[SquarishTriangle (OP)]

Why is it that milk straight from a cow is yellowish, but cow's milk from the shop is white? Do they actually deliberately take the beta carotene out of it...is it just denatured or reacted with another substance during the processing...or is there another explanation?

What do you think?

SqD.

[JimBob]

It has stuff in it - like cells: fat cells and proteins and amino acids and all sorts of other organic things needed for a good growth rate of the calf, the introduction of antibodies to diseases, histamines, hormones - lots of good stuff. It is really just modified sweat.

It may be white as it contains a high amount of calcium and Vitamin C, necessary for calcium uptake, used for the growth of bones in the newborn animal.

[chris]

This is a really interesting observation and is down to the same science that makes snow appear to be white, despite water and ice both being transparent.

I suspect that the milk you've been buying at the supermarket has been homogenised.

Milk contains a lot of fat (about 5-20%, depending upon the animal species that has made it), and this fat usually forms quite large globules. If bottled in this form it would rapidly separate into two layers, one oily and fat-rich, the other watery and protein-rich.

This can be prevented by squirting the milk through a very fine sieve under pressure, which has the effect of breaking up the globules of fat so that they form lots of much smaller "micelles". These consist of a fat-rich core surrounded by protein. The proteins arrange themselves so that their polar (water-loving) regions are pointing outwards into the surrounding water, which helps the fats to suspend themselves in the water, whilst the oily parts of the protein are oriented towards the fatty core.

So why is the homogenised milk whiter?

This is because when light passes from water into oil it changes speed, and this causes its path to bend. The more micelles that are present, the more the light passing through ricochets about until, eventually, it ends up coming back out of the substance again, rather than passing straight through.

So, once it's been homogenised, and contains lots of tiny oily micelles bending the light paths in all directions, more visible light wavelengths are reflected back out of the liquid, making it look whiter.

Chris

[SquarishTriangle (OP)]

Fascinating...and unexpected!

Would a glass of low fat milk, or perhaps diluted milk, then be slightly less white and more yellow than the fuller fat variety?

Thinking about that again, diluting milk obviously does make it less opaquely white...but it does seem to take a fair amount of water to make the whiteness unnoticeable.

[lyner]

When it's low fat (yuk) and diluted it even looks bluish!
Rayleigh scattering by small particles is stronger for short wavelenths (hence the sky is blue) so it's probably for the same reason for milk, once it is diluted enough.
Dairy products with more fat in  - cheese, butter and clotted cream (yum yum) look pretty yellow.
Basically 'nice' must be yellow, if you want some real Science.
Remember the 'goodness', floating on the top of home made soup? Same thing; it's yellow.

[RD]

Fascinating...and unexpected!

Would a glass of low fat milk, or perhaps diluted milk, then be slightly less white and more yellow than the fuller fat variety?

Thinking about that again, diluting milk obviously does make it less opaquely white...but it does seem to take a fair amount of water to make the whiteness unnoticeable.

A pinch of powdered milk added to a fishtank of water can be used to demonstrate a sunset...
   
http://www.smartcenter.org/ovpm/october-05.html

[...and the Naked Scientists also have instructions on how to do this as a "kitchen science" experiment:

http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/why-is-the-sky-blue/ - Ed]