Conversion of Muscle to Meat

1
Conversion of Muscle to Meat
2
Muscle Structure
3
Sarcomere Structure
4
Thick and Thin Filaments
5
Actin and Myosin Association
6
Calcium ions - The Trigger for Contraction

• Concentration of calcium in muscle fiber cytosol
  is regulated by calsequestrin (protein) binding
  within the sarcoplasmic reticulum
• Resting muscle calcium 5 x 10-8 M
• At contraction calcium 5 x 10-6 M

7
Transverse Tubules and Sarcoplasmic Reticulum

• The sarcoplasmic reticulum initiates muscle
  contraction by releasing calcium ions when
  prompted to do so by the transverse tubular
  system.
• Transverse tubules may conduct electrical action
  potentials from the surface of the muscle fiber
  deep into the interior of the fiber.
• Communication between a transverse tubule
  carrying an action potential and the sarcoplasmic
  reticulum is mediated by protein bridges between
  the adjacent membranes of the sarcoplasmic
  reticulum and the transverse tubule.

8
Initiation of Contraction

• Voluntary activity from the brain or reflex
  activity from the spinal cord computes that a
  contraction is needed 
• The impulse is passed down the spinal cord to a
  motor neuron, and an action potential passes
  outwards in a spinal nerve, carried by an axon
  linking the motor neuron to all its muscle fibers
• The axon branches to supply all its muscle fibers
  (motor unit), and the action potential is
  conveyed to a neuromuscular junction on each
  muscle fiber
• At the neuromuscular junction, the action
  potential causes the release of packets or quanta
  of acetylcholine into the small space (synapse)
  between the axon and the muscle fiber

9
Contraction (continued)

• Acetylcholine causes the electrical resting
  potential of the muscle fiber membrane to change,
  and this then initiates a new action potential
  that passes in both directions along the surface
  of the muscle fiber
• The action potential spreads deep inside the
  muscle fiber, carried by transverse tubules   
• Where transverse tubules touch parts of the
  sarcoplasmic reticulum, the sarcoplasmic
  reticulum releases calcium ions
• The calcium ions cause the movement of troponin
  and tropomyosin on their thin filaments, which
  then enables the myosin molecule heads to "grab
  and swivel" their way along the thin filament
  (i.e., filaments slide past each other for muscle
  contraction)
• Muscle contraction requires a constant stream of
  energy from ATP hydrolysis

10
ATP Function in Contraction

• Provides energy source for contraction through
  the action of Ca-ATPase (found in globular heads
  of myosin)
• ATP ? ADP Pi
• Hydrolysis is also required for calcium transport
  back into the SR during relaxation

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Pathways for ATP Generation

• Glycolysis (anaerobic respiration)
• 2 net ATP molecules per glucose molecule
• TCA Cycle (aerobic respiration)
• 34 ATP molecules per glucose molecule

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Glycolysis
13
Glycolysis (continued)
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Pyruvate to Acetyl-CoA
15
TCA Cycle
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Oxidative Phosphorylation

• Oxidation of NADH with phosphorylation of ADP
  to form ATP are processes supported by the
  mitochondrial electron transport assembly and ATP
  synthase
• ADP Pi ? ATP

17
Salvage Pathways to Generate ATP

• Creatine Kinase (cytosol)
• creatine-P ADP ? ATP creatine
• Adenylate Kinase (cytosol)
• ADP ADP ? ATP AMP

18
Muscle Relaxation

• After muscle contraction is no longer required,
  it is turned off by the sarcoplasmic reticulum
  sequestering all the calcium ions it just
  released.
• Sustained muscle contraction or tetanus is the
  result of the fusion of individual muscle
  twitches.
• The peak tension generated by a single twitch
  occurs a few milliseconds after the action
  potential on the muscle fiber membrane, when
  about 60 of the maximum calcium ion release has
  occurred.

