Effect of refrigeration on texture of meat 49 flooded the microfilaments, thus separating them from each other once more and enabling them to slide freely over each other in response to any externally applied force Two features of the calcium-pumping mechanism are of special impor- tance in the present context. First, it is likely that the calcium storage vesi- cles are somewhat leaky, even in resting muscle, so that the calcium pump has to operate continuously, albeit slowly, to keep the intrafibrillar Ca concentration at its low resting level. Second, the calcium pump has an extremely high temperature coefficient, so that at 10"C it works at 1/200th and at 2C at only 1/1000th of the rate at the body temperature of about 38C (Bendall, 1974). Passive diffusion(leakage) out of the pump would only be reduced at 10C to about half the value at 38C. Thus, there is ncreasing chance of net Ca- leakage into the myofilaments as the tem perature falls, the effect becoming dramatic below 10C. Such leakage stimulates the contractile ATP-ase, bringing about the shortening charac- teristic of cold shortening and increasing the production of ADP. The latter in its turn would then stimulate the reactions of ATP synthesis mentioned earlier, so that the timescale in Fig 3. 1 would become shorter and shorter the lower the temperature. This explains the anomalous temperature dependence of the time for half change of ATP, shown in Fig. 3.2 The contracture, which occurs when a rapidly frozen muscle is thawed resembles cold contracture in that it sets in while the level of contractile fuel(ATP) is still high. However, it differs because the amount of work done and force developed are much higher. Withthaw shortening the tem- perature is raised through the calcium release'danger zone from 0 to 10C, whereas in cold shortening it is reduced through this zone. The rate of contracture depends entirely on the rate of thawing. Rapid thawing of a reely suspended, unloaded muscle strip causes very dramatic shortening. often to less than 40% of the frozen'length 3.1.2 Preventing shortening Rapid chilling has many practical advantages but increases the danger of cold shortening. As discussed in Chapter 2 the breakdown of glycogen lactic acid occurs at different speeds in different species. In lamb and beef, the rate is low and the pH falls slowly. Hence, it is only too easy to cool car- casses of these animals, at least on the surface, below 10C when the pH is above 6.2 and such carcasses are extremely vulnerable to cold shortening In pork, the rate of breakdown of glycogen is more rapid and under noderate chilling regimes, cold shortening will not occur. However, pig muscle can cold shorten, and with fast chilling, for example using sub-zero air temperatures, cold shortening has been clearly demonstrated. Another point that should be made is that at an early stage, the surface of the carcass will reach the same temperature as that of the air. Since the air temperature used in chilling is commonly below 10C, there exists theflooded the microfilaments, thus separating them from each other once more and enabling them to slide freely over each other in response to any externally applied force. Two features of the calcium-pumping mechanism are of special impor￾tance in the present context. First, it is likely that the calcium storage vesi￾cles are somewhat leaky, even in resting muscle, so that the calcium pump has to operate continuously, albeit slowly, to keep the intrafibrillar Ca2+ concentration at its low resting level. Second, the calcium pump has an extremely high temperature coefficient, so that at 10 °C it works at 1/200th and at 2 °C at only 1/1000th of the rate at the body temperature of about 38 °C (Bendall, 1974). Passive diffusion (leakage) out of the pump would only be reduced at 10 °C to about half the value at 38°C. Thus, there is an increasing chance of net Ca2+ leakage into the myofilaments as the tem￾perature falls, the effect becoming dramatic below 10 °C. Such leakage stimulates the contractile ATP-ase, bringing about the shortening charac￾teristic of cold shortening and increasing the production of ADP. The latter in its turn would then stimulate the reactions of ATP synthesis mentioned earlier, so that the timescale in Fig. 3.1 would become shorter and shorter the lower the temperature. This explains the anomalous temperature dependence of the time for half change of ATP, shown in Fig. 3.2. The contracture, which occurs when a rapidly frozen muscle is thawed, resembles cold contracture in that it sets in while the level of contractile fuel (ATP) is still high. However, it differs because the amount of work done and force developed are much higher.With ‘thaw shortening’ the tem￾perature is raised through the ‘calcium release’ danger zone from 0 to 10 °C, whereas in cold shortening it is reduced through this zone. The rate of contracture depends entirely on the rate of thawing. Rapid thawing of a freely suspended, unloaded muscle strip causes very dramatic shortening, often to less than 40% of the ‘frozen’ length. 3.1.2 Preventing shortening Rapid chilling has many practical advantages but increases the danger of cold shortening. As discussed in Chapter 2 the breakdown of glycogen to lactic acid occurs at different speeds in different species. In lamb and beef, the rate is low and the pH falls slowly. Hence, it is only too easy to cool car￾casses of these animals, at least on the surface, below 10 °C when the pH is above 6.2 and such carcasses are extremely vulnerable to cold shortening. In pork, the rate of breakdown of glycogen is more rapid and under moderate chilling regimes, cold shortening will not occur. However, pig muscle can cold shorten, and with fast chilling, for example using sub-zero air temperatures, cold shortening has been clearly demonstrated. Another point that should be made is that at an early stage, the surface of the carcass will reach the same temperature as that of the air. Since the air temperature used in chilling is commonly below 10°C, there exists the Effect of refrigeration on texture of meat 49