3.1 - Postmortem Changes in Muscles

Muscle will not become meat if it doesnt:

go through the post mortem process

Muscle =

tissue in live animals

Meat =

for consumption

4 major factors influencing homeostasis (muscle) =

• Oxygen concentration

• Metabolism

• pH

• Temperature

• Energy

Rigor mortis =

indicates the end of the postmortem process

Homeostasis = 

In the living state, all organs and systems within the body interact to maintain an environment under which each can perform its functions efficiently

Homeostasis has a Narrow range of physiological conditions for:

• Oxygen concentration

• pH

• Temperature

• Energy supply

Maintenance of this physiological state is termed:

homeostasis

Removal of blood or ___ marks the beginning of postmortem changes

exsanguination

Postmortem – Oxygen Concentration

• Drop in blood pressure

• Homeostatic control mechanisms kick in

What Homeostatic control mechanisms kick in?

• Protective/survival mechanism to compensate

• Increase in heart rate to maintain circulation

• Peripheral vessels constrict to prioritize blood flow

• 50% of remaining blood redirected to vital organs

hemoglobin =

protein in blood and biggest function is to transport oxygen from the blood into muscle tissue

Since blood goes to sustaining vital organs:

muscle doesn't get much blood and doesn't have oxygen supply

Muscle NOT converted to meat until:

metabolism stops

• no oxygen = oxidative phosphorilation shuts down

Oxygen supply depleted:

TCA cycle and electron transport failing

Homeostatic maintenance: shifting to

anaerobic metabolism

• no matter if it's type 1 or type 2 tissue, it'll all switch to anerobic metabolism

Anaerobic =

• When lacking oxygen

• Mostly glycolysis

• Less energy-efficient

• Accumulation of lactic acid

Anaerobic metabolism continues until:

glycogen stored in the muscle is depleted

• glucose can be easily use, but glycogen needs to be broken down before it can be used

• you have a lot more glycogen in the body than glucose

Decline of muscle pH =

• Accumulation of lactic acid

  • Greatly variable rate of decline

  • lactic acid production varies by muscle fiber type and animal species type and this in turn affects the rate of ph decline

• Greatly variable rate of decline 

Normal decline pattern = 

• Gradually from 7.0 to 5.7 within 6 to 8 h postmortem 

  • this refers to large animals like pigs and cattle

  • this process happens a lot faster in chickens

• Ultimate pH: 5.5 within 24 h postmortem

WHAT ELSE CAN IMPACT PH CHANGE?

• when ATP is produced from ATD, heat and hydronium ions are generated

  - negative log of H+ concentration is pH

  - more H+ ions generated = decrease in pH

Is it really just anaerobic metabolism?

even though you cannot continue oxygen supply to the muscle, there is still a little bit of aerobic happening because of the small bit of oxygen leftover in the muscle

To improve water holding capacity:

salt or another emulsifyer needs to be added before that meat can be sold

Why does muscle temperature change?

• Rate of metabolism (heat production)

• Size and location of muscles – large vs. small?

• Predominate muscle fiber type – oxidative vs. glycolytic?

• Duration of metabolism – long vs. short?

• Fat insulation – thick vs. thin or fat vs. lean animals

• Slaughter: handling, stress, stun/stick

How does temp change? – briefly increasing postmortem – WHY?

• this inc is because when the muscle is trying to produce atp, it generates a lot of heat

• when heat is generated in a living animal, the heat can be dissapated throughout the body

• after death, muscle is still trying to generate atp and that heat will be localized where the atp will be reduced

more glycogen =

metabolism lasts longer

oxidative =

energy efficient — this generates less heat then glycolytic process

Gly =

only 2 ATPs but a lot of heat

thick layer of fat prevents:

• heat from coming out

  - ex. thick back fat in pigs

  - the muscle under that layer of fat will have a higher temp

stress =

generate more atp & this generates more heat

rate of temp dec =

depends on size of carcass

- large carcass = longer time of temp dec

Temp inc only lasts:

a very short time

Consequences of pH decline & temp inc:

• Microbial growth

• Protein denaturation

• Oxidation

  • because enzymes work better at higher temp

Why is energy depleted?

oxidative phosphorylation stops so ATP production stops a lot

Two sources of ATP =

• Glycolytic/anaerobic metabolism

• Creatine phosphate

• both of these methods are not energy efficient and don't produce much atp

How is ATP used?

