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Espresso under the microscope: emulsion, oils, 9 bar pressure

Espresso looks like a simple, small coffee, but beneath this cup hides fascinating physics and chemistry. What distinguishes espresso from every other brewing method is the combination of high pressure, an emulsion of oils and dissolved gas, which create a thick, intense drink of velvety texture and characteristic crema. Understanding what really happens during those twenty-odd seconds of brewing changes the way we look at espresso. In this post we will look into espresso as if under a microscope: we will explain the role of 9 bar pressure, what the emulsion of oils is, where crema comes from, why it disappears and why espresso differs so much from filter coffee. It is science that makes every cup more than just a strong drink.

What makes espresso espresso

Espresso is not simply strong coffee, but a distinct extraction method of unique physics. What makes espresso special is passing hot water through tamped, finely ground coffee under high pressure, in a short time. This pressure changes everything: it lets you draw from the coffee substances and effects that no other method can. The result is a concentrated, thick drink of full body, velvety texture and a characteristic golden crema on the surface. Espresso is not just a solution of dissolved compounds, like filter coffee, but a complex suspension of oils, gas and particles. Understanding this difference is the key to grasping why espresso tastes and looks completely different from other coffees. Beneath the seemingly simple cup hides a complicated world of physics and chemistry worth knowing, to fully appreciate this drink and what makes it so special.

9 bar pressure

The heart of espresso is pressure, standardly around nine bar, that is nine times atmospheric pressure. It is precisely this that distinguishes espresso from gravity methods like filter. 9 bar pressure does two key things simultaneously. First, it dissolves carbon dioxide into the liquid under pressure, in accordance with Henry’s Law, by which gases dissolve more readily the higher the pressure. Second, it emulsifies the coffee oils, which are normally not soluble in water. These two effects, impossible with ordinary brewing, are the essence of espresso. The value of 9 bar is not random: it is a proven compromise that allows proper extraction without excessive bitterness or channeling. Too high a pressure can cause flow problems, too low gives a weak, watery coffee. Pressure is the foundation on which the whole physics of espresso and its unique character rest.

The emulsion of oils

One of the most important effects of high pressure is the emulsification of coffee oils. The key is that the oils contained in coffee, mainly lipids and triglycerides, are insoluble in water. In ordinary brewing, like filter, these oils are largely retained or do not reach the drink. In espresso high pressure changes the situation: forcing water through the coffee, it breaks the oils into tiny droplets and disperses them into the drink, creating an emulsion. These oil droplets are suspended in the liquid thanks to surface tension, pressure and naturally present compounds with emulsifying properties. It is precisely this emulsion of oils that gives espresso its characteristic, full, velvety body and rich texture, which clear filter coffee lacks. The emulsification of insoluble oils is one of the foundations of what makes espresso special. It is literally a suspension of fat in water, possible only thanks to pressure, and one of the secrets of the creamy consistency of this drink.

Dissolved CO2

The second key effect of pressure is the dissolving of carbon dioxide into the drink. Freshly roasted coffee contains a lot of CO2, trapped in the bean after roasting. During brewing under 9 bar pressure, in accordance with Henry’s Law, a significant part of this gas dissolves into the hot water, because high pressure favours the solubility of gases. This dissolved CO2 plays a key role, especially in the formation of crema. As long as the drink is under pressure in the machine, the gas stays dissolved. The situation changes dramatically at the moment the espresso leaves the high-pressure environment of the portafilter. That is why the freshness of the coffee matters so much for espresso: freshly roasted coffee, rich in CO2, gives a better, more abundant crema. Dissolved carbon dioxide is an invisible but key player in the physics of espresso, linking the brewing pressure with what we see on the surface of the cup. It is this, escaping, that creates the characteristic foam.

Crema - a colloid of gas and oils

Crema, the golden foam on the surface of espresso, is a direct effect of the processes described. From a scientific point of view crema is a colloidal emulsion: bubbles of CO2 gas suspended in a matrix of emulsified coffee oils and water, stabilised by surface-active compounds extracted from the coffee. The mechanism is precise: when the pressure drops at the exit from the portafilter, the previously dissolved CO2 rapidly comes out of solution and nucleates, that is forms bubbles, around the droplets of emulsified oil, building the crema foam. The stability of this foam depends on melanoidins, proteins and other surface-active compounds that coat the gas bubbles and prevent them from merging and popping immediately. Crema is thus not a random decoration, but a complex physical structure, joining gas, oil and water. It is visible evidence of what happens in espresso under pressure. We write more about crema itself and what it says about coffee in our post on crema in espresso.

