Meat Science, Muscle Structure, and Meat Safety

Fundamentals and Types of Meat

• Definition of Meat: Animal tissue used as food for human consumption.

• Primary Classifications of Meat:

  • Red Meat: Includes beef, pork, veal, lamb, and horse.

  • Poultry: Includes chicken, turkey, duck, and goose.

  • Fish and Seafood: Includes all edible aquatic species.

  • Other Meat Sources: Game meat, as well as grubs and worms.

• Production Leadership: Chicken is the leading producer among all meat sources globally.

Muscle Structure and Histology

• Primary Muscle Types:

  • Smooth Muscle: Located within the walls of the digestive tract; involuntary and non-striated.

  • Involuntary Striated Muscle: Cardiac muscle forming the heart; contains visible striations (lines/layers).

  • Voluntary Striated Muscle: Skeletal muscle attached to bones; features alternating light and dark bands under microscopic examination.

• Muscle Cell Anatomy (Muscle Fibers):

  • Prefix Etymology: The prefix "sarco-" denotes muscle.

  • Myofibers: Structural units of muscle cells.

  • Sarcolemma: The excitable outer cell membrane enclosing the muscle fiber.

  • Nuclei Distribution:

  • Voluntary striated muscle cells contain multiple nuclei.

  • Involuntary striated and smooth muscle cells contain a single nucleus.

  • Myofibrils: Long, intracellular fibers of contractile filaments located within myofibers that enable the muscle cell to perform mechanical work.

  • Sarcoplasm: The liquid, cytoplasm-equivalent portion of the muscle cell.

• Connective Tissue Sheaths:

  • Endomysium: A thin connective tissue layer immediately surrounding an individual muscle cell.

  • Perimysium: Connective tissue that wraps around groups of several muscle cells to form muscle bundles (fascicles).

  • Epimysium: Connective tissue sheath that surrounds the entire muscle body.

• Microscopic Filament Architecture:

  • Actin: Thin protein filaments that form light bands.

  • Myosin:** Thick protein filaments that form dark bands.

  • Sarcomere: The smallest functional, contractile unit of muscle tissue.

  • Composed of several hundred filaments.

  • Defined structurally as the segment located between two adjacent $Z$ lines ($Z$ bands).

Physiology of Muscle Contraction and Relaxation

• Sliding Filament Theory:

  • Contraction is regulated directly by calcium ion ($Ca^{++}$) concentration.

  • During contraction, the individual lengths of actin and myosin filaments do not change; instead, thin filaments slide past thick filaments.

  • Sarcoplasmic Reticulum: A specialized organelle that sequesters and holds $Ca^{++}$ around the myofibrils.

• Role of Adenosine Triphosphate (ATP):

  • Relaxation: ATP binds to keep actin and myosin filaments physically separated.

  • Contraction: ATP hydrolysis provides the necessary chemical energy to drive filament sliding.

• Energy Metabolism:

  • Anaerobic Reactions: Yield energy without oxygen.

  • Aerobic Reactions: Yield energy in the presence of oxygen, producing a higher total energy yield compared to anaerobic pathways.

Post-Mortem Changes, Rigor Mortis, and Aging

• Rigor Mortis:

  • Defined as the post-mortem stiffening of the carcass caused by intense shortening of muscle fibers.

  • Phases of the Rigor Mortis Curve (Tension vs. Time):

  • Onset ($O$): Muscle remains relaxed and meat is at its most tender state.

  • Completion ($C$): Muscle reaches maximum contraction, peak tension, and maximum toughness.

  • Resolution ($R$): Muscle stiffening releases and meat becomes progressively more tender due to enzymatic breakdown of muscle proteins.

• Autolysis (Self-Breakdown):

  • Driven by endogenous proteolytic enzymes, specifically cathepsins and calpains.

  • Enzymatic activation occurs when muscle $pH$ declines to $pH = 5.6$.

  • Water-Binding Capacity: Drops significantly as muscle $pH$ decreases.

• Meat Aging:

  • Process that enhances both flavor and tenderness post-mortem.

