2 Drip production in meat refrigeration The quality of fresh meat exposed for retail sale is initially judged on its appearance. The presence of exudate or 'drip,, which accumulates in the container of prepackaged meat or in trays or dishes of unwrapped meat, substantially reduces its sales appeal(Malton and James, 1983). Drip can be referred to by a number of different names including"purge loss, ' press loss'andthaw loss' depending on the method of measurement and when it is measured In general, beef tends to lose proportionately more drip than pork or lamb. Since most of the exudate comes from the cut ends of muscle fibres. small pieces of meat drip more than large intact carcasses. The protein concentration of drip is about 140mgmI-, about 70% of that of meat itself. The proteins in drip are the intracellular, soluble proteins of the muscle cells. The red colour is due to the protein myoglobin, the main pig of mea The problem of drip loss is not however confined to retail packs. The meat industry uses large boneless primal cuts, which are packed in plastic bags, for distribution throughout the trade. These may be stored under refrigeration for many weeks before use and during this time a consider able volume of drip may accumulate in the bag Not only does this exudate look unattractive, but it also represents an appreciable weight loss to the user when the meat is subsequently removed from its container Excessive drip could have a small effect on the eating quality of meat Perceived juiciness is one of the important sensory attributes of meat. Dryness is associated with a decrease in the other palatability attributes, especially with lack of flavour and increased toughness(Pearson, 1994) However, moisture losses during cooking are typically an order of
2 Drip production in meat refrigeration The quality of fresh meat exposed for retail sale is initially judged on its appearance. The presence of exudate or ‘drip’, which accumulates in the container of prepackaged meat or in trays or dishes of unwrapped meat, substantially reduces its sales appeal (Malton and James, 1983). Drip can be referred to by a number of different names including ‘purge loss’, ‘press loss’ and ‘thaw loss’ depending on the method of measurement and when it is measured. In general, beef tends to lose proportionately more drip than pork or lamb. Since most of the exudate comes from the cut ends of muscle fibres, small pieces of meat drip more than large intact carcasses. The protein concentration of drip is about 140 mg ml-1 , about 70% of that of meat itself. The proteins in drip are the intracellular, soluble proteins of the muscle cells. The red colour is due to the protein myoglobin, the main pigment of meat. The problem of drip loss is not however confined to retail packs. The meat industry uses large boneless primal cuts, which are packed in plastic bags, for distribution throughout the trade. These may be stored under refrigeration for many weeks before use and during this time a considerable volume of drip may accumulate in the bag. Not only does this exudate look unattractive, but it also represents an appreciable weight loss to the user when the meat is subsequently removed from its container. Excessive drip could have a small effect on the eating quality of meat. Perceived juiciness is one of the important sensory attributes of meat. Dryness is associated with a decrease in the other palatability attributes, especially with lack of flavour and increased toughness (Pearson, 1994). However, moisture losses during cooking are typically an order of
22 Meat refrigeration magnitude higher than most drip losses during refrigeration. Consequently, small differences in drip loss will have little affect on eating quality The potential for drip loss is inherent in fresh meat and is influenced by many factors. These may include breed, diet and physiological history, all of which affect the condition of the animal before it is slaughtered. After slaughter, factors such as the rate of chilling, storage temperatures, freezing and thawing can all influence the drip produced The mechanism of drip formation has been well described by Taylor (1972), Bendall (1974)and Penny(1974)and form the basis of this chapter To understand how drip occurs, it is useful to have a basic understanding of the biochemistry of meat. This includes the structure of muscle, the changes that take place after death and where water is held in the muscle The factors affecting drip production through the refrigerated cold chain can then be quantified 2.1 Biochemistry of meat 2.1.1 Structure of muscle The structure of muscle has been well described by Voyle(1974)and forms the basis of this section. Meat consists mainly of skeletal muscles which all have a similar structure. Figures 2.1-2. 