(ADP). As mentioned in Chapter 3, the reaction is in effect a hydrolysis and is reversible:
ATP + H2O !"ADP + H3PO4.
This is a simplified outline of the reaction. For example, ATP is actually complexed to magnesium ions. ADP can itself be hydrolysed to adenosine monophosphate (AMP). Perhaps surprisingly, bearing in mind its pivotal role in the operation of muscles, the concentration of ATP in the tissue is very low – only enough for a few brief ‘twitches’ or contractions. A typical concentration would be 5–7 mmol kg"1 of muscle and this would support contraction for only a few seconds at most. However, the concentration is maintained effectively constant by the reaction:
CP + ADP !"C + ATP.
In this, creatine phosphate, or phosphocreatine (CP), reacts with ADP to form ATP and creatine (C). The reaction is reversible but the equilibrium is to the right at neutral pH so that, immediately ATP is removed, more is generated so long as CP is available. The reaction is catalysed by the enzyme creatine kinase (CK) which is abundant and active in muscle. Therefore, the ATP used in contraction is restored almost instantly. The levels of CP in muscle are much higher than those of ATP (Jeacocke, 1984), although they can drop during exhaus- tive exercise. They are replenished by a ‘reversal’ of the above reaction during a recovery period when ATP is generated from muscle glycogen, or from blood-borne metabolites, and the ATP:ADP ratio rises. The levels of glycogen in most muscles (10–20 mg g"1) are sufficient for many thousands of ‘twitches’. A full account of the biochemistry of energy production in muscles can be found in Newsholme and Start (1973).
around 5.5. The ultimate pH (pHu) finally reached varies between muscles. For example, in some ‘red’ muscles of the pig the ultimate pH may be closer to 6.0. The actual pattern of acidification also varies considerably. Typical patterns are illustrated in Fig. 5.3.
The process of acidification normally takes 4–8 h in pigs, 12–24 h in sheep and 15–36 h in cattle (Dransfield, 1994b). In poultry meat the initial pH fall may be relatively rapid. For example, in turkeys the pH in the breast muscle can have fallen to 6 by 10–15 min post mortem.
However the pHu is often quite high (5.8) compared with that in many muscles in red meat species. The pHu is inversely proportional to the concentration of lactate and the initial glycogen concentration becomes limiting below about 10 mg g"1muscle. This is illustrated for beef in Fig. 5.4. If glycogen is not limiting, the production of lactic acid ceases when the enzyme systems will no longer function at the low pH.
Excellent, detailed accounts of the changes that occur in muscle after death are given in Bate-Smith and Bendall (1956) and Bendall (1973).
Fig. 5.3. The patterns of acidification of the m. longissimus dorsi in pigs and cattle. The curves are the average of 15 cattle and 30 pigs. The vertical bars are standard errors.
Importance of acidification to the characteristics of meat
The muscle proteins tend to denature as the pH falls. This leads to a reduction in their power to bind water. Also, the myofibrillar proteins, myosin and actin, reach their isoelectric point. This is the pH at which the protein molecules have no net electrical charge and tend to lose the water that is normally bound to them. Both these phenomena lead to exudation of fluid from the muscle fibres. When the meat is cut the fluid also exudes onto the cut surface, which becomes moist. Eventually this exudate may produce drip, which can collect in the meat container or be lost, leading to weight loss. The change in the proteins increases the light scattering properties of the contractile elements of the muscle fibre. The meat changes from being relatively dark and translucent in the living animal to being opaque and paler (Fig. 5.5). These relation- ships between the physical characteristics of meat and pH are discussed in detail in Bendall and Swatland (1988). A detailed review of the effects of post-mortem changes occurring in muscles on their water- holding capacity and appearance is given in Offer et al.(1989).
The development of rigor mortis
In a resting muscle, ATP serves to keep the muscle in a relaxed state by preventing the formation of actomyosin. The muscle can also be stretched if put under tension. Only when ATP is hydrolysed to ADP does contraction occur. The ATP concentration is maintained by the breakdown of glycogen until lack of substrate or unsuitability of Fig. 5.4. The relationship between (a) initial glycogen and (b) final lactate
concentration, and ultimate pH in the m. longissimus dorsi of the ox. Reprinted from Warriss et al.(1984), with permission from Elsevier Science.
conditions, particularly pH, for the enzymes, inhibits glycolysis. The level of CP then falls as it is used to regenerate ATP from ADP.
Eventually however, the supply of regenerated ATP fails. Rigor mortis occurs when the ATP level falls below the very low level (~ 5 mmol
kg"1) required to maintain relaxation. When this happens, the actin
and myosin molecules of the thin and thick filaments combine irreversibly to form actomyosin and extensibility of the muscle is lost.
Cross-bridges form permanently and there is in effect a very weak contraction. Each muscle fibre goes into rigor very quickly once ATP is depleted, but the variation between individual fibres leads to a more gradual development of stiffness in the whole muscle as more and more fibres become inextensible (Fig. 5.6).
The time of onset of rigor will obviously relate to factors affecting the level of glycogen and creatine phosphate at death and the rate of post mortem muscle metabolism. For example, in animals that have undergone violent exercise at death, or in which glycogen has been depleted by longer-term stress preslaughter, rigor occurs faster. The rate of development will be reduced if the carcass is cooled quicker. Note that rigor onset is determined only by the availability of ATP, not the pH value of the muscle. It is possible to have rigor in muscle in which the pH is still high if the animal has been exhausted preslaughter. This has been referred to as alkaline rigor. However, the concentration of ATP below which rigor develops varies a little in different muscles and also it may be higher in alkaline rigor. On average, rigor takes different times to develop in the different species, ranging from about 4 h in the chicken to over 24 h in excised beef muscles (Etherington et al.,1987).
There is some evidence that further breakdown of ADP occurs when ATP is exhausted. This may lead to the formation of AMP,
Acidification
(1) Protein denaturation (2) Myofibrillar proteins reach
isoelectric point
Exudation of moisture from
cut surfaces
Wetness/drip loss Increased light
scattering
Opaque/paler appearance
Fig. 5.5. The consequences of muscle acidification for the appearance and water- holding of meat.
inosine monophosphate (IMP), inosine and eventually hypoxanthine.
As more muscles enter rigor mortis the whole carcass becomes stiffer and ‘sets’. This ‘setting’ is to a degree aided by the fat in the carcass becoming firmer as it cools.