Muscle to Meat

Reminder: describe what makes up muscle fibers.

**ATP is required.

1. Excitation-Contraction Coupling

• Nerve Impulse: An action potential arrives at the neuromuscular junction, releasing the neurotransmitter acetylcholine (ACh).

• Membrane Depolarization: ACh binds to receptors on the muscle fiber membrane, sending an electrical wave down T-tubules.

• Calcium Release: This electrical signal triggers the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum into the cytoplasm.

2. Cross-Bridge Formation

• Uncovering Binding Sites: Calcium ions bind to troponin C on the thin actin filament.

• Shifting Tropomyosin: This binding causes tropomyosin to move, exposing the myosin-binding sites on the actin filament.

• Attachment: Energized myosin heads (holding ADP and inorganic phosphate) attach to the exposed sites on actin, forming a cross-bridge.

3. The Power Stroke and Sliding

• Power Stroke: The myosin head releases the phosphate and ADP, snapping backward and pulling the actin filament toward the center of the sarcomere.

• ATP Detachment: A fresh molecule of ATP binds to the myosin head, causing it to detach from actin.

• Re-energizing: ATP is split into ADP and phosphate (ATP hydrolysis), resetting the myosin head into a high-energy "cocked" position to repeat the cycle.

Rigor Mortis - no ATP available, myosin heads bind with actin, do not unbind, cross bridge cycle stays in place which is what causes the rigidity

What is the first way ATP is generated?

1. Creatine phosphate - in resting muscle, excess ATP transfers its energy to creatine = energy reserve. When muscle first starts to contract it can release ATP for about 15 secs of energy

• Energy Depletion: During high-intensity, short-duration activities (like sprinting or heavy lifting), muscle cells rapidly consume adenosine triphosphate (ATP), breaking it down into adenosine diphosphate (ADP) and an inorganic phosphate.

• Phosphate Transfer: The enzyme creatine kinase takes a high-energy phosphate group off creatine phosphate and transfers it directly onto an ADP molecule.

• Result: This single reaction converts ADP into ATP and leaves behind free creatine:

What is the second way ATP is generated?

2. Glycolysis - anaerobic process to break down glucose to ATP and pyruvic acid. Converted to lactic acid when oxygen levels are low.

• Remember, is an anaerobic process, glucose to ATP and pyruvate, when oxygen is low, lactic acid is formed

What is the third way ATP is generated?

3. Aerobic Respiration - Breakdown of glucose in presence of oxygen to carbon dioxide, water and ATP = 95% of ATP required

What happens when an animal is slaughtered?

1. Circulation Stops

   1. Aerobic → anaerobic since oxygen requirements go down (Lack of blood flow and oxygen), cells consume ATP faster than it can be produce which causes lactic acid to built up

   2. Aiming for pH down to 5.5, above this we get meat quality defects

      1. At lower pH, have increased water content compared to a higher pH

   3. As body become more acidic, certain enzymes become active (Autolytic) → break down muscle fibers leading to proteolysis which breaks down the myofibrillar protein (Z plates)

   4. This causes tenderization

   5. Releases AA and peptides, reacts with sugars, changes flavors, increases tenderness

1. Death → muscle becomes tender → rigor (Rigor mortis) → process below occurs (Conditioning → returns to tenderness)

1. Oxygen Intake Stops

BUT body tries to maintain homeostasis

What pre-mortem characteristics might affect tenderness and meat quality?

• Breed

• Genetics

• Energy level of diet

• Type of muscle

• Single muscle or double muscle

• Stress level

What features of the post mortem period can improve meat quality?

•Storage conditions

• Temperature, humidity, airflow

• Length of time ageing - longer = more tender but need good aging facilities

•Processing conditions

• Tenderizing techniques - enzymes and certain blades are used

• Speed - how quickly they are processes, too slowly - spoilage

What is DARK FIRM DRY (DFD) meat?

• Caused by chronic stress more long-term (Loading, travel, periods without food)

• pH in 12-24h post-mortem is 6.0 to 6.8 (target pH 5.4-5.5)

• Glycogen stores depleted PRIOR tensing ataximuscle

• Increased metabolic rate due to cortisol release - depletes glycogen in muscle, not enough glycogen for lactic acid production as has been used up well before, lactic acid is by-product of glycogen use → Less glycogen = less lactic acid = reduced pH drop → Proteins stay intact, water-holding capacity stays intact, intracellular water reflects less light

• DFD -  High pH results in relatively little denaturation of proteins, water is tightly bound (Due to high pH) and little or no exudates is formed so it is very dry. This is because there is little or no shrinkage of the myofilament lattice and the differences in refractive index of the myofibrils and sarcoplasm are reduced so it appears very dark red. 

