Microfoam

Microfoam is finely textured milk used for making espresso-based coffee drinks, particularly those with latte art. It is typically made with the steam wand of an espresso machine, which pumps steam into a pitcher of milk.
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, traditionally used for cappuccinos.
Microfoam is shiny, slightly thickened, and should have microscopic, uniform bubbles. It is not as viscous or "foamy" as macrofoam – it is better described as "gooey" and resembles melted marshmallows or wet paint. There have been a variety of names used for this ideal standard, such as "microfoam", "velvet milk", "microbubbles", and so forth.
The decorative application of microfoam is called latte art, which involves making patterns in espresso-based drinks. Microfoam is essential for this as the microscopic bubbles give definition and stability to the patterns, which are harder to achieve with macrofoam which disperses more readily. Latte art is traditionally associated with lattes, as the name suggests, but can also be used in cappuccinos and other drinks.
A cappuccino made with microfoam is sometimes called a "wet" cappuccino. However, cappuccinos typically use thicker macrofoam, with a layer of dry foam floating on the top of the drink. Latte macchiato is another drink which generally has separate layers of dry foam and liquid milk, but microfoam is occasionally used instead. 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 (a "coffee-free" cappuccino), though this can instead be made with dry foam.
As it requires a skilled barista to produce microfoam (especially when used for latte art), it is a sign of attention to quality, and a defining characteristic of the third wave of coffee.
Microfoam is usually created with the steam wand of an espresso machine. This is the quickest method and provides precise control over the timing and depth of air injection. Alternative methods are rarely as effective for producing microfoam, but some are acceptable for macrofoam. These include whisking, shaking, and hand pumps. Dedicated electric milk frothers may also be used, usually consisting of a motorized whisk.
When using a steam wand, the volume and type of foam is controlled by the barista during the steaming process, and loosely follows these steps:
The details of the above method vary between baristas, and are influenced by the machine and the desired outcome.
The basic requirements for formation of foam are an abundance of gas, water, a surfactant, and energy. The steam wand of an espresso machine supplies energy, in the form of heat, and gas, in the form of steam. The other two components, water and surfactants, are naturally occurring ingredients of milk. Varying the balance of these factors affects the size of bubbles, the foam dissipation rate, and the volume of foam.
Microfoam may be represented simply as a metastable liquid-gas colloid of milk and air, consisting of gaseous bubbles suspended in the liquid milk. In reality, the suspension is more complex because milk consists of two different colloids itself - an emulsion of fat and a sol of protein. In fact, these two colloids are what enable milk to form such a mechanically strong foam which does not collapse under its own weight. The interaction between fat and air creates a structure of microscopic bubbles strong enough to support itself, and even be submerged (i.e. suspended within the liquid milk).
Like in whipped cream, air bubbles are initially stabilized by the protein β-casein, prior to their adsorption of fat. This adsorption causes destabilization of the bubbles, because the fat molecules are amphiphilic (i.e. they have polar and non-polar ends), competing with protein molecules which are more conducive to bubbles. The denaturation of milk fat occurs around 40 °C (104 °F), so milk at higher temperatures is not significantly affected by this problem. At higher temperatures, the protein β-lactoglobulin enables the foam to maintain its structure and is the prime factor in the formation of foam. This can be shown trivially by adding various quantities of skim milk powder which contains a high concentration of β-lactoglobulin.
Since fat reduces the likelihood of bonding at the surface of bubbles, it follows that fat content in milk is inversely proportional to its frothing potential. Whilst this is true, an excessive fat constituent also enables larger bubbles, leading to macrofoam rather than microfoam. As a result, most baristas prefer to use whole milk rather than skim milk, due to its tendency to form smaller, more homogeneous bubbles.
Several studies have confirmed that the foamability of pasteurized whole milk, measured by the volume of foam produced, reaches a minimum at 25 °C (77 °F). This value is higher for raw milk - around 35 °C (95 °F). The dip in foamability occurs due to fat globules consisting of both solid and liquid phases at this temperature. Solid fat crystals in a globule may penetrate the film which separates them from the surrounding air, causing spreading of the membrane material which is then adsorbed onto air bubbles. At temperatures above the minimum foamability temperature, the volume of foam steadily increases, which has been attributed to the trends of decreasing viscosity and surface tension with temperature.
If milk is heated above 82 °C (180 °F), it becomes scalded and its texture is compromised. Microfoam cannot exist in overheated milk due to the missing tertiary structure in the protein. When milk is scalded, the suspended protein casein becomes denatured and cannot maintain the intermolecular bonds necessary for microfoam.
The stability of milk foam, measured by the half-life of its volume, is also greatly influenced by temperature. For pasteurized whole milk, stability increases with temperature up to about 40 °C (104 °F), then rises steeply until 60 °C (140 °F), where it starts steadily decreasing. Skim milk generally produces more stable foam, owing to its lower concentration of micellar casein. For regular pasteurized, homogenized whole milk, steamed at 70 °C (158 °F), the half-life is roughly 150 minutes. However, microfoam tends to separate into layers more quickly than it reduces in volume, so baristas usually steam milk immediately before serving it. This is especially important when serving latte art which may degrade within minutes.
When using a steam wand, a slight but audible hissing sound occurs when the air enters the milk, mainly due to microscopic cavitation. A louder screaming sound may be heard if the steam orifice becomes blocked or the machine cannot pump enough air.