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Post-mortem changes in muscle and its conversion to meat

33 important questions on Post-mortem changes in muscle and its conversion to meat

What can you tell about energy metabolism in muscles in living animals

Major function of muscles = contract
ATP = energy required for

Contraction
Maintaining functional integrity of muscle

Sarcoplasm & mitochondria enzyme systems ensure ATP supply to contractile elements
ATP required to fuel Ca pump of sarcoplasmic reticulum (SR)

What happens in the fed state and what in the fasting state of animals [energy metabolism in muscles]

In fed animals, levels of circulating FFAs are low and glucose is mostly used

In the fasting state FFAs are metabolised

Glycogen & glycose are broken down easily by the same process.

Which involves operation of three interrelated processes, namely

Glycolysis
Oxidative decarboxylation
Oxidative phosphorylation

Lead to complete oxidation of 1 molecule glucose by 6 molecules of oxygen to 6 molecules CO2 & 6 molecules water
C6H12O6 + 6O2 + 6H2O

Where takes glycolysis place?

Glycolysis takes place in sarcoplasm

Enzymes that catalyse other processes located in mitochondria

The mobilisation of glycogen (when needed)

Muscle glycogen needs to be broken down rapidity to release energy for muscle contraction

Phosphorylase activation catalyses 1st stage glucogenolysis

External stressor > adrenaline > adenylate cyclase > Cyclic AMP > Protein kinase > Phosphorylase b > Phosphorylase a > glycogen breakdown

How is energy stored [energy storage]

ATP stored engel as energy-rich phosphate bond
Energy liberated by releasing phosphate - reaction = hydrolysis & reversible
ADP can be hydrolysed to AMP
ATP - very low concentrations in tissue
Concentration maintained effectively constant by Lohamann reaction

CP + ADP <-> C + ATP

Creatine phosphate (CP) reacts with ADP to form ATP & creatine
Reaction is reversible but equilibrium is to right at neutral pH

How does acidification of muscles work after animal is killed?

Death - oxygen supply to muscles ceases when blood circulation stops
Metabolism --> anaerobic & ATP only regenerated through breakdown of glycogen by glycolysis (anaerobic glycolysis)
Glycogen = broken down --> LA accumulates as no longer removed by blood system --> muscle gradually acidifies
Unstressed animal - pH value falls from 7.2 to 5.5

pHu (ultimate) reached - differs between muscles & species
Pattern of acidification may also vary between muscles & species

Process of acidification takes
4-8h in pigs
12-24h in sheep
15-36h in cattle

If glycogen not limiting - LA proaction ceases when enzyme systems will no longer function at low pH

What are the two important characteristics of meat affected by acidification?

Water-holding capacity (WHC)
Colour

Influence of decrease pH on structure of muscle constituents
WHC decreases so that drip/exudate is lost - especially if muscle is cut
Meat relatively dark & translucent in living animal / recently dead to being paler & opaque

What is the effect of meat acidification on WHC?

± 3/4 of meat = water

Muscle cut after regor mortis (RM) - some water will be lost & contribute to exudate/drip (dilute solution of mainly sarcoplasmic proteins)

Drip formation

= function of WHC of meat

Drip lost when unpackaged

= weight loss

Collect in packaging

= aesthetically unattractive

There are three compartments that need to be recognised when talking about the effects on WHC, namely

Extracellular space outside muscle fibres

contain 5-10% of total water in muscle

Intracellular stage outside myofibrils

filed with sarcoplasm - contains sarcoplasmic proteins - creatine kinase & myoglobin

Space within myofibrils

contain 80% of water in muscle

Water is held in different ways within muscle (3)

± 10% of total water = bound very strongly to muscle proteins
Relatively large volume held by attraction to bound water
Smaller amount is "free" water held weakly

Muscle proteins denature as pH falls = reduced water binding power

How is water lost from proteins?

It is squeezed out from myofibrils into sarcoplasm increasing "extra-myofibrillar" volume

Diluted sarcoplasm fluid is expelled from muscle fibre and is collected in extracellular space (to flow out muscle)

When meat is cut: fluid exude on to cut surface --> eventually exudate may drive from meat surface

What is rigor mortis development

Variation between individual fibres lead to more gradual development of stiffness in whole muscle as more fibres become inextensible

ATP keeps muscle in relaxed state by preventing actomyosin formation
figure: Relationship between ATP depletion & the onset of rigor (with muscle extensibility)

Where is the rigor onset time related to?

Rigor onset time relate to factors affecting level of glycogen & CP at death & rate of post-mortem muscle metabolism

Stress & exercise
Rate of development reduced if carcass cooled quicker

Where is the rigor mortis determined by?

Is determined by ATP availability not with high pH of muscle

Possible to haver rigor in muscle when pH still high (proteins not denatured) if animal is exhausted pre-slaughter then you have alkaline rigor

Low pH: unsustainability of conditions (pH for enzymes) inhibits glycolysis - ATP concentration is not maintained

Concentration of ATP below which rigor develops varies in different muscles

What can you tell about meat quality and rigor mortis development?

