What Causes Frozen Semi-Skimmed Milk to Change Color?

[cmb]

When you freeze semi-skimmed milk, it goes yellow, and when you unfreeze it, it goes back to white.

I would tend to have assumed that the two colours were the same (i.e. the 'white' is just a very very light shade of the same yellow colour) so maybe the question is why the shade of it changes.

Why do we get this change of appearance?

 

[xts]

My ad hoc explanation (they often occur to be wrong!)

As you freeze milk, relatively large the crystals of pure ice are forming, taking out water - thus making the liquid part to be a kind of condensed milk or coffee cream. Those are yellowish too. Light comes through large, transparent crystals of ice, getting scattered only on yellowish condensed milk.

 

[JeffKoch]

Frozen adolescent yak urine turns royal purple when it freezes, so go figure.

Actually I think it's due to separation of fatty components due to different freezing temperatures. To test this you could try freezing different kinds of milk - whole, lowfat, nonfat, etc. I would guess that nonfat milk doesn't turn yellow, but I haven't done the experiment myself.