• Muscle contraction

• Metabolism

Muscle contraction atp use:

to attach to myosin heads to get the heads to detach from the actin — this needs to happen for the muscle to relax

What is the consequence of energy depletion?

Rigor sets in when ATP depletes

• Rigor mortis = when ATP runs out

• this means actin and myosin heads stay together and don't separate

Rigor Mortis:

“stiffness of death”

RM Delay phase =

immediately after exsanguination

RM onset phase =

within hours, species-dependent

• ATP is still being generated using glycogen

RM completion phase =

within 24 h, species dependent

What happens at resolution of rigor?

- tension slowly dec

- dec tension & inc extensibilty = this happens because of enzymatic activity

What happens at rigor completion?

- almost 100% of binding sites (myosin and actin binding sites) are used

- muscle is contracted to length of sarcomere is shortest

- muscle tension will be the largest because sarcomere is shortest and muscle stays at contracted status

- the actomyosin bonds will never separate and they'll stay like that

Smaller the animal:

shorter the delay time before onset of rigor mortis

Delay time before onset of rigor mortis: beef

6-12

Delay time before onset of rigor mortis: lamb

6-12

Delay time before onset of rigor mortis: pork

¼ - 3

Delay time before onset of rigor mortis: turkey

<1

Delay time before onset of rigor mortis: chicken

< ½

• this happens in about 15 minutes

Delay time before onset of rigor mortis: fish

<1

Rigor Mortis vs. Normal Contraction

• Rigor: 100% binding sites used

• Normal contraction: 20% binding sites used

Resolution

decrease in tension with time postmortem

• Actomyosin bonds are NOT broken

What is the aging process?

• aging = mainly to improve the tenderness of the meat

• beef carcass esp needs to be aged -> 7-21, sometimes up to 28 days

• not just desmin, but the other proteins are also degrated over time, like titin and nebulin

Changes in muscle structural integrity:

Subtle degradation right after exsanguination

Most notable change in muscle integrity:

48-72h post-mortem

During rigor resolution, which proteins at the Z disk are degraded?

• Nebulin

• Titin

• Desmin

• Vinculin

• Calpains

REMEMBER: which proteins are NOT degraded in aging?

actin & myosin

What causes resolution of rigor?

Enzymatic activities

Postmortem proteolytic degradation of myofibril proteins: Calpains

enzyme system = calpain system -> calcium dependent enzymes

Calpain system =

2 calcium-dependent enzymes and a specific inhibitor

• m-calpain

• μ-calpain:

• Calpastatin

m-calpain:

millimolar (high) calcium conc

• degrading the proteins degrades the integrity of the sarcolemma and the sr and this then releases a lot of calcium

• that is when this calpain comes into work as well

μ-calpain:

micromolar (low) calcium con

• this starts to work when only a little bit of calcium is released

Calpastatin: what is it?

• you can't let the calpain work all the time

• this is a calpain inhibitor

• this is especially important when the animal is alive

• does't really work much in dead animal

• calpain works until calcium is unavailable in dead animal

The calpain system is:

Activated by Ca++ released from mitochondria and SR during storage

• calcium is stored in sarcolemma

• during postmortem, you don't really lose much calcium

Failure of protective mechanisms = 

• Connective tissues: slightly degraded (these tissues have some protein)

• Cell membranes: slightly degraded – why this is important to meat quality?

• Lymphatic system: fail – why this is important for meat safety?

• Circulating white blood cells

Microbiological risks (not just because temperature increase, what else) =

• Spread, Proliferation, Contamination

• a lot of time it's because of the processing protocols or practices

  • this is why ssops and HACCP is really important

  • also carcass needs to be maintained at a low temp to reduce most bacterial growth

Proteolysis begins:

muscle degradation, tenderness