26 Meat refrigeration Table 2.1 Typical time for rigor onset Type of meat Development time Range(h) for rigor(h) 10-20 Source: Offer et al. 1988. the rate of rigor development increases with temperature, like many other metabolic processes. The rate increases three to four times for each 10C rise in this range. As a result of this fall in pH a number of enzymes change their activity ome lose it by changing their three-dimensional structure and some nhance their activity, i. e especially liposomal enzymes which are necessary or the conditioning process(Honikel, 1990). In the course of the break down of energy-rich compounds(shortly before they get used up) the onset of rigor occurs which increases the rigidity of the meat, i.e. the meat tough ens. Conditioning reduces the toughness as the number of rigid longitudi nal and transversal cross-links in the myofibres are reduced by enzymic action(Honikel, 1990) The conditions for the onset and development of rigor have a profound influence on the tenderness, juiciness and water-holding capacity of meats While factors such as species, breed, age, nature of muscle, ante- and post- mortem treatments, and so on all have an influence, temperature is prob- ably the most important Conditions of exhaustion or stress before slaughter can cause changes in the degree of glycolysis producing detrimental effects to the meat. Animals subjected to severe exhaustion shortly before slaughter use up their glyco gen reserves thus less lactic acid is formed pre mucin high pH(6.0-6.5)dark meat, often described as dark, firm and dry(dFD) meat DFD problems an occur in pork, mutton, veal and beef. By convention all pork above pH 6.0/6.2 is classified as DFD meat(Honikel, 1990). Drip losses from DFD meat are less than from normal meat (Offer et aL., 1988) A second cause of shrinkage is protein denaturation In life, muscle pro- teins are stable for many days at 37C and pH7. However, after death the musculature, especially in the interior of the carcass, cools relatively slowl nd becomes acidic. Under this combination of high temperature and low pH, some proteins especially myosin, the principal protein of muscle, slowly denature. If sufficient myosin is denatured, the myofibrils shrink about twice as much as usual and the meat is pale, soft and exudes drip more quickly and in greater amounts than usual Consumers react unfavourably against the unattractive paleness of this pale, soft and exuding(PSe)meat
the rate of rigor development increases with temperature, like many other metabolic processes. The rate increases three to four times for each 10 °C rise in this range. As a result of this fall in pH a number of enzymes change their activity. Some lose it by changing their three-dimensional structure and some enhance their activity, i.e. especially liposomal enzymes which are necessary for the conditioning process (Honikel, 1990). In the course of the breakdown of energy-rich compounds (shortly before they get used up) the onset of rigor occurs which increases the rigidity of the meat, i.e. the meat toughens. Conditioning reduces the toughness as the number of rigid longitudinal and transversal cross-links in the myofibres are reduced by enzymic action (Honikel, 1990). The conditions for the onset and development of rigor have a profound influence on the tenderness, juiciness and water-holding capacity of meats. While factors such as species, breed, age, nature of muscle, ante- and postmortem treatments, and so on all have an influence, temperature is probably the most important. Conditions of exhaustion or stress before slaughter can cause changes in the degree of glycolysis producing detrimental effects to the meat. Animals subjected to severe exhaustion shortly before slaughter use up their glycogen reserves thus less lactic acid is formed producing high pH (6.0–6.5) dark meat, often described as dark, firm and dry (DFD) meat. DFD problems can occur in pork, mutton, veal and beef. By convention all pork above pH 6.0/6.2 is classified as DFD meat (Honikel, 1990). Drip losses from DFD meat are less than from normal meat (Offer et al., 1988). A second cause of shrinkage is protein denaturation. In life, muscle proteins are stable for many days at 37 °C and pH 7. However, after death the musculature, especially in the interior of the carcass, cools relatively slowly and becomes acidic. Under this combination of high temperature and low pH, some proteins especially myosin, the principal protein of muscle, slowly denature. If sufficient myosin is denatured, the myofibrils shrink about twice as much as usual and the meat is pale, soft and exudes drip more quickly and in greater amounts than usual. Consumers react unfavourably against the unattractive paleness of this pale, soft and exuding (PSE) meat. 26 Meat refrigeration Table 2.1 Typical time for rigor onset Type of meat Development time Range (h) for rigor (h) Beef 18 8–30 Lamb 12 10–20 Pork 3 0.6–8 Source: Offer et al., 1988
