Microfoam

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Cappuccino milk froth.jpg
"Macrofoam" (pictured), or milk frothed with larger air bubbles, is traditionally used in cappuccinos and does not allow for latte art, whereas "Microfoam" is used in caffè lattes and allows for the making of latte art.

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.

Contents

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.

Characteristics

Microfoam is shiny, slightly thickened, and should have microscopic, uniform bubbles. [1] It is not as viscous or "foamy" as macrofoam [2] – 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", [3] "microbubbles", and so forth.

Applications

Microfoam is primarily used for making latte art, such as this rosette. Latte art.jpg
Microfoam is primarily used for making latte art, 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. [4] 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. [5] 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. [6] 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

Milk being steamed. This milk would be too "foamy" for latte art, due to too much air (large bubbles) and the air not being sufficiently mixed into the milk. Steamed milk.jpg
Milk being steamed. This milk would be too "foamy" for latte art, due to too much air (large bubbles) and the air not being sufficiently mixed into the milk.

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. [4] Dedicated electric milk frothers may also be used, usually consisting of a motorized whisk. [7]

When using a steam wand, the volume and type of foam is controlled by the barista during the steaming process, [1] and loosely follows these steps:

  1. 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, [8] 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.
  2. 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). [8] This creates a turbulent vortex or "whirlpool" in the vessel. [9] 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 70 °C (158 °F), at which point the steaming is finished. [6] [10]
  3. Lastly, the milk is poured from the pitcher into a cup, usually already containing espresso. Methods for pouring vary widely depending on the type of drink and personal technique (see Latte art § Styles).
In a "dry" variation of a cappuccino, the foam is light (high air-to-milk ratio) and floats on top on the espresso. Latte art requires heavier, "wetter" foam. Classic Cappuccino.jpg
In a "dry" variation of a cappuccino, the foam is light (high air-to-milk ratio) and floats on top on the espresso. Latte art requires heavier, "wetter" foam.

Notable variations

The details of the above method vary between baristas, and are influenced by the machine and the desired outcome.

Chemical and physical properties

An example of properly steamed milk, considered ideal to pour latte art Nearly texturized milk.jpg
An example of properly steamed milk, considered ideal to pour latte art

The basic requirements for formation of foam are an abundance of gas, water, a surfactant, and energy.[ citation needed ] 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. [11] Varying the balance of these factors affects the size of bubbles, the foam dissipation rate, and the volume of foam. [12]

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. [6] 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). [13]

Interaction of fat and protein

Like in whipped cream, air bubbles are initially stabilized by the protein β-casein, prior to their adsorption of fat. [14] 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. [15] The denaturation of milk fat occurs around 40 °C (104 °F), so milk at higher temperatures is not significantly affected by this problem. [15] At higher temperatures, the protein β-lactoglobulin enables the foam to maintain its structure and is the prime factor in the formation of foam.[ citation needed ] 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. [15] 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. [15]

Microfoam is unstable, decomposing into a layer of "dry" foam over a layer of liquid milk, as in this latte macchiato. Latte macchiato with coffee beans.jpg
Microfoam is unstable, decomposing into a layer of "dry" foam over a layer of liquid milk, as in this latte macchiato.

Effect of temperature

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). [12] [13] [16] 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. [13] 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. [17]

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. [18] When milk is scalded, the suspended protein casein becomes denatured and cannot maintain the intermolecular bonds necessary for microfoam. [19]

The stability of milk foam, measured by the half-life of its volume, is also greatly influenced by temperature. [13] 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. [13] However, microfoam tends to separate into layers more quickly than it reduces in volume, so baristas usually steam milk immediately before serving it. [10] This is especially important when serving latte art which may degrade within minutes.

