1ASSIGNMENT 1 CFB 20102 FOOD CHEMISTRY (THE FORMATION OF RIGOS MORTIS IN MEAT )

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.

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 developed the

Muscle cannot shorten.

2ASSIGNMENT 1 CFB 20102 FOOD CHEMISTRY (THE FORMATION OF RIGOS MORTIS IN MEAT )

DEFINITION OF RIGOS MORTIS

Rigor mortis (Latin: rigor "stiffness", mortis "of death")

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.

3ASSIGNMENT 1 CFB 20102 FOOD CHEMISTRY (THE FORMATION OF RIGOS MORTIS IN MEAT )

THE EVENTS THAT OCCUR DURING RIGOR MORTIS FORMATION.

4ASSIGNMENT 1 CFB 20102 FOOD CHEMISTRY (THE FORMATION OF RIGOS MORTIS IN MEAT )

PHASE OF RIGOS MORTIS

Involved three main phase which is ;

Delay phase — while there is plenty of ATP in the muscle (complexed with Mg++), the muscle

will remain in the relaxed state and no crossbridges between the thick and thin myofilaments will

occur.

Onset phase — As stores of ATP and Creatine Phosphate (CP is used to rephosphoryate ADP to

ATP) are used up, rigor bonds between the thick and thin myofilaments are formed. As more

bonds are formed, the muscle loses extensibility.

Completion — When all of the CP is gone, the muscle has no way of regenerating ATP. Thus,

full rigor mortis will set in.

Muscle ATP, a high energy compound used by muscle to perform work, content directly

relates to muscle extensibility (the ability to contract and relax). In pre-rigor muscle where ATP

is high, the tissue is soft, extensible, and reacts like living muscle. When the ATP content

declines to critically low levels after death, the tissue will become firm and inextensible.

It is extremely important to know that during the delay phase of rigor (Fig. II-6), muscle is

uniquely susceptible to environmental conditions, some of which may be good, some bad. For

example, carcasses can be electrically stimulated during slaughter to speed the onset of rigor - a

desirable event in beef and lamb, where as electrical stimulation of pork carcasses usually lowers

pork quality. If pre-rigor muscle is frozen or hot-boned before any rigor onset has occurred, the

consequences can be very undesirable. The importance of timing of some processing conditions

cannot be overstressed.

5ASSIGNMENT 1 CFB 20102 FOOD CHEMISTRY (THE FORMATION OF RIGOS MORTIS IN MEAT )

Figure II-6. Rigor Mortis as a Function of Meat Chemistry and Time The timing of the

stages of rigor mortis varies between species. Poultry is often in rigor within 1-2 hours, pork

within 4 to 6 hours, beef and lamb within 7 to 15 hours after exsanguination.

The time line from start to onset to completion of rigor mortis for carcasses with normal

quality and pH decline is illustrated in Fig. II-6. Once the ATP reaches a critically low level at

about 1/2 of the original value in a resting muscle at higher temperatures and 1/3 at less than 15°

C (the ATP arrow), then the muscle starts to get stiff and pre-rigor processing can start with less

likelihood of toughening due to shorter than normal sarcomeres. During the decline in ATP

levels at the onset (arrow) of rigor mortis and when the ATP concentration is about 1/3 to 1/2 of

the original ATP level, some calcium starts to leak out of the sarcoplasmic reticulum (SR). The

elevated calcium stimulates muscle contraction and the sarcomeres shorten to the extent that the

skeleton will allow. The pH is gradually declining due to the increase of lactic acid. In rested

animals, the creatine phosphate (CP) helps immediately to regenerate muscle ATP when there is

a need to maintain high levels. However, many animals have little or no CP in muscles at the

time of slaughter because it has been used up during animal transport and lairage. Therefore,

ATP must be generated from another source of energy production.

Glycogen is the major storage form of energy in muscle and is metabolized to help

supply ATP, thus keeping meat in the pre-rigor state. Animals with lower than normal levels of

6ASSIGNMENT 1 CFB 20102 FOOD CHEMISTRY (THE FORMATION OF RIGOS MORTIS IN MEAT )

muscle glycogen at the time of slaughter will have a shorter delay phase and go into rigor faster

than normal (see DFD pH decline curve, Fig. II-4).

If carcasses have an accelerated pH decline (as with severe PSE) due to the rapid

production and consumption of ATP, the accumulation of lactic acid is more rapid and rigor

onset is much faster (less than 1 hr to 4 hr). Unfortunately, the meat temperature is often still

high, which facilitates higher than normal denaturation of protein.

Sarcomere shortening can be counteracted by adding salt to the meat before rigor onset to

the meat to partially inhibit rigor onset, thus improving WHC. Alternatively, pre-rigor excision

could be delayed until the onset of rigor where shortening would not be greater than normal.

At the bottom of Fig. II-6 is three sarcomeres representing the contractile states of the

muscle at the beginning, onset, and completion of rigor. The left, prerigor sarcomere can still

contract and relax, has a limited number of cross bridges, and is very sensitive to some meat

processing conditions. The center sarcomere is shorter indicating that the muscle is shortening

just as far as the skeletal restraint will allow. Calcium is leaking out of the SR (the sarcoplasmic

reticulum) and stimulates muscle contraction and formation of cross bridges between muscle

filaments. The sarcomere on the right represents the final contractile state of post-rigor muscle

where the number of cross bridges between the filaments is greatest, the distance between

muscle protein filaments if the lowest, and the meat is the least tender. Rigor, therefore,

decreases WHC because of less total open space for entrapment and holding of water.

The time muscle remains in the sensitive delay phase of rigor depends upon the chill

room conditions (temperature, air movement, carcass spacing, etc), the fatness and total mass of

the carcasses, muscle location within a carcass, and the type of muscle cells (fibers) within a

muscle. In addition, the state of the animal before death has major impacts on meat quality.

Exhausted or highly excited animals go into rigor faster than rested animals because their total

energy supply (creatine phosphate and glycogen) is less and the critical level of ATP needed to

delay rigor is soon reached. Meat from excited animals at slaughter is very likely to exhibit some

degree of PSE, less WHC, and may have greater spoilage potential (due to its wet surfaces) than

normal pH meat. Meat from exhausted animals at slaughter will have a higher pH and WHC than

7ASSIGNMENT 1 CFB 20102 FOOD CHEMISTRY (THE FORMATION OF RIGOS MORTIS IN MEAT )

normal but often has some undesirable quality characteristics (e.g., its darker color and greater

susceptibility to microbial spoilage (due to the higher pH) even though it is just as wholesome

and nutritious compared with meat with a normal rigor mortis pattern. These undesirable muscle

properties result from the interaction between timing of the events, muscle pH, and carcass

temperature; all three of which ultimately affect WHC.

8ASSIGNMENT 1 CFB 20102 FOOD CHEMISTRY (THE FORMATION OF RIGOS MORTIS IN MEAT )

FACTOR RELATED TO THE CONVERSATION OF MUSCLE OF MEAT

Homeostasis — Maintenance of a physiologically balanced internal environment. This includes

pH, temperature, oxygen concentration, and energy supply. In the living animal, this is an

important process that is activated quickly after death.

Immobilization and Exsanguination — Immobilization is where the animal is rendered

unconscious prior to bleeding and exsanguination is where blood is removed from the animal.

Postmortem pH decline — Postmortem pH decline has a tremendous impact on the color of

meat — whether normal, dark or light. The following describes some of the problems associated

with meat if animals are stressed prior to slaughter:

9ASSIGNMENT 1 CFB 20102 FOOD CHEMISTRY (THE FORMATION OF RIGOS MORTIS IN MEAT )