Microfoam

Microfoam is shiny, slightly thickened, and should have microscopic, uniform bubbles. – 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. == Applications ==

, such as this rosette. 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. 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. == Procedure ==

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: • Air is introduced from the steam wand by immersing only the tip of the wand in the milk. This process is sometimes known as frothing, stretching, or surfing, and usually lasts less than 10 seconds. After the creation of small bubbles, the milk is covered with a soft foam phase which separates from the liquid and floats on top of the milk. • The second stage involves mixing the incorporated air throughout the milk (mixing or texturing), which is achieved by immersing the steam wand more deeply (typically 20–30 mm). This step is necessary to integrate the foam which naturally separates from the liquid phase. During this stage, the milk is also heated to about , at which point the steaming is finished. This method is also used to assess whether grooming is necessary (see above), and is intended to delay separation of the milk. • In order to remove any large bubbles from the surface, some baristas tap the jug on a bench before pouring • Occasionally a barista may use less-than-full pressure from the steam wand, if they are steaming a very small amount of milk (variable pressure is usually only a feature on professional machines) • It is also possible to create microfoam for latte art by using a french press, moving the plunger rapidly to aerate the milk. This method can, with practice, yield close to same consistency as with using a steamwand. • A stovetop steamer is also a viable option for generating microfoam. == Chemical and physical properties ==

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). 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 , so milk at higher temperatures is not significantly affected by this problem. This value is higher for raw milk - around . 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. If milk is heated above , 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. A louder screaming sound may be heard if the steam orifice becomes blocked or the machine cannot pump enough air. == References ==