microfoam
   HOME

TheInfoList



OR:

Microfoam is finely textured
milk Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digestion, digest solid food. Immune factors and immune ...
used for making
espresso Espresso (, ) is a coffee-brewing method of Italian origin, in which a small amount of nearly boiling water (about ) is forced under of pressure through finely-ground coffee beans. Espresso can be made with a wide variety of coffee beans and ...
-based
coffee Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulant, stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world. S ...
drinks, particularly those with
latte art 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. ...
. It is typically made with the steam wand of an
espresso machine 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. The first machine for making espresso was built in ...
, 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 commonly used for
cappuccino A cappuccino (; ; Italian plural: ''cappuccini'') is an espresso-based coffee drink that originated in Austria and was later popularized in Italy and is prepared with steamed milk foam (microfoam). Variations of the drink involve the use of cre ...
s.


Characteristics

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.


Applications

The decorative application of microfoam is called
latte art 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. ...
, 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
latte Caffè latte (), often shortened to just latte () in English, is a coffee beverage 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 ma ...
s, as the name suggests, but can also be used in
cappuccino A cappuccino (; ; Italian plural: ''cappuccini'') is an espresso-based coffee drink that originated in Austria and was later popularized in Italy and is prepared with steamed milk foam (microfoam). Variations of the drink involve the use of cre ...
s 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 __NOTOC__ Latte macchiato () is a coffee beverage. The name means stained or marked milk, which refers to the espresso stain on the milk used. It is a play on "Espresso macchiato". an espresso with a dollop or two of milk or cream. Related dri ...
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 ''Café au lait'' (; ; French for "coffee with milk") is coffee with hot milk added. It differs from white coffee, which is coffee with cold milk or other whiteners added. Variations Europe In Europe, ''café au lait'' stems from the same c ...
, 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 Third-wave coffee is a movement in coffee marketing emphasizing high quality. Beans are typically sourced from individual farms and are roasted more lightly to bring out their distinctive flavors. Though the term was coined in 1999, the approach o ...
.


Procedure

Microfoam is usually created with the steam wand of an
espresso machine 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. The first machine for making espresso was built in ...
. 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,"Steaming with Prosumer Machines"
Coffee Geek
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 creates a turbulent
vortex In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
or "whirlpool" in the vessel. 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. # 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 .


Notable variations

The details of the above method vary between baristas, and are influenced by the machine and the desired outcome. * It is common to briefly switch on the steam wand before using it, in order to flush any condensed water from the plumbing and preheat the steam wand itself. The same is often done after steaming milk, to remove milk residue. * On machines with pivoting steam wands, the wand should be between 10° and 30° from vertical. However, some baristas tilt the jug relative to the steam wand, whilst keeping the wand almost vertical. * If milk has been over-aerated (i.e. the froth is too thick), it may be ''groomed'' by running the tip of a spoon through it. * As a supplementary method of mixing, a barista may swirl the pitcher just before pouring it. 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 A French press, also known as a cafetière, ''cafetière à piston'', ''caffettiera a stantuffo'', press pot, coffee press, or coffee plunger, is a coffee brewing device, although it can also be used for other tasks. In 1923 Ugo Paolini, an Ital ...
, moving the plunger rapidly to aerate the milk. This method can, with practice, yield close to same consistency as with using a steamwand. * Portable stovetop steamer is also a viable option for generating microfoam.


Chemical and physical properties

The basic requirements for formation of
foam Foams are materials formed by trapping pockets of gas in a liquid or solid. A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the reg ...
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 In chemistry and physics, metastability denotes an intermediate Energy level, energetic state within a dynamical system other than the system's ground state, state of least energy. A ball resting in a hollow on a slope is a simple example of me ...
liquid-gas
colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...
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 An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althoug ...
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).


Interaction of fat and protein

Like in whipped cream, air bubbles are initially stabilized by the protein ''β''-casein, prior to their
adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a f ...
of fat. This adsorption causes destabilization of the bubbles, because the fat molecules are
amphiphilic An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compo ...
(i.e. they have
polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates * Polar climate, the c ...
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. 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 show trivially by adding various quantities of skim
milk powder Powdered milk, also called milk powder, dried milk, or dry milk, is a manufactured dairy product made by evaporating milk to dryness. One purpose of drying milk is to preserve it; milk powder has a far longer shelf life than liquid milk and do ...
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.


Effect of temperature

Several studies have confirmed that the foamability of
pasteurized Pasteurization American and British English spelling differences#-ise, -ize (-isation, -ization), or pasteurisation is a process of food preservation in which packaged and non-packaged foods (such as milk and fruit juices) are treated with mi ...
whole milk, measured by the volume of foam produced, reaches a minimum at . This value is higher for
raw milk Raw milk or unpasteurized milk is milk that has not been pasteurized, a process of heating liquid foods to kill pathogens for safe consumption and extending the shelf life. Proponents of raw milk have asserted numerous supposed benefits to consu ...
- 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. 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 , it becomes scalded and its texture is compromised. Microfoam cannot exist in overheated milk due to the missing
tertiary structure Protein tertiary structure is the three dimensional shape of a protein. The tertiary structure will have a single polypeptide chain "backbone" with one or more protein secondary structures, the protein domains. Amino acid side chains may int ...
in the protein. When milk is scalded, the suspended protein
casein Casein ( , from Latin ''caseus'' "cheese") is a family of related phosphoproteins (CSN1S1, αS1, aS2, CSN2, β, K-casein, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of ...
becomes denatured and cannot maintain the intermolecular bonds necessary for microfoam. The stability of milk foam, measured by the
half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
of its volume, is also greatly influenced by temperature. For pasteurized whole milk, stability increases with temperature up to about , then rises steeply until , 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 , 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.


Sound

When using a steam wand, a slight but audible hissing sound occurs when the air enters the milk, mainly due to microscopic
cavitation Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, cal ...
."How To Steam Milk"
latteartguide.com
A louder screaming sound may be heard if the steam orifice becomes blocked or the machine cannot pump enough air.


References


External links


CoffeeGeek - ''The Milk Frothing Guide''



Difford's Guide - ''The science of milk in coffee''

Kruss Scientific - ''Foamability of different milk types, and stability and structure of the foam produced''

International Dairy Journal - ''The influence of temperature on the foaming of milk''
{{Coffee Milk Coffee_chemistry