Sound

When using a steam wand, a slight but audible hissing sound occurs when the air enters the milk, mainly due to microscopic cavitation. [20] [21] A louder screaming sound may be heard if the steam orifice becomes blocked or the machine cannot pump enough air. [22]

Related Research Articles

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<span class="mw-page-title-main">Barista</span> Person who prepares and serves coffee drinks

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Latte art is a method of preparing coffee created by pouring microfoam into a shot of espresso and resulting in a pattern or design on the surface of the latte. It can also be created or embellished by simply "drawing" in the top layer of foam. Latte art is particularly difficult to create consistently, due to the demanding conditions required of both the espresso shot and milk. This, in turn, is limited by the experience of the barista and quality of the espresso machine. The term also applies to other beverages containing milk foam, such as cappuccino and hot chocolate.

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References

  1. 1 2 "Steaming with Prosumer Machines" Archived 2010-03-27 at the Wayback Machine , Coffee Geek
  2. ↑ "Barista Technique:Frothing Milk", home-barista.com
  3. ↑ "Latte Art 101" Archived 2008-08-12 at the Wayback Machine , espressovivace.com
  4. 1 2 "Frothing Milk: A Guide". Coffee Revolution. 28 December 2017. Retrieved 19 May 2019.
  5. ↑ Andrews, Christos (20 October 2018). "How to Make a Cappuccino". Coffee Geek.
  6. 1 2 3 Illy, Andrea; Viani, Rinantonio (2005). Espresso Coffee (2 ed.). Elsevier. ISBN   0123703719.
  7. ↑ "What is a Milk Frother? (With pictures)".
  8. 1 2 3 "Milk Techniques: A Training Guide" (PDF). Darkwoods Coffee. Retrieved 19 May 2019.
  9. ↑ "Milk Frothing Guide - Frothing for Newbies & Intermediates". CoffeeGeek. Archived from the original on 23 December 2010. Retrieved 19 May 2019.
  10. 1 2 3 4 5 6 7 Rao, Scott (2008). The Professional Barista's Handbook. Elsevier. ISBN   9781605300986.
  11. ↑ "Milk structure". University of Guelph. Retrieved 31 May 2019.
  12. 1 2 "Foamability of different milk types, and stability and structure of the foam produced" (PDF). Kruss Scientific. March 2016. Retrieved 31 May 2019.
  13. 1 2 3 4 5 Kamath, Sapna; Huppertz, Thom; Houlihan, Avis V.; Deeth, Hilton C. (October 2008). "The influence of temperature on the foaming of milk". International Dairy Journal. 18 (10): 994–1002. doi:10.1016/j.idairyj.2008.05.001. ISSN   0958-6946.
  14. ↑ Hui, Y.H. (1992). "1. Chemistry and Physics". Dairy science and technology handbook. Vol. 1. Wiley. ISBN   9780471187974.
  15. 1 2 3 4 Fick, Karen. "The science of milk in coffee". Difford's Guide. Retrieved 20 May 2019.
  16. ↑ Sanmann, F.P.; Ruehe, H.A. (1930). "Some factors influencing the volume of foam on milk". Journal of Dairy Science. 13 (13 ed.): 48–63. doi: 10.3168/jds.S0022-0302(30)93502-3 .
  17. ↑ Shinde, U.P.; Cougule, S.S.; Dighavkar, C.G.; Jagadale, B.S.; Halwar, D.K. (June 2015). "Surface Tension as a Function of Temperature and Concentration of Liquids" (PDF). International Journal of Chemical and Physical Sciences. 4. ISSN   2319-6602.
  18. ↑ "Denaturation". Science in Context. 2006-04-03.
  19. ↑ Collins, Clare (9 December 2016). "The chemistry behind amazing meringue and perfect cappuccino". The Conversation. Retrieved 19 May 2019.
  20. ↑ "How To Steam Milk", latteartguide.com Archived May 2, 2013, at the Wayback Machine
  21. ↑ "SCAE Diploma: Barista Foam standards" (PDF). Flair Academy. Retrieved 19 May 2019.
  22. ↑ Grant, Keith (2 March 2011). ""Screaming" noise while steaming milk?". Seasoned Advice. Retrieved 19 May 2019.
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