4 show in diagrammatic form the levels of organisation of the components which together form a muscle. The gross levels of organisation can be resolved with the unaided eye, and it may be observed that each muscle is separated from its neighbour by a sheet of white connective tissue-the fascia. This gives support to the tional components of the muscle and connects it to the skeleton thr tendinous insertions. The connective tissue consists mainly of collagen and in some muscles includes elastic fibres. In cross-section(Fig. 2.1)a muscle appears to be subdivided into tissue bundles surrounded by thin layers of connective tissues. These bundles consist of a number of very long, multinucleated cells or fibres each sur- rounded by a thin layer of connective tissue. Each fibre is about as thick as a hair of a young child and may be several centimetres in length. Fibres are normally elliptical in cross-section and have blunt tapered ends(Fig 2.2) Fibre thickness varies between muscles within an animal as well as between species. It is also dependent on age, sex and nutritional status. As an example, the fibres of the eye muscle(M. longissimus dorsi) of an 18-month old steer are about 40um in diameter Each fibre is surrounded by a typical lipoprotein membrane, the sar- colemma, which in its native state is highly selective in its permeability to solutes. The space within the sarcolemma is mostly occupied by smaller lon- gitudinal elements, or myofibrils, each about 1 um in diameter. Figure 2.3 shows part of a single myofibril in longitudinal section. Figure 2. 4 repre
magnitude higher than most drip losses during refrigeration. Consequently, small differences in drip loss will have little affect on eating quality. The potential for drip loss is inherent in fresh meat and is influenced by many factors. These may include breed, diet and physiological history, all of which affect the condition of the animal before it is slaughtered. After slaughter, factors such as the rate of chilling, storage temperatures, freezing and thawing can all influence the drip produced. The mechanism of drip formation has been well described by Taylor (1972), Bendall (1974) and Penny (1974) and form the basis of this chapter. To understand how drip occurs, it is useful to have a basic understanding of the biochemistry of meat. This includes the structure of muscle, the changes that take place after death and where water is held in the muscle. The factors affecting drip production through the refrigerated cold chain can then be quantified. 2.1 Biochemistry of meat 2.1.1 Structure of muscle The structure of muscle has been well described by Voyle (1974) and forms the basis of this section. Meat consists mainly of skeletal muscles which all have a similar structure. Figures 2.1–2.4 show in diagrammatic form the levels of organisation of the components which together form a muscle.The gross levels of organisation can be resolved with the unaided eye, and it may be observed that each muscle is separated from its neighbour by a sheet of white connective tissue – the fascia. This gives support to the functional components of the muscle and connects it to the skeleton through tendinous insertions. The connective tissue consists mainly of collagen and in some muscles includes elastic fibres. In cross-section (Fig. 2.1) a muscle appears to be subdivided into tissue bundles surrounded by thin layers of connective tissues. These bundles consist of a number of very long, multinucleated cells or fibres each surrounded by a thin layer of connective tissue. Each fibre is about as thick as a hair of a young child and may be several centimetres in length. Fibres are normally elliptical in cross-section and have blunt tapered ends (Fig. 2.2). Fibre thickness varies between muscles within an animal as well as between species. It is also dependent on age, sex and nutritional status. As an example, the fibres of the eye muscle (M. longissimus dorsi) of an 18-monthold steer are about 40mm in diameter. Each fibre is surrounded by a typical lipoprotein membrane, the sarcolemma, which in its native state is highly selective in its permeability to solutes.The space within the sarcolemma is mostly occupied by smaller longitudinal elements, or myofibrils, each about 1mm in diameter. Figure 2.3 shows part of a single myofibril in longitudinal section. Figure 2.4 repre- 22 Meat refrigeration
Drip production in meat refrigeration 23 Fig 2.1 Diagrammatic representation of cut surface of muscle to show bundles of fibres(source: Voyle, 1974) 腫‖D Fig 2.2 Single muscle fibre Diagrammatic representation of morphology as seen by direct microscopy(source: Voyle, 1974). sents a single muscle fibre in cross-section, showing myofibrils and asso ated structures that are referred to below Each myofibril is enwrapped in a thin vesicular structure the sarcoplas- mic reticulum, which is involved in the transmission of the nervous impulse to the contractile elements. The characteristic striated appearance of each muscle fibre, represented in Fig. 2. 