Milk proteins Flashcards
(48 cards)
Protein functionality refers to:
physical and chemical properties that influence the performance of proteins in food systems during processing, storage, preparation, and consumption.
Ignoring nutrition and other bioactivity!
Performance = solubility, thermal stability, gelation, emulsifying, foaming, fat binding, water binding, …
List the functional roles of food proteins in food systems.
[list some of]
- Solubility
- Viscosity
- Water binding
- Gelation
- Cohesion-adhesion
- Elasticity
- Emulsification
- Fat and flavour binding
What is the mechanism of solubility?
Hydrophilicity
What is the mechanism of viscosity?
Water binding; hydrodynamic size and shape
What is the mechanism of gelation?
Water entrapment and immobilization, network formation
What is the mechanism of cohesion-adhesion?
Hydrophobic, ionic, and hydrogen bonding
What is the mechanism of elasticity?
Hydrophobic bonding; disulfide crosslinks
What is the mechanism of emulsification?
Adsorption and film formation at interfaces
What is foaming?
Interfacial adsorption and film formation
What is the mechanism for fat and flavour binding?
Hydrophobic bonding; entrapment
Where do we get milk proteins from?
What does milk look like under a microscope?
Casein micelles ~50-100 nm in diameter
Describe the composition of milk.
Describe the concentration of proteins in bovine milk.
Much more casein than whey protein.
Describe the structure of casein.
- Family of phosphoproteins (phosphorylated via post-translational modification)
- Functionally defined as proteins that precipitate ≤ pH 4.6
- Intrinsically disordered proteins
Describe the properties of casein proteins. [4]
- Number of proline residues: disrupt secondary structures, leading to flexible, open conformation
- Number of intra-molecular -s-s- bonds: Lack disulfide cross-linkages, meaning casein will not form rigid 3D structures, but instead will remain highly hydrated and flexible.
- Phosphorus content: Phosphate groups attached to serine residues; contributes to casein negative charge and ability to bind calcium
- Sensitivity to calcium: Highly sensitive, which impacts ability to form micelles; calcium bridges help hold micelles together; kappa-casein which is less sensitive, stabilizes micelles by preventing uncontrolled precipitation (too much free calcium can lead to aggregation and precipitation - curd formation)
- Isoelectric point: Caseins have a low pI, meaning they will retain a negative charge in milk which has a pH ~6.7.
These properties make caseins unique among milk proteins, allowing them to form micelles, stabilize emulsions, and function as excellent nutritional and structural components in dairy-based products.
Compare alpha-s1-casein, beta-casein, and kappa-casein.
- αs1-Casein: Major casein protein (~40% of total casein), plays a key role in calcium binding and micelle formation, highly hydrophobic, and involved in cheese curd formation.
- β-Casein: More surface-active, contributes to micelle hydration, forms aggregates in cold milk (cold gelation).
- κ-Casein: Acts as a stabilizer, prevents micelles from aggregating; crucial in cheese-making as rennet cleaves it, triggering coagulation.
Compare alpha-s1-casein, beta-casein, and kappa-casein functions in milk and role in cheese-making.
Describe alpha-s1-casein.
- Highly negative (24 at pH 6.7)
- Crucial to binding calcium nanoclusters.
Describe beta-casein.
- Greater charge in N-terminus.
- β-Casein has a highly hydrophilic N-terminus (positively charged) and a hydrophobic C-terminus.
The greater charge in the N-terminus of β-casein is critical for its solubility, micelle stabilization, temperature-dependent dissociation, emulsification properties, and digestion behavior, making it functionally distinct from other caseins.
Describe kappa-casein.
- Ser149 is phosphorylated (SerP149), contributing to κ-casein’s negative charge.
- The C-terminal region of κ-casein contains negatively charged glutamic acid (Glu) residues. Contributes to the hydrophilic nature of the C-terminal tail, allowing κ-casein to interact with water and stabilize micelles. This region extends into the surrounding liquid, forming a protective shell around micelles. The hydrophilic C-terminus is cleaved by chymosin during cheese-making, triggering curd formation.
- The glycosylation of Thr133 makes κ-casein more hydrophilic, reinforcing its stabilizing role at the micelle surface.
- When chymosin (rennet) cleaves κ-casein, it cuts the molecule at Phe105-Met106. The N-terminal fragment (para-κ-casein, residues 1-105) remains attached to the casein micelles and stays in the curd.
This allows the casein micelles to aggregate into a gel, forming the structure of cheese. Para-κ-casein is hydrophobic, which helps it contribute to curd formation. - The C-terminal fragment (residues 106-169), which contains the highly charged, glycosylated region, is released into the whey after chymosin cleavage. This soluble glycomacropeptide (GMP) does not participate in curd formation and remains in the whey fraction.
- By removing the hydrophilic, stabilizing C-terminal region, chymosin destabilizes the casein micelles, allowing them to aggregate into curds.
Significance of:
SerP at 149
Kappa-casein
Limited phosphorylation keeps κ-casein surface-active and less sensitive to calcium.
Significance of:
Glu towards C-terminus
Kappa-casein
Creates a strong negative charge, stabilizing micelles by electrostatic repulsion.
Significance of:
O-glycosylation at Thr133
Kappa-casein
Enhances water solubility, contributes to steric hindrance, and stabilizes casein micelles.
