[Q] Frozen milk colour

[paigetheoracle]

Why does milk turn yellow when it freezes?

[CraigD]

Milk is a suspension of microscopic protein (casein) and fat particles, in protein (whey) and sugar (lactose) -rich water. Because fat stores various nutrients, especially orange-yellow colored carotene (a good source of which is carrots, but found in most vegetables and, of course, fats), milk fat has a slightly to very yellow color. This is why butter (the real kind, made for milk) is yellow, and buttermilk more yellow than skimmed milk.

 

I found neat explanations, diagrams, micrographs, and links at this “milk under the microscope” webpage.

 

My guess is that, when milk is frozen, more of the yellowish fat globules than the smaller casein micelles are squeezed out of the crystallizing water ice, so the surface of the frozen body (contained inside the plastic bottle) appears more yellow than the original, unfrozen one.

 

You could test this hypothesis several ways: frozen whole milk should be yellower than frozen skimmed/fat-free milk; cutting a milk-popsicle should reveal a cross-section with more yellow on the outside than the inside; a frozen milk-popsicle should feel buttery-slick on the outside. You might even be able to scrape off the outer layer, and get something passably like butter.

 

Also, I’ve a guess that if you melt frozen milk, the fat globules separated to its surface will remain separate, and you’ll get a layer of buttermilk floating on top of skimmed milk.

 

A good opportunity for some kitchen science experiments, here ;)

[paigetheoracle]

Milk is a suspension of microscopic protein (casein) and fat particles, in protein (whey) and sugar (lactose) -rich water. Because fat stores various nutrients, especially orange-yellow colored carotene (a good source of which is carrots, but found in most vegetables and, of course, fats), milk fat has a slightly to very yellow color. This is why butter (the real kind, made for milk) is yellow, and buttermilk more yellow than skimmed milk.

 

 

I found neat explanations, diagrams, micrographs, and links at this “milk under the microscope” webpage.

 

 

My guess is that, when milk is frozen, more of the yellowish fat globules than the smaller casein micelles are squeezed out of the crystallizing water ice, so the surface of the frozen body (contained inside the plastic bottle) appears more yellow than the original, unfrozen one.

 

 

You could test this hypothesis several ways: frozen whole milk should be yellower than frozen skimmed/fat-free milk; cutting a milk-popsicle should reveal a cross-section with more yellow on the outside than the inside; a frozen milk-popsicle should feel buttery-slick on the outside. You might even be able to scrape off the outer layer, and get something passably like butter.

 

 

Also, I’ve a guess that if you melt frozen milk, the fat globules separated to its surface will remain separate, and you’ll get a layer of buttermilk floating on top of skimmed milk.

 

 

A good opportunity for some kitchen science experiments, here :)

 

Well Craig, sorry to get back to you so late but the milk I froze wasn't the one I normally buy. It seemed 'watery' after (no visible separation once defrosted) but now having used it out of the fridge, rather than the freezer, I can see that is its normal state, rather than a consequence of it being frozen and then defrosted.