2, may be observed by direct microscopy The finer details of structure, represented in Figs 2.3-2.4, can only be resolved by electron microscopy Between the myofibrils are small particles, the mitochondria, which provide the energy for contraction via oxidative processes. The myofibrils are bathed in a fluid, the sarcoplasm, which contains many soluble enzymes. These are mostly concerned with the process of glycolysis by which lactic acid is produced in the oxygen-free post-mortem muscle. The myofibrils occupy about 74% of the total fibre volume. The myofibrils are packed with contractile microfilaments of actin and myosin which, in cross-section, may be seen to be arranged in a hexagonal lattice. The interdigitating sliding action of these filaments when stimulated to contract is suggested by the longitudinal view represented in Fig. 2.3.A fibril contains about 16% contractile protein and about 84% water in which are dissolved small solutes such as adenosine triphosphate(ATP)
sents a single muscle fibre in cross-section, showing myofibrils and associated structures that are referred to below. Each myofibril is enwrapped in a thin vesicular structure the sarcoplasmic reticulum, which is involved in the transmission of the nervous impulse to the contractile elements. The characteristic striated appearance of each muscle fibre, represented in Fig. 2.2, may be observed by direct microscopy. The finer details of structure, represented in Figs 2.3–2.4, can only be resolved by electron microscopy. Between the myofibrils are small particles, the mitochondria, which provide the energy for contraction via oxidative processes. The myofibrils are bathed in a fluid, the sarcoplasm, which contains many soluble enzymes. These are mostly concerned with the process of glycolysis by which lactic acid is produced in the oxygen-free post-mortem muscle. The myofibrils occupy about 74% of the total fibre volume. The myofibrils are packed with contractile microfilaments of actin and myosin which, in cross-section, may be seen to be arranged in a hexagonal lattice. The interdigitating sliding action of these filaments when stimulated to contract is suggested by the longitudinal view represented in Fig. 2.3. A fibril contains about 16% contractile protein and about 84% water in which are dissolved small solutes such as adenosine triphosphate (ATP), Drip production in meat refrigeration 23 Fig. 2.1 Diagrammatic representation of cut surface of muscle to show bundles of fibres (source: Voyle, 1974). Fig. 2.2 Single muscle fibre. Diagrammatic representation of morphology as seen by direct microscopy (source: Voyle, 1974)
24 Meat refrigeration Fig. 2.3 Part of myofibril. Diagrammatic representation to show filament-array in longitudinal section with adjacent structures(source: Voyle, 1974) 8包 商88 ③原寓 Fig. 2.4 Cross-section of single fibre, showing myofibrils and other structures (source: Voyle, 1974) the fuel for contraction, but from which the larger enzyme molecules are The fluid within the fibrils is distributed between the microfilaments of the hexagonal lattice. After rigor in a muscle at rest length the filament lattice volume decreases and releases fluid into the spaces between the myofibrils, i.e. into the sarcoplasm. The permeability of the sarcolemma also hanges after rigor, and fluid, generally referred to as'drip', escapes into the extracellular space. The extent to which this happens depends upon the ultimate level of pH attained by the post-rigor muscle
the fuel for contraction, but from which the larger enzyme molecules are excluded. The fluid within the fibrils is distributed between the microfilaments of the hexagonal lattice. After rigor in a muscle at rest length the filament lattice volume decreases and releases fluid into the spaces between the myofibrils, i.e. into the sarcoplasm.The permeability of the sarcolemma also changes after rigor, and fluid, generally referred to as ‘drip’, escapes into the extracellular space. The extent to which this happens depends upon the ultimate level of pH attained by the post-rigor muscle. 24 Meat refrigeration Fig. 2.3 Part of myofibril. Diagrammatic representation to show filament-array in longitudinal section with adjacent structures (source: Voyle, 1974). Fig. 2.4 Cross-section of single fibre, showing myofibrils and other structures (source: Voyle, 1974)
