MILK: THE SCIENCE
There is a bit of a science to steaming milk as with the rest of espresso making. Here it is in a simplified explanation:
Cold milk is made of:
Fat: The good stuff.
Sugar: (Lactose), made up of galactose and glucose.
Protein: A long chain of amino acids which, when in cold milk are all wound up. The hydrophobic molecules of the protein are buried inside the coil because they do not like water.
When you heat the milk…
Fat: Gets thinner and melts.
Sugar: Breaks down into smaller simpler sugars which have a sweeter taste. Optimum temperature for maximum sweetness is 60 degrees celsius.
Protein: The coil loosens up and the hydrophobic molecules become exposed and look for somewhere to go. The hydrophobic molecules go into the bubbles produced by air and are encapsulated by a skeleton of protein.
Start with milk below 7 degrees and introduce air until 38 degrees. This step will lock the air into tiny bubbles that we call micro-foam. Once the temperature of the milk is lost (38 degrees), tilt the jug up and maintain a whirlpool, increasing the temperature but no longer injecting air. Keep steaming until you reach 60 degrees, this is where the milk is at its sweetest. Heating the milk any further will allow for souring and scalding.