Effect of refrigeration on texture of meat 47 the criterion(see second arrow in Fig 3. 1). From 38C down to 25C the duration increases in the manner for a normal chemical reaction(cf curves 1 and 2). Below this point, however, the experimental points diverge more and more from the predicted line; in other words, the processes take place more quickly. At about 10C the experimental curve actually inverts, so that the rate of chemical change at 2C is greater than at 15C. Such anomalous temperature dependence can only mean that new reactions are occurring with increasing intensity as the temperature is reduced The clue to the nature of the new reactions is given by curve 3, which represents the total work the muscle does during shortening From 16C ur to 38C the total work increases about 2.5-fold, but even so it is very small. By contrast, it increases by a similar amount by going only from 16 to 9C and eight-fold by going to 2C. Quite clearly, therefore, the new reactions intervening below 9-10C are somehow concerned with the increased muscle shortening The shortening occurring below 10C is usually described as"cold short ening or ' cold contracture. In some muscles, it can develop a force of between 1 and 2 Ncm-2. which is between 4 and 8% of the total force devel- oped in a fully stimulated contraction of living muscle. It is supposed to set n because the trigger for contraction is itself highly temperature sensitive and fires spontaneously to an increasing extent as the temperature is reduced below10° This trigger has been shown to be the release of calcium ions, Ca +,from the sarcoplasmic reticulum (Bendall, 1974; Jeacocke, 1986; Offer et al 1988). During use, muscle cells are triggered to contract by calcium ions (Ca)liberated from internal stores within the muscle cell. Although the early stages of activation in muscle contraction in life and cold shortening In a carcass liffer, the final stage, the release of calcium ions, is the same. In resting muscle, the intrafibrillar level of free Ca2* is very low. Most of the total store of intracellular calcium(about 10 Mole)is locked up in highly specialised structures which enwrap each of the 1000 or so fibrils within a muscle fibre(see Fig. 3.3). These structures which are part of the so-called sarcoplasmic reticulum(SR) have transverse connections(SR(T)) with the outer membrane or sarcolemma() of the fibre, so that when a nervous impulse from the motor nerve(MN) arrives at the motor end-plate (EP)it travels in both directions along the sarcolemma and invades the muscle fibre itself via the myriads of these transverse tubules. These tubules are in contact with the longitudinal elements (SR(L) of the SR which enwrap each fibril(see upper fibril in Fig. 3.3). The contact is made via the triad junctions(T)where two dense structures, the so-called lateral cis- terne, are closely opposed to the transverse tubules (Sr(T). It is thought that the lateral cisternae are the storehouse for Ca in the resting muscle In many muscles there are pairs of cisternae at the level of the Z-discs(z) of each sarcomere, so that in a fibril that is 10cm in length there are about 40000 transverse connections and pairs of cisternaethe criterion (see second arrow in Fig. 3.1). From 38°C down to 25°C the duration increases in the manner for a normal chemical reaction (cf. curves 1 and 2). Below this point, however, the experimental points diverge more and more from the predicted line; in other words, the processes take place more quickly.At about 10 °C the experimental curve actually inverts, so that the rate of chemical change at 2°C is greater than at 15°C. Such anomalous temperature dependence can only mean that new reactions are occurring with increasing intensity as the temperature is reduced. The clue to the nature of the new reactions is given by curve 3, which represents the total work the muscle does during shortening. From 16 °C up to 38 °C the total work increases about 2.5-fold, but even so it is very small. By contrast, it increases by a similar amount by going only from 16 to 9°C and eight-fold by going to 2 °C. Quite clearly, therefore, the new reactions intervening below 9–10 °C are somehow concerned with the increased muscle shortening. The shortening occurring below 10 °C is usually described as ‘cold short￾ening’ or ‘cold contracture’. In some muscles, it can develop a force of between 1 and 2 N cm-2 , which is between 4 and 8% of the total force devel￾oped in a fully stimulated contraction of living muscle. It is supposed to set in because the trigger for contraction is itself highly temperature sensitive and fires spontaneously to an increasing extent as the temperature is reduced below 10 °C. This trigger has been shown to be the release of calcium ions, Ca2+ , from the sarcoplasmic reticulum (Bendall, 1974; Jeacocke, 1986; Offer et al., 1988). During use, muscle cells are triggered to contract by calcium ions (Ca2+ ) liberated from internal stores within the muscle cell. Although the early stages of activation in muscle contraction in life and cold shortening in a carcass differ, the final stage, the release of calcium ions, is the same. In resting muscle, the intrafibrillar level of free Ca2+ is very low. Most of the total store of intracellular calcium (about 10-3 Mole) is locked up in highly specialised structures which enwrap each of the 1000 or so fibrils within a muscle fibre (see Fig. 3.3). These structures which are part of the so-called sarcoplasmic reticulum (SR) have transverse connections (SR(T)) with the outer membrane or sarcolemma (S) of the fibre, so that when a nervous impulse from the motor nerve (MN) arrives at the motor end-plate (EP) it travels in both directions along the sarcolemma and invades the muscle fibre itself via the myriads of these transverse tubules.These tubules are in contact with the longitudinal elements (SR(L)) of the SR which enwrap each fibril (see upper fibril in Fig. 3.3). The contact is made via the triad junctions (TJ) where two dense structures, the so-called lateral cis￾ternae, are closely opposed to the transverse tubules (SR(T)). It is thought that the lateral cisternae are the storehouse for Ca2+ in the resting muscle. In many muscles there are pairs of cisternae at the level of the Z-discs (Z) of each sarcomere, so that in a fibril that is 10cm in length there are about 40 000 transverse connections and pairs of cisternae. Effect of refrigeration on texture of meat 47