Chapter 3. Slaughter, and

Biochemistry of Meat in pre and post-Slaughter

Meat technology

By: Mohammed Sabah

An-Najah National UniversityFaculty of Agriculture

At the time of slaughter, animals should be

healthy and physiologically normal. Slaughter animals should be adequately rested. They should be rested, preferably overnight, particularly if

they have travelled for some times over long distances. However, pigs and poultry are usually slaughtered on arrival as time and distances travelled are relatively short and holding in pens is stressful for them.

Animals should be watered during holding and can be fed, if required.

The holding period allows for injured and victimised animals to be identified and for sick animals to be quarantined.

Animals should never be beaten nor have their tails twisted.

When ready for slaughter, animals should be driven to the stunning area in a quiet and orderly manner without undue fuss and noise

1. Preparing livestock for slaughter

In pre-slaughter muscle tissue, as long as the animal is alive and therefore breathing, muscle contraction and relaxation can take place in an aerobic way in the presence of oxygen.

During muscle movement, the filaments actin and myosin slide into each other and chemically link and unlink, causing the muscles to contract as well as relax. Energy in the form of ATP is required to bind myosin into actin as well as to separate myosin from actin afterwards.

The ultimate source of energy, is a highly complicated process and ATP is normally present in muscle tissue at a level of around 5–6 µ mol per gram of tissue.

2. Biochemical processes in meat pre-slaughter

ATP hydrolyses easily to ADP, a single phosphate unit, and energy.

The chemical energy obtained is utilized for all energy-consuming processes within the living organism as well as for the movement of muscle fibres.

Excess glucose is stored in muscle tissue and in the liver in the form of glycogen as well as CP (creatine phosphate) and is converted into energy if required.

Availability and utilization of ATP break down the actomyosin complex, obtained during contraction, into the separate fibres of actin and myosin once again.

ATP can be quickly synthesized in such situations and CP binds quickly with ADP to form ATP (CP + ADP creatinine + ATP).

The formation of ATP whilst the animal is still alive and breathing can take place via the utilization of glucose, proteins or fat. All those materials can be transformed into acetyl coenzyme A (acetyl CoA), which is the substance that enters the citric acid cycle.

How a muscle contraction is signalled - Animation

Carbohydrates, however, are primarily utilized for rebuilding ATP and glycogen is readily broken down to glucose when required for energy. Fat or proteins are utilized for the formation of ATP when no more carbohydrates are available.

Example of cattle slaughtering lines_Part1 - YouTube

Process of obtining ATP under aerobic condition

Post-slaughter, chemical changes in muscle tissue due to the absence of oxygen result in a situation where actin and myosin are not present as separate fibres any longer but are bound together in the actomyosin complex.

Muscular sugar glycogen, present at the point of slaughter in muscle tissue, is converted anaerobically after slaughter into pyruvate, just as it is during glycolysis whilst the animal is still alive, as this step occurs anaerobically regardless of whether the animal is alive or dead; the absence (or presence) of oxygen does not play a role at this point. Furthermore, any remaining CP in muscle tissue at the point of slaughter is also used during post-mortem glycolysis to turn ADP into ATP under anaerobic conditions.

3. Post-slaughter (rigor mortis)

Rigor mortis is casued by the skelatel muscles contracting and then they are unable to relax, so they become fixed in place. Rigor mortis occurs a few hours after a person or animal.

The major difference between aerobic and anaerobic glycolysis is that the pyruvate obtained from glucose is not converted into acetyl CoA in anaerobic glycolysis as the oxygen required for this step is no longer available. As a result, no acetyl CoA enters the citric acid cycle to create reduced coenzymes such as NADH and FADH2, and oxidative phosphorylation also does not take place. Instead, pyruvate is reduced predominantly to lactic acid (lactate– and H+) under anaerobic circumstances and this is catalysed by the enzyme lactate dehydrogenase.

Anaerobic GlycolysisAerobic Glycolysis 

The level of glycogen in muscle tissue prior to slaughter is around 7–11 g per kilogram of muscle tissue and lactic acid is formed at an amount of 38 molecules per molecule of glucose in post-mortem glycolysis.The lactic acid formed is not transported back to the liver, as happens whilst the animal is still alive, and therefore the concentration of lactic acid within muscle tissue increases steadily after slaughter.

A tiny amount of ATP, however, is still obtained anaerobically post-slaughter. Only three molecules of ATP are obtained from one molecule of glucose during post-mortem glycolysis, compared with 36 molecules of ATP during aerobic glycolysis.

The effects of the three molecules of ATP obtained in an anaerobic state can be observed on the carcass as there is visible fibre movement even though the animal is already dead.

The pH value of meat at the point of slaughter varies depending on the type of animal but is generally between 6.8 and 7.2.

After slaughter, the pH normally drops to around 5.3 – 5.4

Upon completion of post-mortem glycolysis, the final pH value of red meat is slightly higher than that of white meat; WHY?red meat generally contains slightly less glycogen than white meat at the point of slaughter and, as a result, the pH value does not decline as much during rigor mortis.

The major difference between pre and post-slaughter

Figure 3: pH Scale 

If a red meat animal has a low glycogen level at slaughter the meat will have a higher pH and will be darker and harder to cook.

Lactic acid builds up in the muscle over a 16 to 24 hour period post-slaughter. A normal level of lactic acid (pH of 5.6) in the muscle will cause the meat to be bright cherry-red in color when it is exposed to oxygen for a short time period.

Upon completion of post-mortem rigor mortis, the pH value has dropped to around 5.3, and actin and myosin are present as the actomyosin complex, contrary to the situation when the animal was still alive (Table 1). WHC and solubility of muscular protein are greatly reduced as a result of those changes.

The major difference between pre and post-slaughter