lecture note: Conversion of Muscle to 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.

Post mortem pH decline -- Post mortem pH decline has a tremendous impact on the colour of meat -- whether normal, dark or light. The following describes some of the problems associated with meat if animals are stressed prior to slaughter:

Muscle pH problems

In living muscle, energy is stored as glycogen: glycogen to glucose to pyruvate

 

In dying muscle, lactic acid accumulates and lowers pH.

Within 24 hours after death

(1) glycogen -------> lactic acid

(2) muscle pH: 7.0 -------> 5.6 (because of lactic acid)

(3) muscle colour: purple changes to bright red or pink (pH 7.0 -------> 5.6)

pH also is important in determining the water-holding capacity of meat, the ability of meat to retain its water during application of external forces such as cutting, heating, grinding, or pressing. There are three locations of water found in meat: bound (charged hydrophilic groups on the muscle proteins attract water, forming a tightly bound layer), immobilized (has less orderly molecular orientation toward the charged group), and free (held only by capillary forces and their orientation is independent of the charged group). The following graph depicts the relationship between pH and water-holding capacity. Where water-holding capacity is the lowest is the isoelectric point, where the number of positively and negatively charged groups of the myofibrillar proteins is equal. Thus, the charges cancel out and no charge is available to hold the bound and immobilized water.

When the animal is stressed prior to killing, adrenalin stimulates the conversion of glycogen to lactic and pyruvate which enter the liver and Krebs cycle respectively to generate more ATP.

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	adrenaline
Glycogen	------------à	Lactic acid to liver and pyruvic acid to Krebs cycle

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·         Glycogen concentration goes down. Factors involved in this: exhaustion, exposure to cold, excitement, sex (bullock), sudden feed withdrawal, sickness, show steer.

Rigor mortis

Literal translation is "death stiffening". Rigor has three phases:

·         Delay phase -- while there is plenty of ATP in the muscle (complexed with Mg⁺⁺), the muscle will remain in the relaxed state and no cross bridges between the thick and thin myofilaments will occur.

·         Onset phase -- As stores of ATP and Creatine Phosphate (CP is used to re-phosphorylate 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 the entire CP is gone (which implies all stores ATP have been used up), the muscle has no way of regenerating ATP. Thus, full rigor mortis will set in.

·         The contraction and relaxation of muscle constantly requires ATP i.e. to release the actin and myosin filaments from strong binding, ATP is required.

·         In absence of ATP (which happens during post-mortem) the actin and myosin remain bound together as an actomyosin complex.

·         The above conditions lead to rigor mortis.

·         All possible measure should be adopted to ensure that minimum of actomyosin complexes are formed.

·         One of the common practices in the meat industry of hanging carcasses helps the muscles stretch by help of gravitational force.

·         The period before rigor mortis during which muscle is relatively extensible and elastic is called the delay phase.

·         Completion of rigor mortis is signalled by total depletion of creatine phosphate and other sources of rephosphorylation.

Few hours after a person or animal dies, the joints of the body stiffen and become locked in place. This stiffening is called rigor mortis. Depending on temperature and other conditions, rigor mortis lasts approximately 72 hours. The phenomenon is caused by the skeletal muscles partially contracting. The muscles are unable to relax, so the joints become fixed in place.

More specifically, what happens is that the membranes of muscle cells become more permeable to calcium ions. Living muscle cells expend energy to transport calcium ions to the outside of the cells. The calcium ions that flow into the muscle cells promote the cross-bridge attachment between actin and myosin - the two types of fibres that work together in muscle contraction. The muscle fibres ratchet shorter and shorter until they are fully contracted or as long as the neurotransmitter acetylcholine and the energy molecule adenosine triphosphate (ATP) are present. However, muscles need ATP in order to release from a contracted state (it is used to pump the calcium out of the cells so the fibres can unlatch from each other). ATP reserves are quickly exhausted from the muscle contraction and other cellular processes. This means that the actin and myosin fibres will remain linked until the muscles themselves start to decompose.

Rigor mortis can be used to help estimate time of death. The onset of rigor mortis may range from 10 minutes to several hours, depending on factors including temperature (rapid cooling of a body can inhibit rigor mortis, but it occurs upon thawing). Maximum stiffness is reached around 12-24 hours post mortem. Facial muscles are affected first, with the rigor then spreading to other parts of the body. The joints are stiff for 1-3 days, but after this time general tissue decay and leaking of lysosomal intracellular digestive enzymes will cause the muscles to relax. It is interesting to note that meat is generally considered to be tendered if it is eaten after rigor mortis has passed.

