Light scattering. Fats. Micelles. And why whole milk is whiter than fat-free milk (and sweetened condensed milk is yellow).
Why does fat-free milk look more translucent than whole milk?
We see white light or any color because of light scattering. This is what happens when light falls on and into any material, and its wavelengths are reflected and deflected by the particles on and in the material. When the scattered light enters our eyes, we see the objects and their colors around us.
For this to happen, the particles must be large enough to scatter the light’s wavelengths. In milk, these particles are micelles and fat.
According to How Baking Works, “[m]ost of the whiteness of milk is from the scattering of light off casein micelles.” This is true of fat-free, low-fat or whole milk.
Micelles are “clusters of casein molecules” which are made up of the protein caseins bound up with calcium and phosphorus. They are the reason milk is such a rich source of calcium and proteins for babies and bodybuilders alike (read all about it in the 1-Minute NomNom “Making a strong caseins for milk“).
But light is also scattered off the fat in the milk. The less fat, the less light scattering there is. This is why “whole milk appear[s] white and more opaque than fat-free milk“, and low-fat or skimmed milk’s white sits somewhere in between.
The effect of light scattering and color is even more pronounced when the amount of milk fat is increased. For example, sweetened condensed milk has a high fat content because much of its water has been removed. It doesn’t look white; it looks yellowish because the light is now also scattered off the carotenoids (naturally occurring pigments such as yellow and orange) in the milk fat.
So now fortified with the knowledge of light effects, you can get moo-ving to choose your next milk!
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