Drip production in meat refrigeration 27 With beef and lamb, provided the chilling regime is adequate, only a little myosin denaturation occurs probably because the carcass is chilled suffi- ciently before a low pH is reached. PSE meat is therefore not usually a problem with these species, except sometimes in the deep muscle if the carcass has been chilled slowly(Offer et aL., 1988) With pork, however, the pH fall is faster, especially in carcasses of stress susceptible animals. In these carcasses, the ph falls to below 6.0 within 45 min of slaughter when the carcass temperature is above 35C. Myosin denaturation may then be extensive and pig carcasses are vulnerable to the PSE state. As well as stress, this condition may be genetically predetermined (Honikel, 1990) PSE is not an all-or-none phenomenon and the drip loss depends on the extent of myosin denaturation. The drip loss can therefore be controlled to some extent by the chilling regime. Frozen PSE meat exhibits excessive drip loss on thawing(Honikel, 1990) 2.1.3 Water relationships in meat In living muscle, 85-95% of the total water is held within the fibres in dynami equilibrium with the remaining 5-15%(plasma water) outside the fibre walls. Within the fibre, the water is held both by the contractile, myofibrillar, filament proteins, myosin and actin, and by the soluble, sarcoplasmic proteins which include myoglobin and the glycolytic enzymes. The water balance is such that it allows movement of the proteins within the fibre and exchange of metabolites in and out of the fibre without altering the overall amount of water held. Therefore, when a force is applied to a pre-rigor muscle, excised immediately after the death of an animal, very little fluid can be squeezed out. The distribution of space in muscle is shown in Table 2.2 Calculations can be made of the diameters of the capillary-like spaces between the filaments of the myofibril and between sarcoplasmic proteins from which the number of water molecules between nearest-neighbour structures can be deduced. The results are shown in Table 2.3 Table 2.2 Ap ate distribution of th excised muscle Volume as total vol pre-nigor Extrafibre space Intrafibre space 889 Extrafibrillar space 2-24 30-32.5 Intrafibrillar space 5862 ming a 12% reduction in filament lattice volume post rigor. 974
With beef and lamb, provided the chilling regime is adequate, only a little myosin denaturation occurs probably because the carcass is chilled suffi- ciently before a low pH is reached. PSE meat is therefore not usually a problem with these species, except sometimes in the deep muscle if the carcass has been chilled slowly (Offer et al., 1988). With pork, however, the pH fall is faster, especially in carcasses of stresssusceptible animals. In these carcasses, the pH falls to below 6.0 within 45 min of slaughter when the carcass temperature is above 35°C. Myosin denaturation may then be extensive and pig carcasses are vulnerable to the PSE state.As well as stress, this condition may be genetically predetermined (Honikel, 1990). PSE is not an all-or-none phenomenon and the drip loss depends on the extent of myosin denaturation. The drip loss can therefore be controlled to some extent by the chilling regime. Frozen PSE meat exhibits excessive drip loss on thawing (Honikel, 1990). 2.1.3 Water relationships in meat In living muscle,85–95% of the total water is held within the fibres in dynamic equilibrium with the remaining 5–15% (plasma water) outside the fibre walls.Within the fibre, the water is held both by the contractile, myofibrillar, filament proteins,myosin and actin,and by the soluble,sarcoplasmic proteins which include myoglobin and the glycolytic enzymes. The water balance is such that it allows movement of the proteins within the fibre and exchange of metabolites in and out of the fibre, without altering the overall amount of water held. Therefore, when a force is applied to a pre-rigor muscle, excised immediately after the death of an animal, very little fluid can be squeezed out. The distribution of space in muscle is shown in Table 2.2. Calculations can be made of the diameters of the capillary-like spaces between the filaments of the myofibril and between sarcoplasmic proteins from which the number of water molecules between nearest-neighbour structures can be deduced. The results are shown in Table 2.3. Drip production in meat refrigeration 27 Table 2.2 Approximate distribution of the spaces in excised muscle Structure Volume as % total vol. pre-rigor post-rigor Extrafibre space <12 100 Intrafibre space 88–95 Extrafibrillar space 22–24 30–32.5 Intrafibrillar space 66–71 58–62a a Assuming a 12% reduction in filament lattice volume post rigor. Source: Penny, 1974
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 meat
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 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. Denaturation 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 nearestneighbour 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