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 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
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Where takes glycolysis place?
- Enzymes that catalyse other processes located in mitochondria
The mobilisation of glycogen (when needed)
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?
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?
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
- 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?
- Stress & exercise
- Rate of development reduced if carcass cooled quicker
Where is the rigor mortis determined by?
- 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
- 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?
- 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)
- 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?![]()
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?
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?
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