Casein Structure and Composition

Primary Structure of Casein

• All caseins contain both polar and non-polar amino acid residues
• These residues are clustered, creating hydrophobic and hydrophilic regions
• This clustering makes casein good emulsifiers
• An emulsion is a mixture of two or more immiscible liquids

Casein Carbohydrate

• αs1-, αs2-, and β-caseins have no carbohydrates
• κ-casein contains about 5% carbohydrate
• Carbohydrates include sialic acid, galactose, and N-acetylgalactosamine
• These carbohydrates increase solubility and hydrophilicity of κ-casein

Casein Phosphorus

• Milk contains approximately 900 mg phosphorus per liter
• Casein contains about 0.85% phosphorus
• Phosphorus exists in inorganic (soluble and colloidal phosphates), organic (phospholipids, casein, and sugar phosphates, nucleotides) forms
• Phosphorus is nutritionally important because it binds to Ca2+, Zn2+, and other metals
• Phosphorus increases casein solubility and heat stability
• Phosphorus is covalently bound to serine residues in the protein
• Phosphorylation occurs in the Golgi membrane

Secondary and Tertiary Structures of Casein

• Casein has relatively little secondary and tertiary structure
• This is likely due to high levels of proline residues, which disrupt α-helices and β-sheets, especially in β-casein

Lack of Secondary and Tertiary Structure Significance

• Caseins are easily broken down by proteolysis (which is beneficial for digestion).
• Caseins' open structures make them easily adsorb to air-water and oil-water interfaces
• This makes caseins good emulsifiers and foamers, which are useful in food industries.
• The lack of higher structures likely explains the high stability of caseins to denaturing agents, including heat.

Molecular Size and Hydrophobicity of Casein

• Casein molecules are relatively small, ranging in molecular weight from 20 to 25 kDa
• Caseins are often considered to be hydrophobic

Influence of Calcium on Caseins

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

Action of Rennin on Casein

• κ-casein is the only major casein hydrolyzed by rennet during milk coagulation
• κ-casein exists in solutions as disulfide-linked polymers
• At 4 °C, β-casein exists as monomers; at 8.5 °C, multiple β-casein monomers are associated.
• αs1-casein forms tetramers with a molecular mass of 113 kDa
• These caseins interact to form casein micelles in the presence of calcium

Casein Micelle Structure

• Casein micelles are 95% casein and 5% low-molecular-weight substances (calcium, magnesium, phosphate, citrate)
• Casein micelles are roughly spherical in shape, ranging in diameter from 50 to 500 nm; average diameter is 120 nm
• They are hydrated, binding about 2 g of water per gram of protein.
• The casein micelles have a hydrophobic core surrounded by a layer of κ-casein

Stability of Casein Micelles

• Casein micelles are stable to normal processing, ultracentrifugation, homogenization and 50 C, up to certain calcium concentration levels.
• They aggregate and precipitate at their isoelectric point (pH 4.6)
• Proteinases help to catalyze hydrolysis of certain bonds in κ-casein
• The micelles can destabilize from alcohols, acetone and electrostatic interactions.

Principal Micelle Characteristics

• κ-Casein, which makes up 15% of total casein, is critical to micelle structure and stability
• Micelle structure has pores where proteins occupy about 25% of its total volume
• Chymosin and similar proteinases hydrolyze most κ-casein
• When heated in milk, κ-casein interacts with other proteins (β-lactoglobulin) and forms disulphide-linked complexes.
• Change in pH and temperatures can modify the structure and stability of casein micelles

Micelle Subunit Structure

• κ-casein content in submicelles varies
• Deficient κ-casein submicelles are located inside the micelle and concentrated κ-casein submicelles are located on the surface
• Other caseins (αs1, αs2, and β) can be exposed on the surface of the micelles as well

Micelle Stability Mechanisms

• Frequent collisions between closely-packed micelles do not normally cause them to separate since they are stabilized by
  • Surface charge (-20 mV at pH 6.7): this is not sufficient for maintaining stability on its own
  • Steric stabilization: protruding κ-casein "hairs" prevent aggregation

Casein Preparation Methods

• Whey protein products are prepared via
  • Separation through ultrafiltration,
  • Ion-exchange chromatography,
  • Demineralization (electrodialysis and ion exchange),
  • Thermal evaporation,
  • Crystallization,
  • Thermal denaturation and precipitation using filtration/centrifugation/spray-drying

Explore the primary structure of casein, including its amino acid residues, carbohydrate content, and phosphorus significance. This quiz covers the features that make casein effective as an emulsifier and its nutritional importance. Test your understanding of these essential components in dairy science.