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The other day, my kid brother asked me this:

What pigment gives milk a white color?

Now, I found that particular question quite interesting since, as a student of Biology, I already had a rough idea as to what constitutes (cow's) milk: Water, sugars, fats, and a whole bunch of vitamins and minerals. Yet as far as I know, none of them play a "pigment"-like role anywhere else in nature... which has left me deeply perplexed.

So what pigment is responsible for milk's characteristic white color?

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There is no "pigment" that makes milk white.

According to Wikipedia:

A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption.

Going by that definition of a "pigment", then there is no (white) pigment in milk. In fact (as @Ivan points out in the comments) there is no such thing as a "white pigment", because that would require the "pigment" to absorb no visible radiation (if it absorbed all visible radiation, it'd appear black)

That's because the real reason for the white color of milk is a physical phenomenon called Scattering. (Source: Optics, 4th edition, E. Hecht)

enter image description here

(Diagram's from Wikipedia)

Milk is an emulsion of fat dispersed as these really tiny droplets in water. These fat globules (and some milk proteins) scatter light of all wavelengths in the visible spectrum. This is why milk appears white

This is the same reason (albeit different cause) as to why clouds often appear white; except here, the scattering is due to suspended water droplets and dust particles.

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    $\begingroup$ I'd rather stress there is no pigment that makes anything white. $\endgroup$ – Ivan Neretin Jan 21 '17 at 10:51
  • $\begingroup$ Isn't the chief reason for the white color the fact that fats/proteins are imbued with calcium? (homogeneously). Roughly every calcium compound in water renders a white solution. Note heterogenous milk is a little yellowish, which you would expect as the color of typical fats/oils. $\endgroup$ – khaverim Jan 21 '17 at 15:47
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    $\begingroup$ Nope. Solutions of calcium compounds are mostly colorless. Arguably, there is no such thing as white solution. $\endgroup$ – Ivan Neretin Jan 21 '17 at 16:01
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    $\begingroup$ @khaverim chemistry.stackexchange.com/questions/66060/… $\endgroup$ – paracetamol Jan 22 '17 at 2:45
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    $\begingroup$ Precipitates in water*, I should have said. $\endgroup$ – khaverim Jan 22 '17 at 2:52

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