13 Rigor & Cold shortening

13.1 Introduction

Rigor mortis is a loss of muscle extensibility marking the conversion of muscle to meat. In other words, living muscles can be stretched and they return to their resting length when released.  Meat cannot be stretched and has very little elasticity.  A strong attempt to stretch a length of meat will merely rip it.

Just before a muscle sets in rigor mortis, it may attempt to shorten. Refrigeration increases the shortening - giving rise to the name cold shortening.  But even cold shortening is weak relative to contraction of a living muscle.  A muscle will only contract if there are no skeletal restraints.  What does this mean?  Consider a beef animal walking into the abattoir on all four legs - a leg at each corner of the body.  We slaughter it and suspend it by its hindlimbs.  The muscles ventral to the vertebral column are stretched and cannot shorten before rigor mortis develops.  But the muscles dorsal to the vertebral column have no skeletal restraints and are free to contract - either from the very weak  shortening just before rigor develops, or the slightly stronger cold shortening caused by refrigeration.

Why is this important?  Because shortening decreases sarcomere length and increases the overlap of thick and thin myofilaments. This increases the toughness of the meat.  Nobody likes tough meat.  It is important to understand how to minimize cold-shortening.

The key point to grasp is - the sarcomere can only shorten if it still has ATP and has not yet developed rigor mortis. An exhausted muscle has minimal glycogen, therefore minimal post-mortem re-synthesis of ATP, therefore it develops rigor mortis early. Once rigor has developed - the muscle cannot shorten.  We are safe.  We cannot make meat tough by rapid refrigeration.

Cold-shortening is a very complex phenomenon.  The most likely cause is the effect of low temperature on the sarcoplasmic reticulum. The sarcoplasmic reticulum works hard using ATP to sequester calcium ions. When it is cooled - it begins to fail.  Calcium ions remain in the cytosol.  The muscle slowly contracts.  Cold shortening is more severe in red muscles than in white muscles because red muscles have a weak sarcoplasmic reticulum.

13.2 Rigor mortis

The conversion of muscles to meat is completed when muscles have depleted their energy reserves or have lost the ability to utilize remaining reserves. In living muscles at rest, an ATP molecule binds to each myosin molecule head and in this condition the myosin head is said to be "charged". In resting muscle, further developments between the actin and myosin of thin and thick myofilaments are prevented by the intrusion of tropomyosin molecules. Contraction in living muscle is initiated by the release of calcium ions from the sarcoplasmic reticulum, and followed by the removal of the tropomyosin  intrusion. As a muscle contracts, charged myosin molecules heads attach to actin molecules, ATP is split to ADP with a release of energy, and the myosin molecule head swivels to cause filament sliding. The myosin molecule head, which is still attached to its site on the actin, can only detach itself if a new ATP molecule is available to be be bound. When muscle is converted to meat, myosin molecule heads remain locked to actin and even passive filament sliding is impossible.

13.3 Rigorometer


       
A rigorometer is a device for measuring the loss of extensibility as rigor mortis develops. The first rigorometer was developed in the late 1930s by one of the pioneer meat scientists (Bate-Smith) and was used extensively by another pioneer (Bendall). The Rigorometry gently loads the bottom of a hanging muscle strip and records the resulting extension. If a suitable load is placed on a muscle strip still containing ATP, the free end drops as the muscle stretches and, when the load is removed, the muscle returns to its original length. But when ATP is no longer available, the muscle is only very slightly extensible. Muscle strips are maintained in an anaerobic atmosphere to prevent aerobic surface resynthesis of ATP.     For a short time after exsanguination, ATP may be resynthesised from CP. After CP has been used up, the length of time before the occurrence of rigor mortis depends on the amount of glycogen available within the muscle and on the survival of glycolytic enzymes.

 

In the diagram above, the muscle strip (M) is loaded with a weight (W) as the platform (P) drops. Whether or not the muscle extends when loaded is detected by the transducer (T).


    A number of features appear on a rigorometer trace (as above). The delay period is when ATP is still being resynthesised, the rapid phase is when individual myofibres run out of ATP, and the final post-rigor phase is when a sufficient number of myofibres have set in rigor mortis to prevent any further extensibility. Sometimes, the muscle may slowly and weakly contract as it goes into rigor. The behavior of muscle strips in a rigorometer and of whole muscles in a carcass, is greatly affected by the condition of the animal at the time of slaughter. These differences may be seen on the rigorometer as follows.

 

(1) An animal calm during slaughter gives a long delay, a slow rapid phase, and a decrease in unloaded length at body temperature but not at room temperature.

 

(2) An animal struggling during slaughter gives a short delay, a short rapid phase, and a decrease in unloaded length at body temperature but not at room temperature.

 

(3) An exhausted animal gives an extremely short or non‑existent delay period, a very short rapid phase, and shortens its unloaded length at both body and room temperature.

