When water
meets coffee.
A three-level journey to understand what happens inside the coffee puck, from the first drop to the physics of flow.
What happens when water reaches the coffee puck?
This is the first question in our journey through espresso extraction: what happens to the ground coffee at the exact moment hot water reaches the puck?
Complex physical and chemical processes take place during extraction. To understand them, the first step is to examine some characteristics of ground coffee and the way water reaches it and flows through it.
The coffee puck is a porous medium. It contains void spaces between the particles, and water must move through these spaces. Two concepts therefore become essential: porosity and permeability.
The void space available within the puck.
The ability of the medium to allow a fluid to pass through it.
These two characteristics are closely related, but they are not the same. The puck structure changes according to particle shape and size, grind setting and the pressure applied during tamping.
The way water initially permeates the coffee bed also plays a crucial role. This is where pre-infusion comes into play: the very first moments of brewing, when water begins to interact with the grounds.
Understanding these first seconds helps explain why water does not always travel through the puck in the same way and why small changes in preparation can alter extraction behaviour.
Want to understand why it happens? Continue with In Depth →Inside the puck: porosity, permeability and grinding
To understand the first contact between water and coffee more precisely, we must consider the puck as a porous medium whose structure controls fluid flow.
Permeability
Absolute permeability is an intrinsic property of porous media such as ground coffee and expresses the medium’s ability to allow a fluid to pass through it.
During extraction it can vary significantly depending on the blend, grind setting, roast profile and pressure applied during tamping. The way and timing with which water initially permeates the coffee bed are also decisive.
Porosity
Porosity is defined as the ratio between the volume of voids, or pores, and the total volume of the material considered: in our case, the ground-coffee puck.
Pores can be classified as macropores, mesopores and micropores. Their presence and distribution depend on particle shape, arrangement and size.
What changes the puck?
It determines pore shape and size and depends greatly on the type of grinder used and the grind setting.
It is related to particle shape and tamping level. Stronger tamping compacts the particles, reduces porosity and restricts water flow between layers.
It depends on the grind setting and affects both porosity and permeability. For Italian Espresso, the document normally indicates a range between 300 and 400 µm, with an unavoidable fraction of extremely fine particles known as fines.
The force used to compress the puck with the tamper is crucial in establishing the relationship between permeability and porosity.
The key distinction
In short: porosity represents the volume of available void spaces; permeability represents the fluid’s ability to pass through those spaces.
When water reaches the puck, it does not encounter a uniform mass but a complex structure whose geometry influences flow. To describe this phenomenon more precisely, we must enter the physics of permeability.
Want to explore the physics of the phenomenon? Continue with the Technical level →The physics of flow: permeability, viscosity and Darcy’s law
In technical terms, puck permeability does not simply describe the presence of void spaces, but the behaviour of the porous medium as fluid passes through it.
Unlike porosity, permeability is described in the document as a vector quantity and is the geometric factor in the proportionality constant that, according to Darcy’s law, links the pressure gradient to fluid flow rate.
The two elements of fluid flow
- ViscosityIt depends on the physicochemical characteristics of the fluid—in our case, water at specific temperatures and pressure gradients.
- Intrinsic permeabilityIt is determined solely by the structure of the porous medium—in our case, ground coffee.
The microscopic structure
The factors affecting absolute permeability are closely linked to the microscopic structure of the medium, including tortuosity and pore size.
Porosity is a scalar quantity. Permeability is instead described mathematically as a tensor, because resistance to water flow can vary with flow direction.
This distinction leads to a fundamental point: knowing the volume of void spaces is not enough. We must also consider the fluid’s ability to pass through them and the direction in which flow develops.
From first contact to pre-infusion
The document emphasises that the way and timing with which water initially permeates the coffee bed play a crucial role. This phenomenon is summarised by the term pre-infusion.
This leads to the next topic: when water saturates the coffee bed, the system’s behaviour requires us to consider non-Newtonian fluids, soft matter, anisotropy, temperature and pressure gradient.
Pre-infusion and changes in the puck
Next topic: the first seconds of brewing.
IN PREPARATION
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