• Is a welfare issue

Economically, why does DFD meat matter?

• Consumers eat with their eyes! - do not find it as attractive, less likely to buy

• Reduced shelf life

• Microbes can grow more easily due to increased water retention and decreased of spoilage competing bacteria that use glycogen

• Not as good of flavour

• Proteins can't break down, not as tender

• Lactic acid cannot interact with the by-products to create flavour

What is PALE, SOFT, EXUDATIVE (PSE) meat?

• Acute stress more short-term (Electric goads to make animals move , prolonged lairage time, fighting before stunning)

• Typically a problem in pigs, not ruminants

• pH in 45m post-mortem is <6 (pH~4 *target pH 5.4-5.5)

• Animal in acute stress, high adrenaline, often very hot due to high metabolic rate, Glycogen stores depleted FASTER after slaughter while carcase is hot

• Low pH and high temperature denatures proteins quickly → Causes decrease in water holding capacity → Increased exudate on cut surtace, all the water seeps out, increased exudate on the surface, increased light reflection giving pale appearance

• Meat shrinks a lot after cooking due to fluid loss

• PSE -  Rapid glycolysis also generates signifcant amounts of heat with increasing carcass temperature. In combination with low pH early postmortem this temperature rise denatures muscle proteins, resulting in an undesirable pale colour and low water-holding capacity of meat. Myosin denatures and shrinks, causing a reduction of the filament spacing. Thus, water is expelled from the cells and lost in the form of drip.

Economically, why does PSE meat matter?

• Consumers eat with their eyes!

• Meat loses a lot of water content and continues to be "wet"

• Difficult to cook

• Potential condemnation for lack of consumer interest

• Lost revenue

• Welfare issue as well!

How can we go about preventing DFD/PSE meat and improve tenderness when the animal is alive?

• GENTLE/QUIET HANDLING

• Ramp usage

• No electric goads at loading/lairage, using gentle methods

• Access to food/water

• Breed stress resistance

• Reduce transport times where possible - should be advocating for slaughter as close to farm as possible

• Don't mix animals - keep fighting to a minimum

• Spray pigs with water to keep them cool

• Genetics

How can we improve meat quality at the level of the slaughterhouse?

7C - red meat, 4C - poultry, 3C - offal

• Cold reduces evaporation

• Cold Shortening: carcasses cooled below 10C before rigor mortis, not allowing enough time for enzymes to activate and produce tender meat as they do not work below these cold temperature

• Main feature of ruminants, not so much pigs and poultry

**Beef and lamb, do not chill below 10C for the first 10 hours, usually more 12C, then gradually drop to 7 and below, gradual cooling prevents cold shortening

What is cold shortening?

Cold shortening is a severe muscle contraction and permanent toughening that happens when a fresh animal carcass is chilled too quickly after slaughter

How are chilled carcasses suspensed?

• Bovine carcass suspended through obturator foramen, OR conventional Achilles' tendon.

• Stretching improves tenderness of some of the more expensive muscles around rump, when hung by the tendon

• Hip bone sometimes used for space (for shorter carcass hanging height) - sometimes keeps hip muscles more round, however takes time to hang this way, sometimes less secure → dangerous and can damage meat quality, would have to trim carcass to prevent contamination

What is hot boning?

• What are the advantages and disadvantages?

Hot boning - cutting the meat before the animal has been chilled

Advantages:

> More efficient and quicker refrigeration of meat, since they are in small packages

> Reduced evaporative weight loss (up to 1%), get greater return

> Less dripping in vac-pack, package

> Better colour since does not react with oxygen

Disadvantages:

> Synchronize slaughter and boning, no gap between death and cutting

> Fat difficult to trim, since fat has not set, difficult to handle

> High hygiene standards,

> Electrical stimulation required.

What is electrical stimulation and what does it do?

Electric current passed through the meat once the animal is dead - Once the meat has entered rigor mortis, the muscle structure becomes chemically locked and lose the ability to contract. The hot-boned meat can then be sliced, packaged, and blast-chilled immediately without any risk of shrinking or toughening.

•Electrode in neck - earthed on overhead rail

- also use higher voltage later in the process (can be more effective).

• Used to speed rigor → allows for faster chilling as well

• Meat ages faster

•Stimulates muscle to contract = glycogen/ATP used up = pH falls