Latte art is a romantic display of a barista's skill, with white milk foam blooming into heart or leaf patterns on the surface of dark brown espresso. Latte art is divided into two types: free-pour latte art and etching done afterward. The wonder of the former lies in the pattern taking shape simultaneously as the milk is poured, while the latter can achieve a fuller variety of designs because the foam is stable enough to allow leisurely drawing. From a physics perspective, the core of latte art lies in surfactants—molecules that possess both hydrophilic and hydrophobic groups, which can stabilize the interface between air and liquid to form lasting foam. Milk proteins and coffee lipids together act as surfactants, allowing the two micron-scale foams to remain clearly separated for a long time after mixing, thereby preserving the pattern. This article will take you from the basic principles of surfactants to understand espresso's crema, the formation of hot milk foam, and why foam mixing rather than liquid mixing is the key to successful latte art, and will reveal the wide applications of surfactants in daily life and industry.
Most people who have seen a latte art performance are amazed by the barista's exquisite craftsmanship. Pure white steamed milk is poured into dark espresso, and with the deft movement of the barista's wrist, heart-shaped or leaf-like patterns emerge on the surface of the latte. The scene is both atmospheric and especially romantic.
Latte art can be broadly divided into two categories:
One is free pour, which means "pulling" a pattern while pouring hot milk into espresso;
The other is etching, where milk and coffee are first mixed, and then tools such as chocolate sauce and toothpicks are used to directly "carve" a design onto the latte foam surface.
Comparing latte art and etching, the former is more wondrous, while the latter can present a richer variety of patterns. The reason is that when etching, the foam on the latte surface has a certain stability, enough to support colored sauces such as chocolate sauce and caramel syrup. This not only gives the barista more time, but also allows greater freedom in drawing and composition. The uniqueness of free-pour latte art lies in the fact that the pattern forms simultaneously during the process of preparing the latte.
Regarding the mystery of latte art, an experienced barista might begin by explaining how to extract espresso and steam hot milk foam. However, if we ask from a physics perspective, what exactly is latte art?
This starts with surfactants.
Surfactants, also known as surface-active agents, are a type of substance with an extremely wide range of uses, almost everywhere in daily life. Laundry powder and dishwashing liquid rely on them to remove oil and dirt, children use them to blow bubbles, and without them, many drinks, including milk, would not appear as we know them. When used in the food industry, surfactants are sometimes also called emulsifiers.
Surfactants are usually a class of small organic molecules that possess both hydrophilic and hydrophobic parts. A typical surfactant has this structure: a polar group that can be hydrophilic acts as the "head" (the sphere in Figure B), plus one or more long "tails," which are hydrophobic groups easily soluble in nonpolar media such as oil (generally a somewhat longer carbon chain). Many readers learned in middle school that the phospholipid molecules that make up biological cell membranes also have a similar structure; they carry two hydrophobic tails and can likewise act as surfactants. It should be noted that this image of a surfactant should not be confused with what we commonly call a "tadpole."
If some surfactant is added to water, because the hydrophobic tails do not want to contact water, they draw close to each other and use the hydrophilic heads to separate the water from the tails, lowering the energy of the entire system. As shown in the later figures, this can form various structures: spherical micelles with hydrophilic heads wrapping hydrophobic tails, capsule-like shapes, structures in which two layers of surfactant molecules wrap into a sphere (the Liposome in the figure below), or planar membranes composed of bilayers, etc. This system is believed to be related to the earliest formation of life, namely how primitive cells gradually formed from a primordial soup mixed with various organic molecules such as primitive RNA or DNA [2].
An important property of surfactants is that they can mix two liquids that originally cannot mix, or a liquid and a solid powder, to form an emulsion or colloid. For example, water and oil can form an emulsion by adding a surfactant. Depending on the water-to-oil ratio, the resulting structure can be water-in-oil, oil-in-water, or a multilayered structure with oil and water separated into layers, with the surfactant distributed at the interface between water and oil, maintaining the stability of the whole system. An important application of this property is washing oil stains from clothes.
