May 7, 2026, 8:35 am | Read time: 5 minutes

Study Reveals Formula for Perfect Espresso

Some swear by high-quality portafilter machines, while others prefer a simple stovetop espresso maker (sometimes called a moka pot). But regardless of the device: Espresso is created when hot water is pressed through finely ground coffee at a pressure typically between 9 and 10 bars. This process is precisely what the recently published study examines.

The central question was how quickly the water flows through the so-called coffee puck, a compacted bed of coffee particles. This flow rate significantly influences the contact time between water and coffee, and thus the amount of dissolved substances–and ultimately, the drinking experience. The longer the water remains in contact with the coffee grounds, the more intense and bitter the product becomes.

The flow rate is therefore a crucial factor in espresso preparation. And it must be adjustable through various parameters. However, a comprehensive physical model that describes and links the various influences was lacking until now. This is precisely the goal pursued by the responsible international research team.

Details of the Research

At the core of the investigation were two key properties of the coffee puck: the so-called porosity (the proportion of voids in the material) and the specific surface area–that is, the total contact area between water and coffee particles per volume. Particularly important is the “connected porosity,” which refers to the proportion of pores through which water can actually flow.

Two types of high-quality roasted coffee were used: Tumba from Rwanda, a renowned natural coffee of the “Red Bourbon” variety, and Colombian Guayacán. Both types were ground in 11 different grind sizes–from very fine to coarse. This resulted in a total of 22 samples.

Focus on Particle Size Distribution

First, the researchers analyzed the particle size distribution using an image-based measurement system, where individual coffee particles were photographed and statistically evaluated. Then, a special imaging technique was used to make the internal structure of the coffee with pores and particles visible. Based on this, the scientists conducted flow simulations to determine the water flow through the coffee beds and thus their permeability.

In parallel, the team tested various physical models for water flow in coffee. A particularly reliable approach proved to be one that only considers the interconnected pores in the coffee puck–those pathways through which water can actually flow.

Factors Affecting Flow Rate

Based on their investigation, the researchers were able to develop a kind of formula that describes how water flows through ground coffee.

The study clearly shows: The finer the coffee is ground, the slower the water flows through the coffee puck. This is because smaller particles are packed more closely together, increasing resistance for the water. At the same time, the total surface area in the coffee puck that the water comes into contact with increases–further slowing the flow.

Additionally, the researchers were able to describe their measurement data very well with a physical model. The key is how well the pores in the coffee are connected. Only continuous channels actually contribute to water flow. The irregular, angular shape of the coffee particles also plays a role, as it makes the structure of the coffee puck more complex and influences the water path.

A central concept is the so-called percolation model. Simply put, it describes that only interconnected voids in the coffee are relevant for water flow, while isolated small pores contribute little. Under typical espresso conditions, the water flow remains stable and well-controlled. Only under extreme settings–such as very fine grind, strong tamping, or uneven distribution of coffee–can it become significantly more turbulent and lead to uneven extraction.

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So What Is the Formula for Perfect Espresso?

In line with the study, there is no single magic formula, but a clear physical principle: Espresso is best achieved when water flows evenly through a well-connected network of small pores in the coffee puck. It’s about finding the right balance between too much and too little permeability.

Three factors are particularly crucial:

• the grind size of the coffee
• the amount of coffee used
• and the compaction of the coffee puck in the portafilter

Understanding these parameters allows one to deliberately influence the flow rate and thus control contact time, extraction, and taste.

Assessment of the Study and Limitations

With the insights from the study, the brewing process of espresso can be better understood and more precisely controlled. For everyday use, this means: Small changes in grind size, amount, or compaction have understandable effects on the espresso. These could help make the result more reproducible without replacing the craft.

Yet, the results should not be overestimated. It should be noted that the coffee pucks in the experiment were not always pressed with the same force. This can affect how the coffee is structured in detail in practice. Part of the results also comes from computer simulations. While very precise, they do not capture every detail of real espresso preparation–such as temperature, chemical processes during brewing, or changes in the coffee during the flow.

Additionally, only two similar coffee varieties were examined. Other beans or significantly darker or lighter roasts might behave somewhat differently, even though grind size remains the most important factor in the study. Lastly, very fine coffee particles can still move during extraction and slightly change the structure in the puck. This was considered but not examined in detail.