Drip production in meat refrigeration 25 Lactic acid d Fig 2.5 Reaction of ATP in muscle(source: Bendall, 1972) 2.1.2 Changes after slaughter Muscles of freshly killed mammals are relaxed, soft, extensible and flexible. However, after a short time they become stiff, rigid and contracted. This state is called rigor mortis Muscles obtain the energy they need for contraction by taking up glucose from the blood and storing it in a polymeric form called glycogen. The chemical fuel the muscle cells use is adenosine triphosphate(ATP), which as well as providing the energy required to shorten muscle fibres, acts as a lubricant during contraction preventing cross-linking Muscles power con- traction by hydrolysing this ATP to the diphosphate(ADP)and inorgan phosphate(Pi) but there is only enough ATP in muscle cells to fuel a con traction for three seconds for a sustained contraction the atp has to be resynthesised from ADP and Pi by coupling this energetically unfavourable reaction to the energetically favourable breakdown of glycogen to lactic In muscle after death the rate of breakdown of atp is low but still ppreciable and the muscle draws slowly on its glycogen stores. These are not replenished because there is no longer a blood supply. The lactic acid accumulates and the ph falls from an initial value of about 7 to a final value of about 5.5 to 6.0 When the breakdown of glycogen comes to a halt, the ATP concent tion falls to zero and the force-generating machinery of the muscle stops in mid-cycle causing the muscle to become rigid and inextensible. It is then said to be in the state of rigor mortis(rigor for short) The most important structural change in muscle tissue during the onset of rigor is the formation of actomyosin complex caused by the cross-linking of actin and myosin filaments and muscle contraction brought about by th breakdown of ATP Breakdown of ATP also contributes to the temperature rise(0. 2-2.0C)which is sometimes observed in the deep musculature of pigs and beef animals during the first hour or so after slaughter, as described by Bendall (1972)and measured by Morley in 1974 ormal rigor sets in before glycolysis ends, i.e. before reaching the final pH value. The time that rigor takes to develop(Table 2. 1)is dependent on muscle type, its posture on the carcass, rate of cooling and so on(Offer et al., 1988). Temperature is particularly significant. Between 10 and 37C
2.1.2 Changes after slaughter Muscles of freshly killed mammals are relaxed, soft, extensible and flexible. However, after a short time they become stiff, rigid and contracted. This state is called rigor mortis. Muscles obtain the energy they need for contraction by taking up glucose from the blood and storing it in a polymeric form called glycogen. The chemical fuel the muscle cells use is adenosine triphosphate (ATP), which as well as providing the energy required to shorten muscle fibres, acts as a lubricant during contraction preventing cross-linking. Muscles power contraction by hydrolysing this ATP to the diphosphate (ADP) and inorganic phosphate (Pi) but there is only enough ATP in muscle cells to fuel a contraction for three seconds. For a sustained contraction, the ATP has to be resynthesised from ADP and Pi by coupling this energetically unfavourable reaction to the energetically favourable breakdown of glycogen to lactic acid (Fig. 2.5). In muscle after death, the rate of breakdown of ATP is low but still appreciable and the muscle draws slowly on its glycogen stores. These are not replenished because there is no longer a blood supply. The lactic acid accumulates and the pH falls from an initial value of about 7 to a final value of about 5.5 to 6.0. When the breakdown of glycogen comes to a halt, the ATP concentration falls to zero and the force-generating machinery of the muscle stops in mid-cycle causing the muscle to become rigid and inextensible. It is then said to be in the state of rigor mortis (rigor for short). The most important structural change in muscle tissue during the onset of rigor is the formation of actomyosin complex caused by the cross-linking of actin and myosin filaments and muscle contraction brought about by the breakdown of ATP. Breakdown of ATP also contributes to the temperature rise (0.2–2.0 °C) which is sometimes observed in the deep musculature of pigs and beef animals during the first hour or so after slaughter, as described by Bendall (1972) and measured by Morley in 1974. Normal rigor sets in before glycolysis ends, i.e. before reaching the final pH value. The time that rigor takes to develop (Table 2.1) is dependent on muscle type, its posture on the carcass, rate of cooling and so on (Offer et al., 1988). Temperature is particularly significant. Between 10 and 37 °C Drip production in meat refrigeration 25 Lactic acid Glycogen ATP ADP + Pi Fig. 2.5 Reaction of ATP in muscle (source: Bendall, 1972)
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
