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How the Latte-Whip whips your Latte |
 Protein is the reason why milk foams. Milk is composed of protein, calcium and fat. The lower the fat content of the milk, the more the protein molecules are available for modification. This does not mean that lower fat milk contains more protein, it just means that there is a higher protein to fat ratio.
A protein molecule is shaped somewhat like a strand of spaghetti wound up on a plate. This strand can elongate (stretch out) or tighten, depending on the temperature of the milk. In order for milk to foam, the proteins must break apart and subsequently get together with a neighbouring protein molecule. "Getting together" means that the proteins form temporary bonds with each other. Sort of like breaking a noodle apart and sticking it to another noodle. When the proteins get together, the milk becomes "more solid", and therefore forms a foam. However, these bonds are temporary and inherent unstable. Once the milk is foamed, the foam will eventually collapse (much like any type of foam -- hair mousse, soap foam, etc).
The Latte-Whip beater head makes a foam by creating a mechanical action that breaks the proteins apart and allows them to bond to neighbouring proteins. However, if you whip the milk too much, you shear the proteins (break them up beyond recognition). Once the proteins are sheared, they cannot bond to anything, and the milk will never foam. As well, once milk is foamed once, it will never foam satisfactorily again, because the proteins are damaged.
When milk is very cold, the proteins are very tight. If the proteins are tight, they are less likely to break apart. If the milk is slightly warm (room temperature), the proteins relax and are more likely to break apart and form bonds. If the milk is very hot, the proteins denature (break apart into their components) and they are unable to form bonds. Conclusion: skim milk at room temperature is the best for foaming.
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