Proteins usually contain both hydrophilic and hydrophobic groups, so many proteins can act as surfactants. Mayonnaise in Western cuisine is mainly made from vegetable oil, eggs, lemon juice or vinegar, and other seasonings. The proteins in the eggs act as surfactants, mixing the vegetable oil and water together. Milk is a natural emulsion, and the proteins it contains also wrap around the fat in milk and suspend it in water. If this system is disrupted by external forces or other means, the fat can be collected to obtain butter, and the proteins can be collected to obtain cheese.
Surfactants can also be used to stabilize the interface between air and water—in plain terms, to blow bubbles. When a large number of small bubbles gather together and the water between them gradually drains away, the bubbles come into contact with one another and form a polyhedral structure: foam. At this point, the hydrophobic tails are exposed to the air, while the hydrophilic heads are embedded in a thin layer of water. The presence of the surfactant lowers the surface tension between air and water, allowing this foam structure to be maintained for a period of time, letting children enjoy blowing bubbles, letting photographers capture their appearance, letting scientists study their properties, and also letting baristas create latte art.
In the cup used to make latte art, about one-third of the volume of espresso should first be poured in. This is a strongly flavored coffee obtained by forcing near-boiling hot water under high pressure through finely ground and compacted coffee powder [4]. On the surface of the espresso floats a layer of reddish-brown foamy crema. This layer of crema is foam formed by the fat components in coffee and gas, and the substances acting as surfactants are proteins in coffee as well as phospholipid molecules from cell membranes.
The volume of crema generally accounts for more than 10% of the espresso, and the volume content of gas can account for about half of the crema. Observing the structure of crema under an optical microscope reveals that it contains bubbles, fat particles (usually smaller than 10 micrometers), and some solid particles (such as fragments of coffee bean cell walls) [5]. Do not underestimate this layer of crema; its presence is often regarded as a sign of espresso quality. Moreover, crema itself is also an important factor in forming latte art.
[Crema observed under an optical microscope.]
When making latte art, the hot milk poured into the espresso has already been pre-treated. With the proteins in milk acting as surfactants, and through stirring and other means, a layer of foam made of milk mixed with air (microfoam) forms on the surface of the hot milk.
In the process of mixing hot milk with espresso, two kinds of foam are stirred together: one is foam made of air mixed with milk, and the other is crema foam made of fat and gas on the surface of espresso mixed with espresso. Both foams are relatively stable. Crema foam can generally be maintained for about 10 minutes [5], and foam formed by milk and air can also last for several minutes [8].
After these two foams are mixed, because their particles are both relatively large (micron scale) and are packed against each other, the diffusion process of the particles is very slow. Without stirring, the mixing speed between the two foams will be very slow, so the boundary between the foams can remain clear for a long time. In this way, the pattern made by the barista when pouring milk can be maintained long enough for customers to appreciate.
Some readers may ask, why must foam be mixed together? Why can't liquids of different colors be mixed together?
When we pour milk into coffee, although micron-sized particles still exist inside these two liquids, these particles are not pressed close to one another, and such particles account for only a very small part of the liquid. Even if they do not immediately mix uniformly, because the internal liquid flow is very unstable, the interface where the two liquids mix will quickly become blurred due to diffusion, making it impossible to create a latte art pattern.
In fact, in addition to latte art, surfactants have very important applications in our daily lives and industrial production. When washing clothes and dishes, the hydrophobic tails of surfactants penetrate deep into the stains, while the hydrophilic heads connect closely with water, and under the movement of washing, the stains are wrapped into suspended particles and carried away with the water. In the food industry, surfactants are used to mix different ingredients together to obtain new flavors and textures (such as the mayonnaise mentioned earlier). In industrial production, surfactants can be used to mix different raw materials to facilitate chemical reactions, or to mix different materials to facilitate subsequent processing (such as paint).
The above content is compiled by CoffeeHunters, a coffee news website.