Drip production in meat refrigeration 29 2.1.4 ce formation in muscle tissues In general, freezing and thawing exacerbate drip loss through damage the muscle structure. It is necessary to differentiate between the effects freezing in pre-rigor and post-rigor muscle. For most practical purposes, meat is in the latter condition but there has been considerable interest the rapid freezing of hot,, 1. e. pre-rigor meat. 2.1.4.1 Pre-rigor muscle The freezing of meat immediately after slaughter appears at first sight to be an excellent method of overcoming many of the chilling, hygiene and storage problems of conventional production methods. However, there are wo problems, ' cold shortening thaw rigor', that result in very tough meat and that have to be overcome to make such a process viable. Thaw rigor, or ' thaw contractor' as it is sometimes called, also significantly ncreases drip loss after thawing If the meat temperature falls below 10C before the supply of fuel for ontraction, i.e. ATP, is used up, but freezing has not occurred, the muscle will contract. This phenomenon called'cold shortening was first described by Locker and Hagyard (1963) and is discussed in Chapter 3 of this book. The protein denaturation that results from cold shortening produces a large amount of drip(offer et al., 1988) If very high rates of heat extraction can be achieved, then the meat can be frozen fast enough to stop cold shortening. However, in this case,a more severe shortening, thaw rigor, will occur during thawing. In unre- strained muscle up to 25% of the muscle weight will be lost in the form of drip during thawing(Bendall, 1974). Bendall stated that the problems associated with thaw rigor could be overcome by holding the frozen meat at-3 to -5C for at least 48h. However, such a process is not used commercially 2.1.4.2 Post-rigor muscle Chemical changes after slaughter cause the acidity of the tissue to increase and the pH falls to a level which is normally in the range of 5.5-5.7. This compares with a pH of about 7.0-7.2 in the living tissue. One of the conse quences of this fall in pH is a change in the permeability of the sarcolemma which now permits sarcoplasmic proteins and water to pass more readily out of the cell(Voyle, 1974). When the tissue is slowly cooled below its freezing point, this protein-containing fluid is extracted from the cell to con tribute to the growth of extracellular ice crystals. Loss of fluid from the cell results in an increase in the intracellular salt concentration This in turn causes some denaturation of those proteins remaining within the cell. more rapid rate of freezing will cause the intracellular water, including that in the actin-myosin lattice, to crystallise
2.1.4 Ice formation in muscle tissues In general, freezing and thawing exacerbate drip loss through damage of the muscle structure. It is necessary to differentiate between the effects of freezing in pre-rigor and post-rigor muscle. For most practical purposes, meat is in the latter condition but there has been considerable interest in the rapid freezing of ‘hot’, i.e. pre-rigor meat. 2.1.4.1 Pre-rigor muscle The freezing of meat immediately after slaughter appears at first sight to be an excellent method of overcoming many of the chilling, hygiene and storage problems of conventional production methods. However, there are two problems, ‘cold shortening’ and ‘thaw rigor’, that result in very tough meat and that have to be overcome to make such a process viable. Thaw rigor, or ‘thaw contractor’ as it is sometimes called, also significantly increases drip loss after thawing. If the meat temperature falls below 10 °C before the supply of fuel for contraction, i.e. ATP, is used up, but freezing has not occurred, the muscle will contract. This phenomenon called ‘cold shortening’ was first described by Locker and Hagyard (1963) and is discussed in Chapter 3 of this book. The protein denaturation that results from cold shortening produces a large amount of drip (Offer et al., 1988). If very high rates of heat extraction can be achieved, then the meat can be frozen fast enough to stop cold shortening. However, in this case, a more severe shortening, thaw rigor, will occur during thawing. In unrestrained muscle up to 25% of the muscle weight will be lost in the form of drip during thawing (Bendall, 1974). Bendall stated that the problems associated with thaw rigor could be overcome by holding the frozen meat at -3 to -5 °C for at least 48 h. However, such a process is not used commercially. 2.1.4.2 Post-rigor muscle Chemical changes after slaughter cause the acidity of the tissue to increase and the pH falls to a level which is normally in the range of 5.5–5.7. This compares with a pH of about 7.0–7.2 in the living tissue. One of the consequences of this fall in pH is a change in the permeability of the sarcolemma which now permits sarcoplasmic proteins and water to pass more readily out of the cell (Voyle, 1974). When the tissue is slowly cooled below its freezing point, this protein-containing fluid is extracted from the cell to contribute to the growth of extracellular ice crystals. Loss of fluid from the cell results in an increase in the intracellular salt concentration. This in turn causes some denaturation of those proteins remaining within the cell. A more rapid rate of freezing will cause the intracellular water, including that in the actin–myosin lattice, to crystallise. Drip production in meat refrigeration 29
