Procedure
Creating steamed milk requires the introduction of steam to the milk until a certain amount of foam (also called "froth") is created.
The foam content is controlled by the barista during the steaming process, and involves two phases: first, introducing air ("frothing, stretching") by having the steam tip near the top of the milk, and second, mixing the incorporated air throughout the milk ("mixing, texturing"), which is achieved by having the steam tip immersed more deeply, creating flow or a "whirlpool" in the milk. During the steaming process, the foam phase is both primarily created at the top, and naturally separates from the liquid phase, and rises to the surface. As a result, after the steaming ends, the mixing often continues by swirling the pitcher in a smooth, circular motion until the mixture of foam and warmed milk becomes homogenous, before being added to a drink.
Too much foam, or foam that is not sufficiently mixed with the milk, creates "dry" foam and a totally white surface on a drink (see cappuccino), while not enough foam yields liquid, runny milk and creates a homogeneous, light-brown coffee drink, with no foam or patterning, just like adding warm milk.
To pour latte art, the milk should be shiny, slightly thick, and should have very small, uniform bubbles. It is not particularly "foamy" – it is better described as "gooey" and closely resembles melted marshmallows. There have been a variety of names used for this ideal standard, such as "microfoam", "velvet milk", "microbubbles", and so forth.
The milk must not be overheated to the point that it is scalded, as at this point the enzymes denature and the texture no longer works.
Read more about this topic: Microfoam