Muscle Glycolysis pH Fall and Temperature

The rate of production of lactic acid from glycogen in anaerobic muscle, which causes the postmortem fall in pH depends on muscle temperature (17). The rate in beef ster-nomandibularis muscle is 0.6 pH units per hour at 40°C and 0.15 pH units per hour at 15°C. As the temperature falls below 15°C, the rate increases; this can be attributed to the increased rate of glycogen used during cold shortening. The rate of muscle glycogen depletion can also be accelerated by preslaughter stunning (18), struggle (19), electrical stimulation by a factor of approximately two (20) (Fig. 3) (these effects are important and are covered by the article Meat and electrical stimulation), and mincing (21). Under practical conditions, the rate of glycolysis slowly decreases due to the fall in muscle temperature in the cooling environments usually employed in chilling of carcasses. The rate of glycolysis also varies with muscle fiber type, being faster in beef m. masseter (0.4 pH units per hour at 35°C), which contains slow-twitch fibers, compared with the fast-twitch m. cutaneous trunci (0.2 pH units per hour at 35°C) (22). After electrical stimulation, the rates do not increase proportionately for particular muscles; the m. cutaneous trunci increases by a factor of two, but there is little change in the m. masseter (22). Differences in rates of glycolysis exist between species, with pigs having considerably faster rates than cattle. The rates of glycolysis are related to the basic metabolic rate of the animal, being very fast in small animals such as rats (23) and poultry. Chicken carcasses go into rigor in approximately four hours even when a fast chilling regime is used during processing (24). 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 (25), 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 eliminated. Electrical stimulation followed by rapid chilling of the birds to 2 to 6°C has been used to achieve rapid processing (see the article Meat and electrical stimulation.)

The process whereby muscle goes into rigor is termed conditioning by New Zealand workers, and subsequent holding periods are termed aging. These terms are used throughout this article. Other countries use the term conditioning, or alternatively aging, for the whole process of going into rigor together with further postmortem holding. In general, the difference in terminology is unimportant, except when processes, such as electrical stimulation, are used that profoundly affect the conditioning stage. In this article, electrical stimulation is considered to produce its effects merely through acceleration of glycolysis to ensure that cold shortening is avoided and aging will start at high temperatures. This is not necessarily true in every instance, see the article Meat and electrical stimulation.

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