What Do Oils in Coffee Reveal About an Espresso Shot?

Crema, the golden-brown foam on an espresso, forms from oils in coffee combined with carbon dioxide, melanoidins, and hydrophobic substances released during extraction. A thick, dark-brown layer points to sufficient extraction and heavier mouthfeel, while a thin or pale layer suggests the shot was pulled short or unevenly.

The foam is a fairly recent marker. Espresso with crema traces back to Achille Gaggia and his lever-piston Crema Naturale machine, which replaced the stable high pressure earlier designs lacked; pre-war posters show espresso as plain black liquid. Illy's definition now treats crema as part of what is coffee, describing the surface foam of fine bubbles.

Crema (coffee oil) is often regarded as an important visual indicator of whether an espresso is up to standard; it relates both to mouthfeel thickness and reveals whether extraction was sufficient. From the perspective of Front Street Coffee, this article sorts out the historical origins of crema, Illy's classic definition of espresso, and how the foam layer forms with the help of carbon dioxide, melanoidins, and hydrophobic substances. It also explains why crema dissipates, what the depth of color can indicate, and how extraction time and water temperature affect the oil's performance, helping coffee lovers and baristas understand this golden-brown foam more systematically.

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Truly understanding the essence of crema will greatly help baristas make a delicious espresso. Today, from the perspective of the Chongqing Barista Coffee Training Course, we will break down crema in detail to help you understand espresso more deeply and improve your practical skills.

In the pursuit of a cup of espresso that is not bitter and has excellent flavor, disappointing results can appear almost every day, and coffee crema is a very key reference when judging quality. Whether ordering in a café or extracting at home, an espresso with a thick, dark-brown crema is the state everyone strives to achieve.

Today it is widely believed that espresso topped with crema dates back to Achille Gaggia—the Italian who first designed a coffee machine capable of providing stable high pressure. In fact, if you look at coffee machine advertising posters from before World War II, such as the classic Victoria Arduino poster, you will find that the espresso in the image shows almost no crema, just a cup of black liquid. Gaggia's "Crema Naturale" coffee machine used a lever-piston structure, completely changing mainstream coffee drinkers' perception of espresso, and the first person to be shocked was probably Gaggia himself.

Today, almost any definition of espresso cannot avoid crema. Illy Cafe has become an industry standard-setter in many respects, and their definition of espresso is as follows: "Espresso" is a polyphasic beverage made simply by combining roasted and ground coffee powder with water. The surface of the espresso is covered with a foam made of fine bubbles, and a unique mark resembling tiger-stripe patterns also appears on the surface. Beneath the foam are particles accompanied by emulsified microscopic oil droplets, as well as a solution in which sugars, acids, protein-like substances, and caffeine are dissolved, with bubbles and solids suspended in it.

(Emulsification: a phenomenon in which one liquid is uniformly dispersed as extremely fine droplets in another immiscible liquid. Emulsification is a liquid-liquid interface phenomenon. Two immiscible liquids, such as oil and water, will separate into two layers in a container, with the less dense oil floating on top and the denser water sinking below. If an appropriate surfactant is added and the mixture is stirred vigorously, the oil will be dispersed in the water to form an emulsion; this process is called emulsification.)

(Phase state: that is, the state of matter, or simply phase, also called physical state, referring to a set of states possessed by a macroscopic physical system. Matter in the same state has a simple chemical composition and physical properties, such as density, crystal structure, refractive index, etc. The most common states of matter are solid, liquid, and gas, commonly known as the "three states of matter.")

What is coffee crema? Why does crema disappear?

A series of complex chemical changes occurs during coffee roasting and produces a large amount of carbon dioxide. Most of the carbon dioxide is naturally released while the roasted coffee beans are stored, but a considerable portion remains within the cell walls.

The most direct effect of 9 bar pump pressure is to emulsify the insoluble oils in coffee. In fact, water pressure has another effect: it makes water supersaturated with carbon dioxide, improving water's ability to dissolve carbon dioxide, thereby dissolving far more carbon dioxide than could normally be contained under ordinary atmospheric pressure. This also explains why thousands of fine bubbles form when hot water passes through the coffee puck.

The formation of the foam layer requires bubbles, and also requires those compounds or particles that tend to gather around the bubbles to make the bubble structure more stable and elastic. The coating effect of these chemical components is related to surfactants. Bubble formation is mainly related to melanoidins. Melanoidins is a general term for a series of compounds formed during coffee roasting, and scientists' understanding of them remains limited to this day.

Proteins and melanoidins can coat bubbles because they contain components that repel the water phase, that is, they are hydrophobic. Therefore, they tend to approach gas rather than water. Due to hydrophobicity, these substances are ultimately adsorbed onto the bubbles, forming a bubble layer. There are also other hydrophobic substances in coffee: fats/oils. The reason fats are discussed separately here is that "fat" is the solid form of oil, and both forms exist simultaneously in coffee.

There are two main reasons coffee crema disappears: fats/oils and gravity. After surfactants dissolve in water, gravity pulls downward on the coffee substances present in the foam layer, while also pulling on the substances covering the bubble surfaces, making the bubbles fragile, lacking elasticity, and ultimately bursting quickly. The speed at which the foam layer disappears mainly depends on how fast gravity acts. A properly extracted espresso will have surface foam that lasts longer than that of an under-extracted espresso. This is because a properly extracted espresso has a heavier overall composition, which can be confirmed by the espresso's mouthfeel.

The color of coffee crema: what can it tell us?

If you are after rich crema, Front Street Classic Espresso Blend is recommended. This is a recipe combining three origins—Brazil, Indonesia, and Vietnam—and uses a dark roast to maximize the coffee's aromatic richness, while also making it easy for everyone to extract abundant and dense coffee crema. It is very suitable whether for making an Americano diluted with water or a latte paired with milk, and can be purchased from the Front Street Taobao store or the Front Street Tmall flagship store.

During normal extraction, crema should appear reddish-brown. But when an espresso's crema is lighter in color, or very dark and close to burnt brown, what does that indicate? The answer is partly related to the espresso itself. Because crema is bubbles in the espresso that have trapped carbon dioxide, if the foam layer looks very dark, it can usually also be determined that the color of the espresso liquid itself will likewise be darker.

It is actually very difficult to distinguish differences in the color of coffee liquid unless it is diluted with water before comparison, or observed under a microscope. There is another factor to consider: the foam layer refracts light. Therefore, the foam layer will appear lighter in color than the coffee liquid itself, and the finer the bubbles, the more obvious this effect becomes. Taken together, even if the liquid color is very dark, and its thickness is less than that of espresso, the foam above this liquid will still appear very light in color.

When making espresso, an espresso obtained with a 15-second extraction time usually has only a very light-colored coffee crema. This is because the extraction is insufficient and the water does not have enough time to extract the soluble substances from the coffee, so it also does not have the same viscosity as an espresso made with a 25-second extraction time. Similarly, espresso extracted at too low a water temperature will also show light-colored crema, because low temperatures make it difficult to extract the soluble substances from coffee. Conversely, this also shows that over-extracted espresso will be darker in color than normally extracted espresso, and a burnt dark outer ring will also appear on the surface of the crema.

The above content is compiled by CoffeeHunters, a coffee news website.