  • Requires careful management of time and temperature to balance enzymatic breakdown against microbial growth.

  • Aging Methods and Conditions:

  • Standard Cold Aging: $7-14\,\text{days}$ at $35^\circ\text{F}$.

  • High-Temperature Rapid Aging: $2-3\,\text{days}$ at $60^\circ\text{F}-68^\circ\text{F}$.

  • Exogenous Enzymatic Tenderization: Addition of papain (a plant-derived proteolytic enzyme).

  • Wet Aging: Aging meat retained within its own fluids inside vacuum-sealed packaging.

Chemical Composition and Nutritional Profile

• Proportional Composition of Muscle Tissue:

  • Water: $75\%$

  • Protein: $19\%$

  • Lipids: $4\%$

  • Carbohydrates: $1\%$

  • Minerals: $1\%$

• Lipid Characteristics:

  • Monogastric and Ruminant Animals: Lipids consist of approximately $50\%$ unsaturated fatty acids and $50\%$ saturated fatty acids.

  • Poultry: Lipids consist of approximately $\frac{2}{3}$ unsaturated fatty acids and $\frac{1}{3}$ saturated fatty acids.

  • Feed Influence: Fatty acid profiles in monogastric animals are strongly influenced by dietary feed composition.

• Protein Characteristics:

  • Economic/Nutritive Value Ranking: Protein sources rank third overall in agricultural value: 1. Eggs, 2. Milk, 3. Meat.

  • Human Nutritional Value: All three sources satisfy essential human protein requirements and provide equivalent nutritional value across cuts.

  • Specific Protein Types:

  • Actomyosin: The primary contractile protein complex formed by actin and myosin.

  • Myoglobin: Transports oxygen ($O_2$) and carbon dioxide ($CO_2$) within muscle tissue.

  • Regulatory Proteins: Troponin and tropomyosin (regulate contraction).

  • Metabolic Proteins: Cellular enzymes and nucleoproteins.

  • Connective Tissue Proteins: Collagen, elastin, and keratin (inedible connective tissues).

• Carbohydrate Characteristics:

  • Glycogen: Present in muscle prior to rigor mortis.

  • Lactic Acid: Formed post-rigor as glycogen breaks down anaerobically.

  • Other Carbohydrates: Glucose derivatives.

• Vitamin Profile:

  • Contains key B vitamins: Vitamin $B_{12}$, niacin, riboflavin, Vitamin $B_6$, and thiamine.

  • Species Outliers: Most meats have similar vitamin distributions except for two major species-specific outliers:

  • Beef is rich in Vitamin $B_{12}$.

  • Pork is rich in thiamine.

• Mineral Profile:

  • Soft Tissue/Muscle Minerals: Iron ($Fe$), Zinc ($Zn$), Potassium ($K$), Sulfur ($S$), Sodium ($Na$), and Chlorine ($Cl$).

  • Bone/Shell Minerals: Magnesium ($Mg$), Calcium ($Ca$), and Phosphorus ($P$).

Meat Quality and Organoleptic Characteristics

• Consumer Quality Criteria (Consumer Viewpoint):

  • Color and Texture:

  • Determined primarily by myoglobin concentration.

  • Used Muscles: Frequently exercised muscles accumulate more myoglobin, resulting in a dark color.

  • Unused Muscles: Less active muscles contain less myoglobin, resulting in a light color.

  • Species-Specific Desired Colors:

    • Beef: Bright cherry red.

    • Pork: Grayish pink.

    • Mutton: Light pink.

    • Veal: Pinkish brown.

  • Aroma and Flavor:

  • Subtle organoleptic combination of taste and smell.

  • Driven by volatile compounds liberated by heat during cooking.

  • Influenced by water-soluble and fat-soluble compounds; fats strongly influence specific species flavors.

  • Juiciness:

  • Sensation of wetness produced by the release of meat fluids during chewing.

  • Effect of Cooking: Meat with higher marbling (intramuscular fat) yields a juicier final product.

  • Tenderness:

  • Refers to how easily meat can be chewed.

