Carcass tenderness Shear force values measured over a 3-d aging period are presented in Figure 5. Shear force values in SC group were lower (P≤ 0.03) but no difference was observed between RC and CC (P≥ 0.16) at 12 h (7.47 kg, 7.05 kg, and 6.25 kg, respectively) and 24 h (6.14 kg, 5.84 kg, and 4.89 kg, respectively) postmortem. However, at 48-h postmortem, shear force values in the CC group were lower (P = 0.02) than those in the RC group (5.18 kg and 6.01 kg, respectively). Shear force values of 12-h aged LD were greater than those with longer aging periods. Higher shear force values of meat from early postmortem aging stage could be due to the postmortem contraction of the myofibrillar proteins. Following aging for 2 and 3 d, shear force values of the cooked pork decreased (meat became more tender), presumably as a result of breakdown of muscle myofibrillar structures caused by proteolytic enzyme activity and/or ice crystal formation. However, after 72 h, the mean shear force values of meat from RC and CC carcasses were higher (P= 0.001) than those from SC carcasses (4.71 kg, 4.49 kg, and 3.50 kg, respectively). Relatively high shear force values with RC samples further support previous published results (Gigiel and James 1984; McGeehin and others 2002; Hannula and Puolanne 2004), in which RC was considered to be one of the most important causes of tough meat. Our discovery is also in accordance with the conclusion drawn by Howard and Lawrie (1956) who found that the rate of pH decline was inversely related to meat tenderness. On the contrary, Jones and others (1993) found no significant difference in the shear force value of semimembranosus (SM) muscle applying higher chilling rates (3 h at –40 °C). Dransfield and others (1991) also found no significant increase in toughness of SM muscles in RC system (–15 °C for 3 h, followed by 0∼4 °C).
According to Locker and Hagyard (1963), lowering the muscle temperature below 10 °C within 10-h postmortem will cause cold-induced shortening. The results also indicate that application of RC for pork processing will take the risk of producing tough meat caused by cold shortening.
The work of Koohmaraie and others (1987) showed that at slaughter, all meat with the same pre-slaughter treatments had the same tenderness level, and that differences in tenderness were created in the 1st 24-h postmortem. The rate and extent of postmortem muscle pH and temperature decline can significantly affect meat tenderness (Rees and others 2003).
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Figure 5–. Effects of different chilling methods on Warner-Bratzler shear force of longissimus dorsi muscle (means ± SD, n= 6).