Casein Structure and Composition

Primary Structure of Casein

• Casein (polar and non-polar residues) are not uniformly distributed, but occur in clusters.
• This creates hydrophobic and hydrophilic regions, making casein good emulsifiers.
• An emulsion is a mixture of two or more normally immiscible liquids.

Casein Carbohydrate

• αs1-, αs2, and β-caseins lack carbohydrates.
• κ-casein contains about 5% carbohydrate, including sialic acid, galactose, and N-acetylgalactosamine.
• Glycosylation (carbohydrate addition) increases κ-casein's solubility and hydrophilicity.

Casein Phosphorus

• Milk contains approximately 900 mg phosphorus per liter.
• Casein contains about 0.85% phosphorus (inorganic and organic forms).
• Phosphorus is crucial for several reasons:
  • Nutritional importance: binding with Ca²⁺, Zn²⁺, and other metals.
  • Increased solubility of casein.
  • High heat stability of casein.
  • Crucial in the coagulation of rennet-altered casein.
• Phosphate is covalently bound mainly to serine.
• Phosphorylation occurs in the Golgi membrane.

Secondary and Tertiary Structures of Casein

• Casein has relatively little secondary or tertiary structure (compared to other proteins).
• This is largely due to the high levels of proline residues, which disrupt α-helix and β-sheet formation.

Lack of Secondary and Tertiary Structures

• Caseins are readily susceptible to proteolysis (enzyme breakdown).
• This is beneficial for digestibility and the development of cheese flavor and texture.
• Caseins readily adsorb to air-water and oil-water interfaces.
  • This is due to their open structure, relatively high content of nonpolar amino acids, and uneven amino acid distribution.
• This characteristic gives caseins excellent emulsifying and foaming properties, widely used in the food industry.
• The lack of higher structures contributes to casein's high stability against denaturing agents like heat.

Molecular Size and Hydrophobicity of Casein

• Casein molecules are relatively small, ranging from 20 to 25 kDa.
• Caseins are often considered hydrophobic molecules.

Influence of Calcium on Casein

• αs1- and αs2-caseins are insoluble in calcium-containing solutions.
• β-casein is soluble at high concentrations of Ca²⁺ (0.4M) at temperatures below 18°C.
• κ-casein is soluble in Ca²⁺ at all concentrations.

Action of Rennets on Casein

• κ-casein is the primary casein hydrolyzed by rennets during milk coagulation (cheese making).
• κ-casein exists largely as disulfide-linked polymers.
• At 4°C, β-casein exists as monomers. At 8.5°C, it forms 20-monomer chains.
• αs1-casein polymerizes into tetramers (113 kDa).
• Casein associations occur due to interactions between different casein types and calcium.
• These associations form casein micelles.

Casein Micelle Structure

• Milk contains 10¹⁴-10¹⁶ micelles per milliliter.
• Micelles are large colloidal particles.
• On a dry-matter basis, micelles contain 94% protein and 6% low molecular weight compounds (calcium, magnesium, phosphate, and citrate).
• Micelles are highly hydrated, binding about 2g of water per gram of protein.
• Micelles are generally spherical, ranging in diameter from 50 to 500nm (average 120nm).
• Micelles have a hydrophobic core and К-casein is enriched on the surface.
• The surface area of micelles is vast, heavily influencing their behavior.
• The white color of milk is largely due to scattered light by casein micelles.

Stability of Casein Micelles

• Micelles are very stable at high temperatures and are easily re-dispersed after sedimentation.
• Resistant to commercial homogenization and stable to high Ca²⁺ concentrations (at least 200mM up to 50°C).
• Aggregation and precipitation occur if the pH approaches the isoelectric point (pH 4.6).
• Proteinases (enzymes breaking down proteins) can destabilize them.
• Destabilization is also possible by alcohols, acetone, and electrostatic interactions.
• Destabilization can occur upon freezing.

Principal Micelle Characteristics

• κ-casein (15% of total casein) is critical to micelle structure and stability.
• The micelle has a porous structure; proteins occupy ~25% of the volume.
• Chymosin and similar enzymes hydrolyze significant portions of micellar κ-casein.
• Heating milk with whey proteins can create linked complexes between casein and whey proteins.
• Alcohols and acetone destabilize micelles, and β-casein dissociates from the micelle at lower temperatures.

Micelle Structure (Subunits)

• κ-casein content varies within submicelles, with κ-casein deficient submicelles in the interior.
• αs1, αs2, and β-caseins are also exposed on the surface.

Micelle Stability Mechanism

• Inter-micellar collisions are frequent, but micelles don't normally stay together after collision.
• Micelle stabilization depends on:
  • A surface (zeta) potential.
  • Steric stabilization due to protruding κ-casein "hairs".

Commercial Casein Production

• Whey proteins are used to produce whey protein concentrates (30-80% protein).
• Ion exchange chromatography removes lactose and salts and produces isolates (≥95% protein).
• Other methods include demineralization, thermal evaporation, and crystallization.

This quiz explores the primary structure of casein, including its polar and non-polar residues, as well as its carbohydrate and phosphorus content. Understand how these components contribute to casein's emulsifying properties and nutritional significance. Test your knowledge on the intricate makeup of this important milk protein.