 

(4) A starved animal gives a short delay period, a fairly long rapid phase, and a decrease in unloaded length at body temperature but not room temperature.

 

13.4 Electrical stimulation (ES)

13.5 Involvement of the nervous system.

Even in a simplified laboratory model of carcass stimulation, with a muscle strip and a pair of stimulatory electrodes, the response may be quite complex. Although stimulation may accelerate post mortem metabolism, muscle with an already accelerated rate of metabolism may lose its excitability at a faster than normal rate. Thus, animals with intrinsically fast glycolytic rates may be detected by their reduced electrical excitability.

Unless special precautions are taken to the contrary, muscle strips contain severed intramuscular nerves and neuromuscular junctions among the myofibres. Immediately post mortem, all three components may be excitable with their own particular activation thresholds and, as these change post mortem, it is difficult to identify the point between the axon and the myofibre responding first to ES.

The complete final common pathway from the spinal cord to the muscles survives for many minutes post mortem. 

The excitability of muscle strips decreases progressively post mortem so that either a higher voltage and/or a longer duration stimulus is needed to obtain a constant response, and it is likely that the initial loss of excitability is caused by fatigue in the excitation-contraction pathway. If neuromuscular junctions are pharmacologically blocked in samples taken shortly after animal exsanguination, excitability is decreased. This suggests  the high excitability of muscle strips at this time is caused by intramuscular motor axons and/or their neuromuscular junctions.

ES and Rate of Stimulation

Living muscles or strips taken immediately after animal exsanguination respond to a progressive increase in stimulus frequency by twitching at a correspondingly faster rate, until the twitches merge into a sustained contraction or tetanus, as shown below.

As the time between animal exsanguination and muscle stimulation is increased, muscle strips become progressively less able to maintain tetanus, as shown below.

Immediately after the excitation of axons and myofibres, there follows an absolute refractory period of complete inexcitability to a second stimulus since the response to the first stimulus is still in progress. Next comes a relative refractory period when, if the second stimulus is of sufficient magnitude, excitation may be elicited. The duration of the relative refractory period probably increases progressively post mortem. Many of the stimuli delivered at a high frequency may, therefore, arrive during a relative refractory period and elicit no response. For this reason, the stimulus frequency for ES of carcasses is usually kept low.

As shown in this example,

impulses arriving at 5 impulses per second produced a stronger effect (line goes up, indicating muscle contracted) than impulses arriving at 10 impulses per second.

The pH decline in meat post mortem may render meat less excitable to ES via the nervous system since a low pH reduces the amount of acetylcholine released at the neuromuscular junction. The accumulation of calcium ions by transverse tubules might also be involved in the decreased excitability of muscles after ES.

Red & White Muscle

White muscles have a greater response to ES than red muscles, because anaerobic myofibres respond more readily.

The higher rate of glycogenolysis in white muscles is caused by a high content of phosphorylase, greater activation of phosphorylase and a higher content of creatine phosphate.

The effects of ES on meat are not limited to the actual period of stimulation, but persist afterwards, perhaps because of changes in the sarcoplasmic reticulum. Electrical stimulation causes swelling of the sarcoplasmic reticulum, transverse tubules and mitochondria, together with autolytic ultrastructural changes. Increased binding of glycolytic enzymes to actin filaments also may be involved.

Red and white muscles also differ in the way in which temperature affects the activity of the sarcoplasmic reticulum.

ES and Current Pathway

Current pathway is difficult to assess in whole carcasses. In homogeneous conductors, resistance is proportional to the distance between electrodes, and is inversely proportional to conductor cross sectional area. In homogenous conductors, resistivity (resistance specific to conductor material) normally is measured between opposite faces of a one centimeter cube. However, not only are carcasses interrupted by tracts of fat with a high resistance and by bones with a variable resistance, but muscles themselves are electrically anisotropic (meaning the direction across which measurements are made affects the results). Resistivity is inversely proportional to meat temperature, it tends to be greater across rather than along myofibres, and it may show a transient increase post mortem followed by a progressive decline.

Resistance of a whole carcass is modified by factors such as,

Continuous DC currents only elicit strong stimulation when they are first applied, but interrupted DC currents of square wave impulses may be used to prolong this initial response. However, DC currents of any type soon cause Polarization at the electrode-tissue interface. Polarization increases resistance and decreases responses. A muscle that has almost ceased responding to unidirectional square waves may respond with original intensity if polarity is reversed, as shown below.

. To avoid polarization, square waves of alternating polarity may be used but apparatus to produce these at high voltage may be expensive. Thus, there are some advantages to using apparatus which supplies modified sine wave impulses derived from the regular 50 or 60 Hz commercial power supply. 

13.6 Summary

Further information

Structure and Development of Meat Animals and Poultry. Pages 553- 563.