Microfoam

Microfoam is milk foamed using a steam wand on an espresso machine, used for making espresso-based coffee drinks, particularly those with latte art.

The opposite of microfoam is macrofoam (also called dry foam, in contrast to the wet foam of microfoam), which has visibly large bubbles, a style of milk commonly used for cappuccinos.

The primary application of microfoam is in making latte art – patterns in espresso-based drinks – and is essential for this. Latte art is traditionally associated with lattes (drinks of 240ml or more), as the name suggests, but can also be used in shorter drinks. If cappuccino-sized (150-180ml), this is sometimes known as a "wet" cappuccino, and latte art can also be made even in the small macchiato.

Microfoam may also be added to brewed coffee in a café au lait, and faint latte art can be produced. Microfoam may also be used in a steamer, though this can instead be made with dry foam.

By contrast, microfoam is not necessary for "dry" cappuccinos, which instead have a layer of dry foam floating on top, nor is it necessary or generally used in latte macchiatos, which also typically have separate layers of dry foam and liquid milk.

As producing microfoam, and a fortiori latte art, requires a skilled barista, it is a sign of attention to quality, and a defining characteristic of the third wave of coffee.

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. A slight but audible hissing sound occurs when the air enters the milk. The second stage involves 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. Tools known as milk frothers may be used for this purpose. 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 homogeneous, before being added to a drink.

...
Wikipedia