Meat refrigeration 5 11 三a品 82-39兰 05101520253035 Temperature(C) Fig 3.2 Time for half-change of ATP during rigor in beef LD muscle, plotted against temperature. Initial pH=7.0 in all cases. Curve 1: times for an initial re- action. Curve 2: observed times Curve 3: work done during shortening(source hese processes is the creatine kinase reaction, which resynthesises ATP from its breakdown product, ADP, and phosphocreatine(PC). The second is the complex process of glycolysis in which the energy for resynthesis comes from the breakdown of glycogen to lactate, with concomitant acidi- fication and fall of muscle pH. The ATP supply remains constant only while PC is still available, but begins to fall as soon as glycolysis is left on its own as the sole source of resynthesis. This phase of declining ATP supply is known as the rapid phase of rigor, because it is then that the stiffening (rigor)of the muscle sets in. It is shown by the first arrow in Fig. 3.1 At temperatures above 12C the post-mortem muscle remains in a passive, relaxed state until the ATp supply begins to dwindle at the onset of the rapid phase of rigor. It then begins to shorten actively. At body tem- perature(38C)the shortening can reach 40% or more of the muscle length if unopposed by the force of a load. This so-called'rigor shortening can be overcome by quite small loads and is incapable of doing much work, even at 38C(see Fig 3. 2). The effect of temperature on the duration of the chemical changes during rigor is shown in Fig. 3.2, using the time for half-change of ATP asthese processes is the creatine kinase reaction, which resynthesises ATP from its breakdown product, ADP, and phosphocreatine (PC). The second is the complex process of glycolysis in which the energy for resynthesis comes from the breakdown of glycogen to lactate, with concomitant acidi- fication and fall of muscle pH. The ATP supply remains constant only while PC is still available, but begins to fall as soon as glycolysis is left on its own as the sole source of resynthesis. This phase of declining ATP supply is known as the rapid phase of rigor, because it is then that the stiffening (rigor) of the muscle sets in. It is shown by the first arrow in Fig. 3.1. At temperatures above 12 °C the post-mortem muscle remains in a passive, relaxed state until the ATP supply begins to dwindle at the onset of the rapid phase of rigor. It then begins to shorten actively. At body tem￾perature (38 °C) the shortening can reach 40% or more of the muscle length if unopposed by the force of a load. This so-called ‘rigor shortening’ can be overcome by quite small loads and is incapable of doing much work, even at 38 °C (see Fig. 3.2). The effect of temperature on the duration of the chemical changes during rigor is shown in Fig. 3.2, using the time for half-change of ATP as 46 Meat refrigeration Work done (mJ g–1) during 'rigor' shortening 11 10 9 8 7 6 5 4 0 5 10 15 20 25 30 35 40 Temperature (°C) Time for half change of ATP (h) 1 2 3 5 4 3 2 1 0 Fig. 3.2 Time for half-change of ATP during rigor in beef LD muscle, plotted against temperature. Initial pH = 7.0 in all cases. Curve 1: times for an initial re￾action. Curve 2: observed times. Curve 3: work done during shortening (source: Bendall, 1974)