Download in the meat

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University of Baghdad
College of Veterinary Medicine
Dept. of Vet. Public Health/ Meat Hygiene Division
Meat Hygiene Course/ 5th. Year
2015-2016
Theory Lecture #: 4
2nd.Semester
Muscle Contraction
 Many events happen during contraction to allow for cooperative action of
individual sarcomeres to generate movement.
 Some of the events are important in the understanding of the conversion of
muscle to meat.
These are:
 The first step in contraction is the transfer of the nerve impulse from the brain
to the muscle.
 Through many hormonal and chemical changes an impulse reaches the
organelle that stores calcium in the muscle cell.

This causes the release of calcium into the sarcoplasm.
 The calcium interacts with regulatory proteins in the myofibril to allow
crossbridges to form between the main contractile proteins, myosin and actin.
 The calcium also activates enzymes that start energy metabolism.
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 This allows for coordination of energy metabolism with muscle contraction.
The high energy phosphate compound, adenosine triphosphate (ATP).
 ATP is hydrolyzed to create the power stroke of contraction and causes the
thick filaments to move past the thin filaments and shorten the sarcomere
 Many sarcomeres shortening together are what causes contraction in the
muscle.
 To break the crossbridge formed between myosin and actin, ATP must be
present.

If energy is depleted and no ATP can be manufactured from glycogen than no
relaxation of the muscle occurs.
 If muscle is working slowly, and oxygen is supplied in adequate amounts,
aerobic metabolism and CP breakdown can adequately supply most of its
energy requirements.
 However, when muscle is contacting rapidly, its oxygen supply becomes
inadequate for support of ATP re-synthesis via aerobic metabolism.
 Under these conditions of oxygen shortage, a third mechanism, anaerobic
metabolism, is able to supply energy for a short time.

A major feature of anaerobic metabolism is accumulation of lactic acid.
 The amount of energy available in this anaerobic route is limited.
 Lactic acid accumulation in the muscle lowers its pH, and at pH values of less
than 6.0 to 6.5, the rate of glycolysis is drastically reduced, with a proportional
reduction in ATP re-synthesis. Under these conditions, fatigue develops quite
rapidly.
 During muscle's recovery from fatigue, lactic acid that has accumulated is
transported out of the muscle via the blood stream, and is converted to
glucose in the liver or metabolized to carbon dioxide and water by the heart
(via a specialized enzyme system).
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 ATP and CP, the energy stores, are replenished by the process of normal
aerobic metabolism.
 The recovery process may occur quite rapidly for a slight fatigue, but may
require extended periods if the fatigue is severe.
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• In living muscle, energy is stored as glycogen: glycogen to glucose to pyruvate
Conversion of Muscle to Meat ( Rigor mortis )
Changes of pH
1 Immediately post-mortem (post slaughtering) the muscle contains a small
amount of muscle specific carbohydrate, called glycogen (about 1%), most of
which is broken down to lactic acid in the muscle meat in the first hours (up to
12 hours) after slaughtering.
 This biochemical process serves an important function in establishing acidity
(low pH) in the meat.
 2 The so-called glycolytic cycle starts immediately after slaughter in the muscle
tissue, in which glycogen, the main energy supplier to the muscle, is broken
down to lactic acid.
 The buildup of lactic acid in the muscle produces an increase in its acidity, as
measured by the pH.
 The pH of normal muscle at slaughter is about 7.0 but this will decrease in
meat.
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 In a normal animal, the ultimate pH (expressed as pH24 = 24 hours after
slaughter) falls to around pH 5.8-5.4.
 The degree of reduction of muscle pH after slaughter has a significant effect on
the quality of the resulting meat.
 3 The pH is important for the storage life of meat. The lower the pH, the less
favourable conditions for the growth of harmful bacteria.
 Meat of animals, which had depleted their glycogen reserves before
slaughtering (after stressful transport/handling in holding pens) will not have a
sufficient fall in pH and will be highly prone to bacterial deterioration
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Meat colouring
1 The red pigment that provides the characteristic colour of meat is called
myoglobin.
 Similar to the blood pigment haemoglobin it transports oxygen in the tissues
of the live animal.
 Specifically, the myoglobin is the oxygen reserve for the muscle cells or
muscle fibres.
2 Oxygen is needed for the biochemical process that causes muscle contraction
in the live animal.
 The greater the myoglobin concentration, the more intense the colour of the
muscle.
 This difference in myoglobin concentration is the reason why there is often
one muscle group lighter or darker than another in the same carcass
3 Myoglobin concentration in muscles also differs among animal
species. Beef has considerably more myoglobin than pork, veal or
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lamb, thus giving beef a more intense colour.
 The maturity of the animal (age)also influences pigment intensity, with older
animals having darker pigmentation.

 Under normal circumstances the colour of meat is in the range of red and may
differ from dark red, bright red to slightly red.
 Meat deriving from different species of animals may have rather different
colours, as can easily be seen when comparing beef, and poultry meat.
 The natural colour of fresh meat, except poultry meat, is dark red, caused by
the muscle pigment, myoglobine.
 Fresh meat surfaces which have been in contact with the air for only a short
period turn into a bright red colour because of the influence of the oxygen in
the air.
 Oxygen is easily aggregated to the myoglobin and drastically changes the
colour of the meat surfaces exposed to it.
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 On the other hand, in the absence of oxygen, for example in meat cuts
packaged in impermeable plastic bags, meat surfaces remain or become dark
red again.
 The same conditions generally prevail in the interior of meat cuts which are
not reached by oxygen.
 Changes from dark red to bright red are therefore typical and are normal
reactions of fresh meat.
 Meat which is in the process of losing its freshness, however, no longer shows
a bright red colour, even when intensively exposed to the air, because of the
partial destruction of the red meat pigment which results in a grey, brown or
greenish colour.
 Once these conditions occur the consumer has to decide, after carefully
checking the appearance, together with testing smell and taste, whether the
meat has to be discarded as a whole or whether use can be made of some
parts which so far have not been altered.
 Remarkable changes in the meat colour occur when fresh meat has been
boiled or cooked. It loses its red colour almost entirely and turns to grey or
brown. The reason for this is the destruction of the myoglobine through heat
treatment.
Dr. Zuhair Ahmed
MSc & PhD
Meat Science & Hygiene