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Muscle Relaxation (continued)

• The acetylcholine at the neuromuscular junction
  is destroyed by an enzyme (acetylcholinesterase),
  and this terminates the stream of action
  potentials along the muscle fiber surface
• The sarcoplasmic reticulum ceases to release
  calcium ions, and immediately starts to sequester
  all the calcium ions that were just released
• Without calcium ions, a change in the
  configuration of troponin and tropomyosin blocks
  the action of the myosin molecule heads so that
  they cannot reach the thin filaments any more,
  and contraction ceases
• In the living animal, an external stretching
  force, such as gravity or an antagonistic muscle,
  is required to pull the muscle back to its
  original length

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Rigor

• Muscle contraction requires a constant stream of
  energy from ATP. Before a myosin molecule of a
  thick filament can release itself from an actin
  molecule of the thin filament, it requires new
  ATP
• At death, respiration (and TCA cycle) halts and
  ATP generation is much reduced. ATP is then
  derived from anaerobic respiration (glycolysis)
  and reactions catalyzed by adenylate kinase and
  creatine kinase. These are inefficient ways to
  generate ATP
• In the absence of the TCA cycle, pyruvate is
  converted to lactic acid (via lactate
  dehydrogenase). Lactic acid build up causes a
  decrease in pH (5.6) and this inhibits
  phosphofructokinase, a key regulatory enzyme in
  glycolysis
• ATP generation halts. Without ATP myosin stays
  locked onto actin, even if the muscle is trying
  to relax. Thus, when living muscle finally runs
  out of ATP after slaughter, then rigor mortis
  develops

21
Sarcomere Length and Meat Tenderness

• As rigor develops after slaughter, carcass
  muscles may be stretched or contracted, depending
  largely on their position in the hanging carcass
   
• Relaxed muscles produce meat that is more tender
  than that from contracted muscles
•  

22
Cold Shortening

• Rapid cooling before the start of rigor
  causes muscles to shorten. Sequestering calcium
  ions takes a lot of energy, so when the
  sarcoplasmic reticulum is cooled down, its
  efficiency drops, and it cannot then mop up all
  the calcium ions released by reflex muscle
  activity during slaughter and by leakage through
  the sarcoplasmic reticulum membrane

23
Thaw Rigor

• Freezing of meat before the completion of
  rigor causes extreme shortening when meat is
  thawed, because ice crystals have slashed open
  the sarcoplasmic reticulum allowing massive
  contraction once the system is warm enough to
  respond

24
Resolution of Rigor(Aging or Conditioning)

• Meat tenderness and taste improve if carcasses or
  vacuum packed cuts are conditioned several days
  after slaughter
• Higher temperature allowing a faster rate of
  conditioning
• An increase in ionic strength solubilizes
  myofibrillar proteins (e.g., thick filament)  
• Proteases (e.g., calpain (cytosol), cathepsins B
  and D (lysosomes)) and aminopeptidase break down
  muscle fiber proteins
• Changes in a number of water-soluble compounds
  that affect meat taste, including free amino
  acids, metabolites of ATP, organic acids and
  sugars

25
Effects of Acidic pH in Meat

• Improves meat color (brighter pink)
• Inhibits microbes

26
The Rate of pH Decline Affects Color and Texture
Dark, firm and dry pork
Pale, soft and exudative pork
27
Water Holding Capacity and pH

• The pH of meat at rigor is 5. At this point,
  actin and myosin are irreversibly associated.
  When associated, these proteins bind less water
  than when they are separate.
• Addition of polyphosphates break apart actin and
  myosin and increase water binding (e.g., sausage)
• Water content is important for texture and
  price

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Putrefaction

• Microbes grow on meat and secrete proteases,
  amino acid deaminase, and amino acid
  decarboxylases
• deamination
• tryptophan ? indole
• cysteine ? H2S
• decarboxylation
• lysine ? cadaverine
• tyrosine ? tyramine

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Refrigeration

• Retards microbes
• Retards chemical and enzymatic reactions
• Surface dehydration
• myoglobin oxidation (brown pigment)
• increase enzyme substrate concentration (increase
  activity)
• Cold shortening

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Freezing

• No microbial growth
• Internal dehydration (favors lipid oxidation)
• Ice crystals (large vs. small, consider storage
  conditions)
• Increase in solute concentration
• buffer precipitation can affect pH
• salts can denature proteins and affect texture
• increase enzyme activity (e.g., lipoxygenase)
• Freezer burn
• ice crystals sublime (desiccated tissue)
• oxidation of myoglobin pigment

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Heating

• Destroys microbes
• Inactivates enzymes
• Conversion of collagen to gelatin
• Denature proteins and loss of WHC
• Emulsifying capacity of proteins decreases
• Rupture of adipose tissue and fat redistribution
  (increases palatability)
• Protein and lipid degradation gives rise to amino
  acids and fatty acids (flavor generation)
• Non-enzymatic browning (color and flavor)

32
Curing (Salt Addition)

• Reduction of water activity (inhibits microbes)
• Increase WHC (salt interacts with water)
• Enhance lipid oxidation (low Aw and salt)