Fines Are Part of the Ground-Particle Distribution

Fines are the small-particle tail of a ground-coffee distribution. In espresso work, distributions are often described as bimodal, and the fraction below about 100 micrometres has been used to study the very fine region. Fines are not a separate raw material; they are created as roasted beans fracture during grinding.

Bean Microstructure Influences Fine-Particle Formation

Roasted beans contain a cellular structure with regions that are not perfectly uniform. Under mechanical stress, weaker areas and already-small fragments can break again. Fine-particle formation therefore depends on bean structure, roast condition and the breakage mechanism of the grinder, not on one machine setting alone.

Compression and Shear Create Small Particles

Grinding produces several forms of mechanical loading at once. Particles can be compressed, sheared and fractured, while fragments from an initial breakage can pass through the grinding zone again and become smaller. Fines are consequently an outcome of the complete breakage sequence rather than a product of one cutting event.

Fines Change Permeability and Flow

When fines occupy the voids between larger particles, they can reduce effective porosity and increase resistance to water flow. A 2024 Scientific Reports study linked a higher fine fraction with lower permeability, lower flow and longer espresso extraction time under its experimental conditions. This does not make fines inherently defective; it makes their proportion an important physical variable.

The Grinder and the Bean Both Shape the Result

Burr type, burr spacing, rotational speed, feed rate and burr condition all influence the distribution, but the bean itself is equally important. Different coffees or roast states can respond differently to the same grinding conditions. Comparisons of fine fractions therefore require sufficiently comparable raw material and operating conditions.

Can Fines Be Reduced in Principle?

In principle, fines can be influenced through breakage mechanism, burr geometry and spacing, bean condition and operating conditions, but there is no simple way to eliminate them completely. A more realistic objective is to control the distribution within a range appropriate to the brewing method and keep it stable from one grind to the next.

Measure the Distribution Rather Than Relying on Feel

Hand feel can suggest that two ground samples differ, but it cannot quantify the fine fraction reliably. When the goal is process control, particle-size measurement using a consistent method provides evidence that can be followed over time and compared between operating conditions.

Fine-Particle Control Is a Distribution Problem

Fine-particle control is therefore a distribution problem, not a search for zero fines. Some fines are a natural consequence of breaking a brittle material. The useful variables are their proportion, stability and effect on flow and extraction under the specific brewing conditions.

Fines Can Affect Flow More Than the Median Size

The 2024 study indicates that the proportion of particles below 100 micrometres can be more strongly related to permeability and extraction time than the median alone. This helps explain why two batches with similar median size can still produce different flow behaviour during espresso extraction.

Roast Structure Determines How Beans Break

Roasting changes the internal structure, brittleness and void structure of the bean. Those changes affect crack formation and the way fragments break again during grinding. Fine production is therefore not simply a property of the grinder; it reflects the interaction between roasted material and grinding mechanism.

A Complementary View

Fines should not be treated as an absolute defect. They are a normal part of brittle-particle breakage, and the relevant issue is how much is present and what effect it has in the system. For espresso, higher fine fractions can reduce permeability and flow, but the evidence does not justify treating every amount of fines as undesirable.

A Practical Point to Watch

Principled interventions include changing breakage conditions, controlling bean state and maintaining burr condition, but each has limits. Reducing burr spacing can change both the main fraction and the fine tail, so it should not be treated as an independent fine-control knob.

An Additional Layer of Data

For quality stability, a time series is more informative than an isolated measurement. A gradual rise in fines under otherwise consistent conditions is a useful signal to investigate together with burr condition, bean properties and the grinding process.