Conversion of Muscle to Meat

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Conversion of Muscle to Meat

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1. Conversion of Muscle to Meat

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

11. Pathways for ATP Generation Glycolysis (anaerobic respiration) 2 net ATP molecules per glucose molecule TCA Cycle (aerobic respiration) 34 ATP molecules per glucose molecule

12. Glycolysis

13. Glycolysis (continued)

14. Pyruvate to Acetyl-CoA

15. TCA Cycle

16. 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.

19. 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

20. 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

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

28. 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

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

30. 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

31. 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)

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