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 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 show 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

<span class="mw-page-title-main">Cappuccino</span> Drink made with espresso coffee and steamed milk

A cappuccino is an espresso-based coffee drink that is traditionally prepared with steamed milk foam (microfoam).

<span class="mw-page-title-main">Espresso</span> Type of strong coffee

Espresso is one of the world's most popular coffee-brewing methods. Originating from Italy, the French also made significant contributions via the invention of coffee makers, predecessors of today's espresso machines.

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

A barista is a person, usually a coffeehouse employee, who prepares and serves espresso-based coffee drinks and other beverages.

<span class="mw-page-title-main">Latte</span> Beverage made with espresso coffee and milk

Caffè latte, often shortened to just latte in English, is a coffee drink of Italian origin made with espresso and steamed milk. Variants include the chocolate-flavored mocha or replacing the coffee with another beverage base such as masala chai, mate, matcha, turmeric or rooibos; alternatives to milk, such as soy milk or almond milk, are also used.

<span class="mw-page-title-main">Espresso machine</span> Device used to brew espresso coffee

An espresso machine brews coffee by forcing pressurized water near boiling point through a "puck" of ground coffee and a filter in order to produce a thick, concentrated coffee called espresso. Multiple machine designs have been created to produce espresso. Several machines share some common elements, such as a grouphead and a portafilter. An espresso machine may also have a steam wand which is used to steam and froth liquids for coffee drinks such as cappuccino and caffè latte.

<span class="mw-page-title-main">Caffè macchiato</span> Espresso coffee drink with a small amount of milk

Caffè macchiato, sometimes called espresso macchiato, is an espresso coffee drink with a small amount of milk, usually foamed. In Italian, macchiato means "stained" or "spotted", so the literal translation of caffè macchiato is "stained coffee" or "marked coffee".

<i>Café au lait</i> Drink made with coffee and hot milk

Café au lait is coffee with hot milk added. It differs from white coffee, which is coffee with cold milk or other whiteners added.

<span class="mw-page-title-main">Cortado</span> Espresso with equal amount of warm milk

A cortado is a Spanish beverage consisting of espresso mixed with a roughly equal amount of warm milk to reduce the acidity, although the exact ratios have considerable regional variation. The milk in a cortado is steamed, but not frothy and "texturized" as in many Italian coffee drinks. The cortado is commonly served all over Spain. The word cortado is the past participle of the Spanish verb cortar, in the sense of "dilute", and can refer variously to either coffee or espresso drinks throughout Spanish and Portuguese speaking countries.

<span class="mw-page-title-main">Coffee cup</span> Tableware product

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<span class="mw-page-title-main">Latte macchiato</span> Hot espresso drink with milk

Latte macchiato is a coffee beverage. The name is Italian for "stained milk" or "marked milk", referring to the way the drink is prepared by pouring a shot of espresso into steamed milk. It is a play on espresso macchiato, an older drink consisting of espresso marked with a dollop or two of milk or cream.

<span class="mw-page-title-main">Flat white</span> Drink of espresso coffee with steamed milk

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<span class="mw-page-title-main">Coffee preparation</span> Process of turning coffee beans into a beverage

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<span class="mw-page-title-main">Milk coffee</span> Category of coffee-based drinks

Milk coffee is a category of coffee-based drinks made with milk. Johan Nieuhof, the Dutch ambassador to China, is credited as the first person to drink coffee with milk when he experimented with it around 1660.

Scalded milk is dairy milk that has been heated to 83 °C (181 °F). At this temperature, bacteria are killed, enzymes in the milk are destroyed, and many of the proteins are denatured. Since most milk sold today is pasteurized, which accomplishes the first two goals, milk is typically scalded to increase its temperature, or to change the consistency or other cooking interactions due to the denaturing of proteins.

Thermoplan AG is a manufacturer of kitchen appliances for the restaurant sector. The headquarters of the company are located in Weggis (LU), Switzerland.

<span class="mw-page-title-main">Latte art</span> Type of decoration on coffee

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.

<span class="mw-page-title-main">Frappé coffee</span> Greek iced coffee drink

A frappé coffee, cold coffee, Greek frappé, or just frappé is a Greek iced coffee drink generally made from spray-dried instant coffee, water, sugar, and milk. The word is often written frappe. The frappé was invented in 1957 in Thessaloniki through experimentation by Dimitris Vakondios, a Nescafe representative. Frappés are among the most popular forms of coffee in Greece and Cyprus and have become a hallmark of postwar outdoor Greek coffee culture.

<span class="mw-page-title-main">Milk frother</span> Utensil

A milk frother is a utensil for making milk froth, typically to be added to coffee. It aerates the milk, creating a thick but light foam. Milk frothers were introduced through the use of espresso machines that contained steamed wands that would froth steamed milk. Although created in Italy, the espresso machine and steam wands were exported internationally to other countries, and frothed milk was introduced around the world.

<span class="mw-page-title-main">Raf coffee</span> Coffee drink made using espresso and vanilla sugar

Raf or raf coffee is a popular coffee drink in Russia and the countries of the former USSR, which appeared in the late 1990s. Prepared by adding cream and vanilla sugar to a single shot of espresso and then foaming the mix with a steam heater. The main differences from latte are the use of vanilla sugar and cream instead of milk and the fact that the whole mix is foamed together instead of just milk. Syrup is often used instead of vanilla sugar.

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 ed.). 13: 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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