Component Responsible for Viscosity and White Colour of Milk

Viscosity and white colour of milk is due to:

Correct Answer: Casein protein

Casein protein is primarily responsible for both the viscosity and white colour of milk. Casein exists as a colloidal suspension of large calcium-caseinate-phosphate micelles, which scatter visible light and give milk its characteristic white, opaque appearance.

How Casein Causes White Colour

• Casein micelles are large enough (~20–300 nm) to scatter all wavelengths of visible light equally → milk appears white (Tyndall effect / Mie scattering).
• Buffalo milk is white because buffalo efficiently converts all carotene → colourless Vitamin A, so no yellow pigment remains, and casein's white colour dominates.
• Cow milk is slightly yellow due to residual β-carotene that is not fully converted to Vitamin A.

How Casein Affects Viscosity

• The large casein micelles increase the internal resistance to flow → higher viscosity than water.
• Normal whole milk viscosity at 25°C: ~1.5–2.0 centipoise.
• Homogenization increases viscosity (smaller, more numerous fat globules increase surface area interactions).
• Increasing temperature decreases milk viscosity.

Casein Facts

• Constitutes ~80% of total milk protein.
• Cow milk: 3% casein; Buffalo milk: 4.3% casein.
• Exists as calcium-caseinate-phosphate complex.
• Three types: α-casein (αs-casein = calcium-sensitive + K-casein = calcium-insensitive), β-casein, γ-casein.