Casein Structure and Composition

Primary Structure of Casein

• All caseins have polar and non-polar residues clustered together.
• This creates hydrophobic and hydrophilic regions.
• This characteristic makes casein good emulsifiers.
• Emulsion is a mixture of two or more liquids that do not mix naturally.

Casein Carbohydrate

• αs1-, αs2, and β-caseins have no carbohydrates.
• κ-casein contains about 5% carbohydrates.
• These carbohydrates include sialic acid, galactose, and N-acetylgalactosamine.
• Glycosylation is the process of adding carbohydrates to proteins.
• Carbohydrates increase the solubility and hydrophilicity of κ-casein.

Casein Phosphorus

• Milk contains approximately 900 mg of phosphorus per liter.
• Casein contains about 0.85% phosphorus (by weight).
• Phosphorus exists in both inorganic and organic forms. Inorganic forms include soluble and colloidal phosphates. Organic forms include phospholipids, casein and sugar phosphates, and nucleotides.
• Phosphorus is crucial in milk for several reasons:
  • Nutritional value: It binds to calcium, zinc, and other metals.
  • Solubility of caseins
  • Heat stability of casein
  • Coagulation of rennet-altered casein
• Phosphate is covalently bound mainly to serine.
• Phosphorylation occurs in the Golgi membrane

Secondary and Tertiary Structures of Casein

• Caseins have minimal secondary and tertiary structures.
• Presence of high levels of proline residues disrupts alpha-helices and beta-sheets.

Lack of Secondary and Tertiary Structures

• Caseins are easily broken down by proteolysis.
• This characteristic makes it easier to digest caseins, both nutritionally and in cheese production.
• Caseins readily bind to air-water and oil-water interfaces.
• Their open structure and high concentration of nonpolar amino acids contribute to this ability.
• The uneven distribution of amino acids provides emulsifying and foaming ability, which is important in food industries.
• This quality is related to the lack of secondary and tertiary structure.
• Caseins have high stability towards denaturing agents, including heat.

Molecular Size of Casein

• Caseins are relatively small molecules with weights ranging from 20 to 25 kDa.
• Caseins are typically considered hydrophobic.
• The presence of Ca2+ influences the solubility of caseins.
• αs1- and αs2-caseins are insoluble in calcium-containing solutions.
• β-casein is soluble in high concentrations of Ca2+ (0.4M) at low temperatures (below 18°C).
• κ-casein is soluble in the presence of Ca2+ in all concentrations.

Action of Rennets on Casein

• κ-casein is the only significant casein hydrolyzed by rennet during the primary cheese production stage.
• κ-casein exists mostly in disulfide-linked polymers.
• At 4°C, β-casein exists as monomers; at 8.5°C, it forms 20-mer chains.
• αs1-casein forms tetramers (113 kDa).
• Interactions between caseins result in casein micelles in the presence of Ca2+.

Casein Micelle Structure

• Casein exists in milk as large colloidal particles called micelles.
• Micelles are about 95% casein.
• The remaining 5% are low-molecular-weight species such as calcium, magnesium, phosphate, and citrate.
• Micelles are hydrated, binding about 2.0g of H₂O per gram of protein.
• Casein micelles are generally spherical with diameters from 50-500nm (average 120nm).
• Inside the micelles, a core is composed of a hydrophobic substance. A "hairy" surface layer is rich in κ-casein.
• Surrounding the micelles are other components such as calcium phosphate.

Stability of Micelles

• Micelles are very stable during processing, surviving high temperatures and chemical treatments.
• Micelles can be disassembled by adjusting pH and enzymes.
• Increased or decreased pH and calcium ion concentration can affect stability and cause aggregation or precipitation.
• Denaturing agents such as 40% ethanol affect stability.

Principal Micelle Characteristics

• K-casein (15% of total casein) plays a key role in micelle structure and stability.
• Micelles have a porous structure with proteins occupying 25% of the volume.
• Chymosin hydrolyzes most of the к-casein.
• Heating milk in the presence of whey proteins can modify rennet coagulability and stability.
• Micelle structure can be destabilized by alcohols, acetone, and electrostatic interactions.
• β-casein separates from the micelles as temperature decreases.

Micelle Structure-Subunits

• K-casein concentration varies in submicelles.
• K-casein-poor submicelles are found inside, and K-casein-rich submicelles are at the surface.
• Some αs1, αs2, and β-caseins are exposed on the surface.

Micelle Stability and Zeta Potential

• Micelles in milk are closely packed, leading to frequent collisions yet they do not usually separate.
• Two main stabilizing factors:
  • Surface (zeta) potential (-20mV at pH 6.7). Values near zero indicate instability.
  • Steric stabilization due to protruding K-casein hairs. Sterically hindered interactions, especially related to protein structure and packing, prevent the macromolecules from getting too close.

Commercial Scale Whey Protein Production

• Whey protein-rich products are produced industrially through various methods:
  • Ultrafiltration and spray-drying to create whey protein concentrates.
  • Ion-exchange chromatography can produce whey protein isolates.
  • Demineralization (electrodialysis/ion exchange/thermal evaporation) followed by crystallization of lactose.
  • Thermal denaturation and filtration/centrifugation followed by spray-drying.

Explore the fundamental aspects of casein's structure, including its primary composition and the roles of carbohydrates and phosphorus. This quiz covers the hydrophilic and hydrophobic properties, the significance of glycosylation, and the phosphorus content in milk. Test your knowledge on these critical components of casein.