. Various stimulation treatments cause different degrees of internal heating and will produce different degrees of mechanical damage; for example, 2 Hz is claimed to cause no internal damage (42), whereas 60 Hz causes much more damage (12). Muscle

Stimulation at 30 min co QQ

Stimulation at 30 min

Stimulation at 2 min

Time postmortem (h)

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