Milk Proteins

Milk proteins are a vital component of milk, contributing to its nutritional value, functional properties, and role in dairy product manufacturing. They are broadly classified into two main categories: caseins and whey proteins, with minor proteins also present. Below is a detailed discussion of milk proteins, focusing on their types, structure, composition, and significance, with a focus on cow milk (as it’s the most studied and widely consumed).

Overview of Milk Protein Composition

• Total Protein Content: Varies by species (e.g., 3.2-3.5% in cow milk, 5-6% in sheep milk, 2-2.5% in horse milk).

• Major Fractions:

  • Caseins: ~80% of total protein in cow milk (~2.6-2.8 g/100 mL).

  • Whey Proteins: ~20% of total protein in cow milk (~0.6-0.7 g/100 mL).

  • Minor Proteins: Trace amounts (e.g., enzymes, immunoglobulins).

1. Caseins

Caseins are the predominant proteins in milk, responsible for its opaque white color and ability to form curds. They are phosphoproteins, rich in phosphate groups, and exist as micelles in milk.

Types

• αs1-Casein: ~38% of caseins; highly phosphorylated, contributes to micelle stability.

• αs2-Casein: ~10%; similar to αs1 but less abundant, also stabilizes micelles.

• β-Casein: ~36%; less phosphorylated, more hydrophobic, affects micelle flexibility.

• κ-Casein: ~13%; located on micelle surfaces, stabilizes micelles via its hydrophilic glycosylated tail.

Structure

• Primary Structure: Chains of 169-209 amino acids (depending on type), with high proline content, reducing ordered secondary structures (e.g., α-helices, β-sheets).

• Micellar Form: Caseins aggregate into casein micelles (50-500 nm diameter), spherical structures with:

  • Core: Hydrophobic αs1-, αs2-, and β-caseins, bound by calcium phosphate nanoclusters.

  • Surface: κ-Casein protrudes outward, providing steric and electrostatic stabilization (prevents aggregation).

• Phosphorylation: Phosphate groups on serine residues bind calcium, linking caseins to minerals.

Properties

• Insolubility: Caseins precipitate at pH 4.6 (isoelectric point) or with rennet (e.g., in cheese-making).

• Nutritional Role: Rich in essential amino acids (e.g., leucine, lysine) and a slow-release protein source due to micelle digestion.

• Functionality: Form gels (e.g., yogurt) and curds (e.g., cheese), critical for dairy processing.

Variability

• Species: Higher in sheep milk (~4-5%) than cow milk (~2.6-2.8%).

• Genetics: Variants (e.g., β-casein A1 vs. A2) affect digestibility and product yield.

2. Whey Proteins

Whey proteins remain soluble in milk’s liquid phase after casein precipitation. They are globular proteins with defined secondary and tertiary structures.

Types

• β-Lactoglobulin: ~50-55% of whey (~0.3 g/100 mL); dominant in ruminants (absent in human milk).

• α-Lactalbumin: ~20-25% (~0.1 g/100 mL); key in lactose synthesis.

• Serum Albumin: ~5-10%; derived from blood, minor component.

• Immunoglobulins: ~5-10%; provide passive immunity (higher in colostrum).

• Proteose-Peptones: ~5%; fragments from casein breakdown.

Structure

• β-Lactoglobulin: 162 amino acids, compact globular structure with β-sheets and some α-helices; binds small hydrophobic molecules (e.g., retinol).

• α-Lactalbumin: 123 amino acids, globular with α-helices; calcium-binding site stabilizes its structure.

• Others: Immunoglobulins are large, Y-shaped molecules; serum albumin resembles blood albumin.

Properties

• Solubility: Remain in whey after acidification or rennet action.

• Heat Sensitivity: Denature at high temperatures (e.g., >70°C), forming gels or aggregates (useful in cooking or whey protein powders).

• Nutritional Role: High in branched-chain amino acids (e.g., leucine), rapidly digested, and bioactive (e.g., immune support from immunoglobulins).

Variability

• Species: Horse milk has more whey (~40% of total protein) than cow milk (~20%).

• Lactation Stage: Colostrum is whey-rich (immunoglobulins dominate).

3. Minor Proteins

• Enzymes: E.g., lactoperoxidase (antibacterial), lipase (fat breakdown), alkaline phosphatase (heat indicator).

• Lactoferrin: Iron-binding protein with antimicrobial properties, more abundant in human milk.

• Growth Factors: Trace amounts, support neonatal development.

Factors Affecting Milk Proteins

1. Species: Sheep milk is protein-rich (~5-6%), horse milk is lower (~2-2.5%).

2. Breed: Holstein cows yield more milk but lower protein (~3.2%) than Jerseys (~3.8%).

3. Diet: Protein-rich feed increases milk protein content.

4. Lactation Stage: Colostrum has higher protein (mostly whey), later milk stabilizes casein:whey ratio.

5. Health: Mastitis reduces casein synthesis, increasing whey (e.g., albumin).

Functional and Nutritional Significance

• Nutrition:

  • Caseins provide sustained amino acid release, ideal for growth.

  • Whey offers quick-digesting proteins and bioactives (e.g., lactoferrin boosts immunity).

• Dairy Processing:

  • Caseins drive cheese and yogurt texture via coagulation.

  • Whey proteins enhance foaming (e.g., in meringues) and gelation (e.g., in heat-treated products).

• Health: Peptides from milk proteins (e.g., casomorphins, lactoferricin) may have antihypertensive, opioid-like, or antimicrobial effects.