  • Influencing Factors:

    • Connective Tissue: Higher connective tissue content makes meat less tender (connective tissue is the primary determinant of tenderness).

    • Muscle Fiber Coarseness: Smaller muscle fibers are more tender.

    • Cooking Method: Application of heat makes myofibrils tougher, but renders connective tissue more tender.

    • Temperature and Storage: Higher storage temperatures increase microbial growth.

Factors Affecting Meat Quality

• Pre-Slaughter Factors:

  • Heredity: Tenderness is a heritable genetic trait.

  • Age: Young animals are tender; older animals are tougher but possess more intense flavor.

  • Hormonal Balance: Male versus female hormonal dynamics (e.g., testosterone influence).

  • Feed and Management: Modifies fat deposition and overall carcass quality.

  • Stress at Slaughter: Low stress prior to slaughter is desirable; high stress leads to dark-colored meat.

• Post-Slaughter Factors:

  • Chilling: Ensures meat quality and firms tissue, making the carcass easier to cut.

  • Aging: Monogastric meats contain higher levels of unsaturated fats, making them prone to oxidative rancidity during aging.

  • Freezing: Inhibits the growth of microorganisms.

  • Electrical Stimulation: Makes the progression of rigor mortis uniform throughout the carcass.

Carcass Dressing Percentages

• Definition: Dressing percentage measures carcass weight as a proportion of live animal weight.

• Species Ranking (Low to High):

  • Sheep: $50\%$

  • Cow (Cattle): $62\%$

  • Pig (Swine): $72\%$

Meat Inspection and Regulatory Standards

• Regulatory Body: Food Safety and Inspection Service (FSIS), a branch of the USDA.

• Legislative Origin: Wholesome Meat Act of $1967$, influenced by Upton Sinclair's book, The Jungle.

• Statutory Purpose: Ensure that all meat offered for sale is wholesome, unadulterated, and safe for human consumption.

• Inspection Criteria:

  • Carcass must be free of disease.

  • No evidence of disease in vital organs.

  • Facility must maintain clean, sanitary conditions.

  • Product must have proper labeling and packaging.

Meat Grading Systems

• Purpose and Administration:

  • Established to categorize meat based on specific quality and yield criteria that consumers desire.

  • Administered according to official USDA guidelines.

  • Paid for directly by meat packers (voluntary service).

  • Some meat packers establish their own private grades.

• USDA Beef Grading:

  • Estimates overall palatability and retail yield.

  • Quality Grading: Evaluates marbling and physiological maturity.

  • Common Beef Quality Grades: Prime, Choice, Select, Standard.

  • Yield Grading: Evaluates fat thickness, carcass weight, and area of the ribeye muscle.

  • Yield Grades: Ranging from Grade $1$ to Grade $5$.

• USDA Pork Grading:

  • Combines yield and quality into a single unified grade.

  • Evaluates backfat, muscling, and yield of lean cuts.

  • Pork Grades: Ranging from Grade $1$ to Grade 4$.\n* **USDA Poultry Grading:**\n - Grades: Grade A$, Grade$B$, and Grade$C$.

  • Whole birds sold at retail must meet Grade $A$ standards.

Meat Marketing Categories

• Marketing Levels:

  • Carcass: Whole or halved carcasses of beef, sheep, and swine.

  • Wholesale/Retail: Carcasses fabricated into primal cuts.

  • Religious Guidelines: Meat prepared according to Kosher or Halal standards.

  • Variety Meats: Edible organ meats, such as heart and liver.

Meat Safety, Pathogens, and HACCP

• Major Foodborne Pathogens:

  • Salmonella

  • Escherichia coli (E. coli)

  • Listeria (notable for growing across a different temperature range than other pathogens)

  • Campylobacter

• Recommended Internal Cooking Temperatures:

  • Beef: Minimum internal temperature of $160^\circ\text{F}$.

  • Chicken: Minimum internal temperature of $165^\circ\text{F}$.

• HACCP System: Hazard Analysis Critical Control Point system, designed for identifying and eliminating biological, chemical, and physical hazards in food processing.