Why is milk white? was a question asked in my Summer, 2003 newsletter from the dairy that delivers my milk, Royal Crest Dairy in Denver. The answer given was: "Milk contains casein, a milk protein rich in calcium that is white." Well, whether it is the casein or the calcium that should be white, this answer is incorrect.
The casein is soluble either in the water or the fat of the milk emulsion, and would be colorless and transparent in either case. Calcium, of course, is not white either. Even calcium carbonate is clear and colorless in a good crystal.
The whiteness is due to the scattering of light by the colloidal particles of the milk emulsion. Just the fact that we get white shows that there is no absorption, so whatever is there is transparent. This is true of most whites. The excellent white pigment titanium dioxide is actually made of clear crystals, dispersed as colloidal particles. Nearly all whites are the result of scattering from colloidal-sized particles. Colors, like green, may be produced in transmission by absorption of other colors; no such process can produce white, which in fact is a subjective color, existing completely in the visual sense. White is a very special color.
Composed by J. B. Calvert
Created 16 June 2003