Soluble and insoluble substances

To fully understand espresso, you need to distinguish two kinds of substances that reach the cup. The first is soluble substances: compounds that dissolve in water, responsible for flavour, acidity, sweetness and bitterness. These reach every coffee, including filter. The second is insoluble substances: above all oils and microscopic particles of coffee, suspended in the drink rather than dissolved. It is precisely the presence of these insoluble components, the emulsified oils and the suspension, that distinguishes espresso from clear methods. Espresso is not only a solution, but also a suspension: a combination of what is dissolved with what is suspended. This combination gives it full body, intensity and a velvety texture. Filter coffee, filtered through paper, loses most of the insoluble oils and particles, which is why it is clear and light. Understanding the division into soluble and insoluble is the key to grasping why espresso is so thick and different from other coffees.

Why crema disappears

Everyone who has drunk espresso has noticed that the crema disappears over time. It is a natural consequence of its physics. Crema is not a stable structure, but a temporary emulsion held together by pressure-induced interactions between oils and gases. When the espresso leaves the high-pressure environment of the portafilter, the carbon dioxide begins to escape upward, and the oil-coated bubbles start to collapse. In other words, the force that created the crema, that is pressure and dissolved gas, disappears after leaving the machine, so the foam gradually breaks down. That is why crema is best admired and drunk fresh, right after preparation. Its disappearance is not a fault, but an inevitable physical effect. Understanding why crema is short-lived also helps understand that thick crema in itself is not a guarantee of quality, but only an effect of coffee freshness and pressure. It is an ephemeral, beautiful sign of the processes that have just occurred in the cup.

Espresso versus filter

Since we know the physics of espresso, it becomes clear why it differs so much from filter coffee. Let us set both worlds side by side:

The table shows that espresso and filter are two different philosophies. Espresso, thanks to pressure, contains emulsified oils and a suspension, giving full body and crema. Filter, filtered through paper, is clear and light. It is not a matter of strength, but of the physics of extraction. We write more about the differences in methods in our post on pressure profiling.

What it means in practice

What does this knowledge mean for someone brewing espresso at home? Quite a few practical conclusions. First, the freshness of the coffee matters enormously: freshly roasted coffee, rich in CO2, will give a better crema and fuller espresso, so it is worth buying fresh beans and grinding just before brewing. Second, pressure and even tamping are key: uneven tamping causes channeling and spoils extraction, so it is worth ensuring good technique. Third, crema is a hint, but not an oracle: its presence indicates freshness and pressure, but thick crema does not guarantee good flavour. Fourth, the full body of espresso comes from oils and suspension, so do not filter it unnecessarily. Understanding the physics of espresso helps you consciously aim for a better cup, instead of acting blindly. It is science that translates directly into practice and the quality of your morning coffee. Knowledge of the processes makes you a better barista in your own kitchen.

How to observe it at home

The physics of espresso can be observed with your own eyes, which makes brewing even more interesting. Watch how the espresso flows from the portafilter: it should have the colour of warm, honeyed brown and a consistency resembling flowing honey, which indicates a good emulsion. Observe the formation of crema: a thick, even crema that lingers for a moment is a sign of fresh coffee and proper pressure. Notice how the crema disappears over time, which confirms its unstable, gas-and-oil nature. Compare espresso from fresh and older coffee, to see the difference in crema resulting from CO2 content. Experiment with grind and tamping, observing how they affect the flow and emulsion. Such attentive observation turns brewing into a fascinating live physics experiment. Note your observations, to learn what gives the best results. Espresso under the microscope is not only theory, but also practice that you can study with every cup in your own home.

The key points in a nutshell

Espresso is a distinct extraction method based on the physics of pressure. The standard 9 bar does two key things: it dissolves carbon dioxide into the drink, in accordance with Henry’s Law, and emulsifies the coffee oils, which in themselves are insoluble in water. Crema is a colloidal emulsion of CO2 bubbles in a matrix of oils and water, stabilised by melanoidins and proteins, forming when the pressure drops at the exit from the portafilter. Crema is short-lived, because the gas escapes and the bubbles collapse. Espresso, unlike clear filter, contains emulsified oils and a suspension, which gives it full body. Coffee freshness and even tamping are key. Want to study espresso and record your observations? Keep notes in the GustoNote app. See also our posts on crema in espresso and pressure profiling.