Biochemistry of Casein Proteins

Primary Structure of Casein

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

Casein Carbohydrate

• αs1-, αs2, and β-caseins lack carbohydrates
• κ-casein contains about 5% carbohydrates
• These carbohydrates include sialic acid, galactose, and N-acetylgalactosamine
• The carbohydrates contribute to the high solubility and hydrophilicity of κ-casein

Casein Phosphorus

• Milk contains approximately 900 mg of phosphorus per liter
• Casein comprises about 0.85% phosphorus
• Casein phosphorus includes inorganic (soluble and colloidal phosphates), organic (phospholipids, casein and sugar phosphates, nucleotides)
• Phosphorus is nutritionally important because it binds to calcium, zinc, and other metals
• It also enhances casein solubility and heat stability
• Casein phosphorus is covalently bonded to the protein, primarily to serine
• Phosphorylation of casein occurs mainly in the Golgi membrane

Secondary and Tertiary Structures of Casein

• Caseins have limited secondary and tertiary structures
• This is likely due to high proline residue content, particularly in β-casein
• Proline residues disrupt α-helix and β-sheet structures

Lack of Secondary and Tertiary Structures

• The lack of secondary and tertiary structures makes caseins readily susceptible to proteolysis
• Caseins readily hydrolyze in cheese, contributing to flavor and texture
• Caseins are good at adsorbing to air-water and oil-water interfaces due to their open structure and a high content of nonpolar amino acid residues
• This quality makes them good emulsifiers and foamers in food

Molecular Size of Casein

• Casein molecules are generally small, ranging from 20 to 25 kDa

Hydrophobicity of Casein

• Casein molecules are often considered hydrophobic

Influence of Calcium on Casein

• 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 Rennits on Casein

• κ-casein is the major casein hydrolyzed by rennets, occurring during the primary phase of milk coagulation
• κ-casein exists largely in solution as disulfide-linked polymers at 4°C
• β-casein exists as monomers at 4°C, but as polymers at 8.5°C
• as1-casein polymerizes into tetramers at a molecular mass of 113 kDa
• The presence of calcium promotes the formation of casein micelles

Casein Micelle Structure

• Casein micelles are large colloidal particles comprising 94% protein and 6% low molecular weight substances (calcium, magnesium, phosphate and citrate)
• They are typically spherical with diameters ranging from 50 to 500 nm (average 120 nm)
• Casein micelles contain about 2.0 g of water per gram of protein
• There are roughly 10^14-10^16 micelles per milliliter of milk
• The white color of milk is due to light scattering by casein micelles

Stability of Casein Micelles

• Casein micelles are stable at high temperatures and to common processing methods
• They are stable to high calcium concentrations (up to 200 mM) at temperatures up to 50°C
• They aggregate and precipitate when the pH is adjusted to the isoelectric point (pH 4.6)
• Caseins are destabilized by 40% ethanol at pH 6.7 and by decreasing pH and increasing calcium levels

Principal Micelle Characteristics

• κ-Casein, comprising about 15% of total casein, is crucial to micelle structure and stability
• Micelle structure is porous, with protein occupying about 25% of the total volume
• Chymosin and similar enzymes hydrolyze most of the к-casein in micelles
• Heating with whey proteins forms a complex, potentially modifying rennet coagulability and heat stability
• Alcohol and acetone can destabilize micelles
• β-casein dissociates from micelles with decreasing temperature

Micelle Subunit Structure

• The к-casein content of the submicelles varies, with k-casein-deficient submicelles located within the micelle interior and к-casein-rich submicelles concentrated on the outside
• Some αs1-, αs2- and β-casein are exposed on the surface

Micelle Stabilization

• Micelle stability is achieved by surface (zeta) potential and steric stabilization
• Zeta potential (approximately -20mV at pH 6.7) is likely insufficient for colloidal stability, so steric stabilization from protruding к-casein hairs is important
• Inter-micellar collisions are frequent, but micelles do not usually remain together after these collisions

Whey Protein Rich Product Preparation

• Whey protein-rich products are prepared through various methods on a commercial scale
• These methods include ultrafiltration, ion-exchange chromatography, demineralization, thermal evaporation, and thermal denaturation

Explore the complex structure and properties of casein proteins, including their amino acid composition, carbohydrate content, and phosphorus significance. This quiz delves into the roles these components play in creating emulsions and enhancing nutritional benefits. Test your understanding of casein's biochemistry and its applications in various fields.