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 37C2.1.2 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 inorganic 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 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 concentra￾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 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)