After rigor a muscle can no longer shorten (be stretched) & sarcomere & muscle length is fixed
Sarcomere length is important for determining tenderness & toughness
After onset of rigor = no danger of cold or heat shortening or compromising meat texture

Temperature at which muscles fo into rigor affect the meat tenderness. What is the right temperature and why does this happens

Beef muscles that enter rigor at high temp. (>15*C & particularly >35*C) produce less tender meat vs meat that is going into rigor at low temperature (<15*C)

Partly due to heat shortening
Denaturation of proteolytic calpains (combination of high temp. & low pH)

What is rigor resolution? [explain the process of rigor resolution]

After time there will be progressive resolution of rigor when muscle softens
Resolution = misleading

muscle does not become extensible again as pre-rigor
Thick & thin filaments remain locked by myosin cross-bridges
Tenderisation not caused by filaments regaining ability to slide
Myofibrils structure begins to break down - region of z-disc

What factors determine the rate of tenderisation?

Varies with temp. & in different species

Faster at higer temp.
Every 10*C up = more than doubling of final tenderness achieved

Rate of tenderisation of meat from different species (image)

What is/are the tenderisation rate(s)

Different rates -> different recommended 'ageing' times
Keeping meat at refrigerated temp. = expensive

storage
refrigeration
weight loss

Compromised reached commercially to produce meat with acceptable tenderness in reasonable period of time

image

What can you tell about the ageing process and tenderisation rate?

Ageing process continues irrespective of size of joint or what stage of marketing chain meat has reached
Ageing time include time spent in distribution & retailing
Does not proceed in frozen state
Continue on thawing if normal ageing process was interrupted by freezing
Meat can be aged 'dry' in unpacked state or 'wet' in vacuum packaging

Tenderisation attributable to two types of processes [conditioning]

Changes in connective tissue components of meat
Weakening on myofibrils

Conditioning process occurs in two phases

Rapid phase - changes in myofibrillar component

most important
attachments of thin (actin) filaments to z-discs who some breakdown
Increase in amount water-soluble nitrogen compounds

Slower phase - structural weakening of intramuscular connective tissue

only small changes seen in major connective tissue components such as collagen

What is calpains & cathepsins

Tenderisation from activities of proteolytic enzymes in muscles
Normal role = breakdown & recycling of proteins - occurs continuously in all living tissues

Calpains

Though to be more important (at least in red meat species & poultry)

Cathepsins

May be more important in post-mortem degradation of fish muscle
possibly tenderisation meat kept at high temp.

Explain how calpains affect meat texture

Is activated by calcium ions
Max. Activity in neutral to alkaline conditions (pH 7.5)
Breakdown of myofibrillar component of muscle (troponin-T, Titian (connectin), desmin, actin & myosin)
Originally referred to as Ca-activated sarcoplasmic factor (CASF)
inhibited by endogenous protein calpastatin - high calpastatin activity reduces extent of proteolysis in muscles

Explain how cathepsins affect meat texture

Occur in lysosomes & sarcoplasm
probably released from lysosomes post-mortem
Max activity in mildly acidic conditions
Known to degrade

Troponin-T
some collagen cross-links
Mucopolysaccharides of connective tissue ground substance

Only degrade actin & myosin <pH5 - unlikely under normal conditions

Why is the meat from Bos taurus cattle more tender than the meat from Bos indicus?

Brahman = bos indicus
Angus = bos taurus

That has to do with the fact that proteolysis is greater in Bos taurus. And that has to do with the higher activities of calpains in the meat.
And also because cathepsins, which is the inhibitor, is actually higher in the Bos indicus, which prevents it from tenderisation as quickly as you have with the Bos taurus.

In the Bos indicus tenderisation occurs more slowly.

What are the two main forms of calpains?

m-calpain activated at high Ca ion concentrations
mu-calpains activated at low Ca ion concentrations

What is the key process of tenderisation?

Gradual breakdown of cytoskeleton framework which holds contractile elements (thick & thin filaments) attached & arranged within structure of muscle fibre

What can you tell about muofibrillar components that are broken down by calpains

One change that occurs in myofibrillar proteins during ageing of meat from all species = breakdown of troponin-T
Indicator of progress of conditioning process
Small amount of actin & myosin degraded too
Most breakdown occurs in porteins associated with z-disc & cytoskeleton - particularly Titian (connection) & desmin

What are possible actions of calpains post-mortem

After exhaustion of ATP & development of rigor mortis: membrane systems of SR & mitochondria no longer sequester Ca ions

released into sarcoplasm & bathe myofibrils

Increased Ca concentration activates mu-calpains allowing proteolysis
normally calpains inhibited by being bound to calpastatin
Calpain activity promoted by higher Ca levels (ca ions remove inhibition)
Calpain activity also promoted by higher pH & temp.
Enhance enzyme activity post-mortem by infusing carcass with 0.3 M CaCl
effect of temp = vital (image)

How can vitamin D3 be used to improve meat tenderness?

Vitamin D3 = fat soluble vit. Important for Ca & phosphorous metabolism --> incorporation into bone
Feeding cattle vit D3 --> increased beef tenderness under some conditions

probably by increasing blood & muscle Ca levels
Elevated Ca levels stimulating activity of calpains post-mortem

Beware!

May reduce feed intake & growth rate = commercially uneconomic
Very high dietary intake = toxic

Which characteristic of meat is affected by resolution of rigor?

Results in tenderisation

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Meat Science

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