30 Meat refrigeration 2.2 Measurement of drip Many methods have been used to measure drip loss from meat. Data obtained using different methods can be used to determine trends but the values obtained are not directly comparable The most important factor that affects the measurement of drip is the ra tio of cut surface to weight or volume. It is clear that the free water has to move to the surface before it can drip from the meat and therefore the more cut surface to volume there is, the less distance the water has to travel. In 1956. Howard and Lawrie reported that drip from beef quarters, domes- tic joints and small samples in the laboratory ranged from 0.3 to 1, 1. 2 to 2, nd 4 to 10% of weight, respectively. Howard(1956) showed that pieces with the same cross-section but l and 3cm thick lost 8 and 6% as drip, respectively. The drip is also reduced if the pieces are cut along the direc tion of the fibres rather than across it. Pressure applied to slices or blocks of meat increases the amount of drip and so does an absorbent material placed on the cut surfaces because of the increase in hydrostatic pressure It is therefore important that an appropriate method is used in order to obtain data that are directly applicable to a commercial situation. Weigh ing unwrapped samples of meat provides information on total weight loss However, some of the loss is due to evaporation from the surface, not drip One simple method is to hang the preweighed meat, using a nylon mesh to support it. A polythene bag is then placed round the sample but not in contact with it to prevent evaporation. The system is then kept in a con trolled environment and the sample reweighed after a set time. For experimental purposes more information and better reproducibility can usually be obtained from methods where force is applied rather than the simple method of measuring"free'drip(Penny, 1974). These include the press method of Grau and Hamm(1953)or methods depending on centrifugation 2.3 Factors affecting the amount of drip Some factors that affect the amount of drip are inherent in the animal and include the breed of the animal and the position of the meat within the animal. Treatment of the animal before slaughter, especially in the case of ork, can influence drip production by producing DFD or PSE meat. The conditions in and the length of the refrigerated cold chain will further influ- ence the resulting drip 2.3.1 Animal factors 2.3 Breed In pigs especially, there are large differences in drip loss from meat from different breeds. Taylor(1972)measured drip loss from leg joints from four
2.2 Measurement of drip Many methods have been used to measure drip loss from meat. Data obtained using different methods can be used to determine trends but the values obtained are not directly comparable. The most important factor that affects the measurement of drip is the ratio of cut surface to weight or volume. It is clear that the free water has to move to the surface before it can drip from the meat and therefore the more cut surface to volume there is, the less distance the water has to travel. In 1956, Howard and Lawrie reported that drip from beef quarters, domestic joints and small samples in the laboratory ranged from 0.3 to 1, 1.2 to 2, and 4 to 10% of weight, respectively. Howard (1956) showed that pieces with the same cross-section but 1 and 3cm thick lost 8 and 6% as drip, respectively. The drip is also reduced if the pieces are cut along the direction of the fibres rather than across it. Pressure applied to slices or blocks of meat increases the amount of drip and so does an absorbent material placed on the cut surfaces because of the increase in hydrostatic pressure. It is therefore important that an appropriate method is used in order to obtain data that are directly applicable to a commercial situation. Weighing unwrapped samples of meat provides information on total weight loss. However, some of the loss is due to evaporation from the surface, not drip. One simple method is to hang the preweighed meat, using a nylon mesh to support it. A polythene bag is then placed round the sample but not in contact with it to prevent evaporation. The system is then kept in a controlled environment and the sample reweighed after a set time. For experimental purposes more information and better reproducibility can usually be obtained from methods where force is applied rather than the simple method of measuring ‘free’ drip (Penny,1974).These include the press method of Grau and Hamm (1953) or methods depending on centrifugation. 2.3 Factors affecting the amount of drip Some factors that affect the amount of drip are inherent in the animal and include the breed of the animal and the position of the meat within the animal. Treatment of the animal before slaughter, especially in the case of pork, can influence drip production by producing DFD or PSE meat. The conditions in and the length of the refrigerated cold chain will further influence the resulting drip. 2.3.1 Animal factors 2.3.1.1 Breed In pigs especially, there are large differences in drip loss from meat from different breeds. Taylor (1972) measured drip loss from leg joints from four 30 Meat refrigeration