Meat Protein Structure and Muscle Contraction Mechanics

Meat Composition and Muscle Tissue

Meat is composed of muscle tissue, which includes fibers, connective tissue, lipid tissue, and myoglobin.

Protein Classifications

• Myofibrillar Proteins: Responsible for muscle contraction and relaxation. They represent 50% of the protein content and consist primarily of actin and myosin.
• Connective Tissue Proteins: These fibers hold muscle tissues together. They represent 10-15% of proteins, with collagen and elastin being the most important.
• Sarcoplasmic Proteins: Proteins found in the cell cytoplasm. They represent 25-30% of the main proteins, including myoglobin, the pigment responsible for the color of red meat.

Myofibrillar Protein Details

Myosin

Myosin exists in a higher percentage than actin. Its molecular weight is approximately 5 x 10⁵. It consists of two protein chains wound together, showing an α-helix structure at one end and SH groups at the other. These bulky ends interact with actin and possess ATPase activity.

Actin

Actin is a thin filament existing in two forms:

• Globular (G-actin): Molecular weight of 50,000 to 60,000.
• Fibrous (F-actin): A polymer of G-actin. Each filament consists of two double-helical chains with 300 to 400 monomers oriented in the same direction.

Other Proteins

Troponin, tropomyosin, and α-actin are sensitive to calcium and are involved in the contraction process.

Muscle Contraction Mechanics

Muscle contraction is a cyclic process explained by the sliding filament theory. Changes in ATP, Ca⁺², and Mg⁺² concentrations generate structural changes in actin and myosin.

• When Ca⁺² concentrations are high, tropomyosin is mobilized to interact with myosin and actin, protecting and preventing actin-myosin binding.
• For contraction to occur, at least 10^-7M of Ca⁺² is required, alongside molecules such as creatine phosphate and glycogen.

ATP Procurement

• Anaerobic Glycolysis: Occurs when oxygen is insufficient for aerobic processes: Glucose → 2 Lactate + 3 ATP.
• Adenylate Kinase Action: Regenerates ATP from ADP: 2 ADP → ATP + AMP.
• Phosphocreatine Kinase Action: ATP + Creatine ↔ ADP + Phosphocreatine.

Effects of Cooking Temperatures

• 50°C: Plasma and sarcoplasmic protein denaturation occurs via ionic or hydrogen bonds, leading to aggregation or clotting.
• 63°C: Collagen and elastin partially solubilize and swell but remain largely unchanged.
• 70°C: Actomyosin becomes firmer, less soluble, and reduces its water-holding capacity. pH is modified by the release of specific groups.
• High Heat: Collagen and elastin soften, but actomyosin hardens due to the formation of disulfide bonds.

Processing and Preservation Effects

• Frostbite: Causes protein denaturation and aggregation, rupture of muscle cells, and decreased water-holding capacity, leading to weight loss and drying upon thawing.
• Dehydration: Results in texture hardening and decreased water-holding capacity during rehydration.
• Proteolytic Enzymes and Salts: Plant enzymes (e.g., papain) and microbial enzymes tenderize meat and release peptides that modify flavor. Salts like NaCl or Na₃PO₄ increase water-holding capacity.