What gives milk its white colour?
V. SHAN VIRGIN
An emulsion is a dispersion of one liquid in another immiscible one (e.g. oil in water). Emulsions are used in all walks of life (e.g. paints, creams, syrups etc). Oil droplets are dispersed in water by fragmenting it in presence of a surface-active species such as surfactants or polymers. The diameter of the disperse phase varies from a few nanometre to several microns. Now, colloidal scientists claim that they have been working in nanoscales for several decades.
Milk is a natural emulsion where each fat globule is stabilized by a membrane of phospholipids and proteins. Milk contains 88 per cent water, 3.3 per cent protein (casein protein is the largest in milk that amounts 82 per cent), 3.3 per cent fat, 4.7 per cent carbohydrate and 0.7 per cent ash. The percentage of disperse phase in milk is very low compared with some of the industrial products, which contain up to 70 per cent of disperse phase.
The casein micelles, which are typically a hundred nanometre in diameter, contain aggregates of protein molecules and are bounded with the help of calcium phosphate. The white colour of milk is due to scattering of light from the fat globules and casein micelles. Skimmed milk appears slightly blue because casein micelles scatter the shorter wavelengths (blue compared to red).
Dr. PHILIP JOHN
Scientist, IGCAR, Kalpakkam