8 Meat refrigeration Table 2.3 Diameters of the '" capillary spaces between nearest neighbour elements of the fibre and the number of water molecules accommodated between surfaces of nearest-neighbour protein molecules Elements Diameter of capillary Number of molecules f wate ctIn-myosin 21.5 Myosin-myosin (H Actin-actin (I-zone) 45.3 Sarcoplasmic proteins g Assuming the average molecular weight (MW)= 120000 Da and a mean diameter of Source: Penny. 1974 These show the capillary spaces between the elements are very small so that it seems reasonable that much of the water would be held by surface tension forces. In addition, quite a large proportion of the water should be immobilised by surface charges on the proteins. When a muscle goes into rigor a number of important changes take place, which affect the water balance. As a result of the loss of ATP, the actin and myosin filaments become bonded together and tend to squeeze water out of the filament lattice into the sarcoplasmic space, and possibly also into the spaces between fibres. This squeezing effect is increased as the ph falls from 7.2 in pre-rigor muscle to 5.5-5.8 in post-rigor muscle This is because the proteins are then much nearer the mean isoelectric point of 5.0-5.2 at which their hydration is at a minimum and their packing density maximal (Rome, 1968). This, no doubt, explains Hegartys (1969)finding that muscle fibre diameter decreases during rigor, which also suggests that the fibre wall has become leaky and allowed fuid to escape. Table 2.2 gives the approximate change in the distribution of space which would if the myofibrillar lattice volume was reduced by 12%(Rome, 1968 The loss of water binding by the proteins also depends on the amount of denaturation that has taken place in the post-mortem period. Denatu ration is an irreversible alteration to the structure and properties of the proteins. Denaturation leads to extra loss of water binding and to closer packing of the fibrillar proteins. It is a function of the post-mortem rate of cooling and the rate of pH fall, and increases dramatically at low rates of cooling and high rates of pH fall As a result of all these post-mortem changes, a considerable amount of previously immobilised water is released by the proteins and redistributed from filament spaces to sarcoplasmic spaces within the fibres, and also into the spaces outside the fibres. This released water makes up most of the fluid (drip) which can then be squeezed out of the meatThese show the capillary spaces between the elements are very small so that it seems reasonable that much of the water would be held by surface tension forces. In addition, quite a large proportion of the water should be immobilised by surface charges on the proteins. When a muscle goes into rigor a number of important changes take place, which affect the water balance. As a result of the loss of ATP, the actin and myosin filaments become bonded together and tend to squeeze water out of the filament lattice into the sarcoplasmic space, and possibly also into the spaces between fibres. This squeezing effect is increased as the pH falls from 7.2 in pre-rigor muscle to 5.5–5.8 in post-rigor muscle. This is because the proteins are then much nearer the mean isoelectric point of 5.0–5.2 at which their hydration is at a minimum and their packing density maximal (Rome, 1968). This, no doubt, explains Hegarty’s (1969) finding that muscle fibre diameter decreases during rigor, which also suggests that the fibre wall has become leaky and allowed fluid to escape. Table 2.2 gives the approximate change in the distribution of space which would occur if the myofibrillar lattice volume was reduced by 12% (Rome, 1968). The loss of water binding by the proteins also depends on the amount of denaturation that has taken place in the post-mortem period. Denatu￾ration is an irreversible alteration to the structure and properties of the proteins. Denaturation leads to extra loss of water binding and to closer packing of the fibrillar proteins. It is a function of the post-mortem rate of cooling and the rate of pH fall, and increases dramatically at low rates of cooling and high rates of pH fall. As a result of all these post-mortem changes, a considerable amount of previously immobilised water is released by the proteins and redistributed from filament spaces to sarcoplasmic spaces within the fibres, and also into the spaces outside the fibres. This released water makes up most of the fluid (drip) which can then be squeezed out of the meat. 28 Meat refrigeration Table 2.3 Diameters of the ‘cylindrical’ capillary spaces between nearest￾neighbour elements of the fibre and the number of water molecules accommodated between surfaces of nearest-neighbour protein molecules Elements Diameter of capillary Number of molecules (nm) of water Actin–myosin overlap 21.5 42 Myosin–myosin (H-zone) 38.4 120 Actin–actin (I-zone) 45.3 67 Sarcoplasmic proteinsa 15.3 30 a Assuming the average molecular weight (MW) = 120 000 Da and a mean diameter of 6.52 nm. Source: Penny, 1974