(1) What happens to meat when an animal undergoes a long term and short term stress

prior to slaughter? 

(2)  What are the three different phases of rigor and what the roles of ATP and CP are in

prolonging the delay and onset phases?

From the sequence of events below, it becomes clear that the fall in pH and rise in temperature are both results of the same events.  This is one critical point in conversion of muscle to meat where important measures are required to be taken. If not attended to will lead to —

• If pH continues to fall it will reach a range where the cathepsins get activated and lead to proteolysis.
• A normal pH drop should be from pH 7 to 5.6 - 5.7 in 6 - 8 hrs post mortem and to an ultimate pH in range 5.3 - 5.7 in 24 hrs post mortem.
• In some animals it will remain 6.5- 6.8 during the first hour after exsanguination.
• An early accumulation of lactic acid is linked with rise in carcass temperature which leads to denaturation of proteins. The level of denaturation of proteins is species specific with pork being more susceptible than beef.
• Denaturation will cause –

i)          Loss of protein solubility

ii)        Loss of water holding capacity

iii)       Loss in the intensity of muscle`s pigment coloration

i+ii + iii = pale muscle with highly wet surfaces. 

All above needs to be monitored so that conversion of muscle is guided through regulated values to become meat. This can be done by understanding the status of the animal subjected to slaughter and by proper handling and cooling of the carcasses.

Animal slaughtered

Exsanguination

Blood pressure drops
(In an attempt to maintain the blood pressure and ensure blood supply to vital organs)

i) Heart pumping increases
ii)Peripheral vasoconstriction

Results in stoppage of nutrient supply to muscle and removal
of waste products  and loss of oxygen supply to muscle. Temperature
of carcass rises due to failure of temperature control mechanism.

Myoglobin stores little oxygen which is sufficient for
short period as a result stored oxygen is depleted.

The above sets in anaerobic metabolism
leading to production of lactic acid.

The energy so produced is less and is only sufficient
to maintain structural integrity and temperature.

Lactic acid produced as an end product
of anaerobic metabolism can not be removed
and is thus stored in muscles

The above leads to fall in pH
( the final pH would depend on amount of glycogen in the muscle at the time of exsanguination )

As all homeostatic mechanisms have failed by
this stage means that all defence mechanisms including
those which protect a live animal against microorganisms are lost .
As a result muscle becomes susceptible to microbial invasions.

 Post-mortem changes

 Colour

• The colour of muscle in living animal is bright red due to abundance of oxygen.
• Post-mortem there is shortage of oxygen and colour becomes dark purplish red.
• Fresh meat when cut has dark red colour but on exposure to atmospheric air within few minutes changes to brighter red colour. This happens due to oxygenation of myoglobin.

Protein

• The protein solubility changes due to denaturation caused by low pH and high temperature.
• Proteolytic breakdown is limited.

Water binding

• Due to fall in pH and denaturation of proteins the water binding capacity decreases.

Factors affecting post-mortem changes

1. Stress

Any external or internal stress activates the homeostatic mechanism which generates variable physiological responses so as to tide over the stress symptoms. These are executed through hormones. Most important are –

• Epinephrine – breaks down glycogen stored in liver & muscles, also breaks down fat.
• Epinephrine & norepinephrine – maintains the blood circulation
• Adrenal hormones – provide stress resistance.
• Thyroid hormones - increase metabolic rate.

2. Environmental effects

• Temperature – Too low a temperature to which an animal is not acclimatized will lead to all such changes which generate and conserve heat like shivering, higher rate of metabolism etc. Too high a temperature will not allow an animal to dissipate body heat leading to ATP splitting and glycolysis.
• Humidity – High levels of humidity add to discomfort stress in a cold environment it increases heat loss and in a hot one it makes heat losses difficult.
• Light , sound, and space –
• Darkness – stresses the animal through the efforts made by it to go towards light.
• Less space – not providing an animal free mobility causes stress.
• Unfamiliar sounds – frighten the animal and cause stress. 

Abnormal Post mortem changes

• Different types of stresses cause different of effects on the animal.
• Some of the stresses demand more from activity the muscle requiring more energy use which aerobic metabolism cannot meet fast enough.
• As a result the anaerobic metabolic pathway may also be activated.
• This may lead to generation of lactic acid and if the build is too large it may cause acidosis.
• In pork it is called "porcine stress syndrome"(PSS) which may lead to death.

Read on Pale, Soft, Exudative (PSE) and DFD (Dark, Firm and Dry)