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)