COFFEE FINES

Fines are the microscopic coffee particles inevitably created during grinding. They are the smallest particles in the grind distribution, often as fine as flour. It’s a common belief that better grinders produce fewer fines and a more uniform particle size, but this isn’t always the case—nor is it always desirable. The criteria for a “good grinder” aren’t so simple, and critically, fines are not exclusively a negative influence.

Fines are generated when structurally weak parts of the coffee bean are fractured during grinding. Their influence stems fundamentally from their incredibly small size. This results in a vast surface area, high reactivity, and overwhelmingly high extraction efficiency, all of which help coffee compounds release rapidly.

Fines are a double-edged sword. They not only improve extraction yield but also positively impact body and mouthfeel. However, their high extraction efficiency also increases the risk of over-extraction. Furthermore, fines above a certain size can be physically detected in the mouth, harming the overall flavor quality. Critically, fines can clog the filter or induce channeling, causing the brew to deviate from the barista’s intention. Given their significant impact, it’s no exaggeration to say that mastering coffee extraction is mastering the control of fines.

Increased Extraction Yield

Fines possess an overwhelmingly large surface area. A 100-micrometer particle, for instance, has ten times the surface area of a 1000-micrometer particle for a given mass. Moreover, the paths for water to penetrate and for flavor compounds to diffuse out are significantly shorter. Considering that extraction is essentially a process of dissolving soluble compounds from the coffee’s surface, the reduction in particle size has an effect that goes beyond a simple increase in surface area. The graph below models the estimated extraction efficiency by particle size. While particles under 100 µm have an efficiency of 95%, particles in the 800-1000 µm range don’t even reach 20%.

Fines play the role of rapidly and explosively increasing the extraction yield in the initial phase of brewing. According to one study, fines, which make up about 10% of the ground coffee’s weight, can contribute to over 30% of the total extraction yield. The graph below illustrates the contribution of each grind size range to the total yield. In a particle distribution with a median of 700 µm, the 700 µm particles themselves contribute only about 5% to the total yield, whereas particles under 300 µm account for over 60%. Of that, fines under 100 µm make up 30%, the highest of any single range. Particles larger than 800 µm may constitute over 20% of the total volume, but their impact on the final yield is only about 10%.

Improved Body and Mouthfeel

The very small fines that pass through the filter have the effect of enhancing the coffee’s texture and weight. The smooth, coating sensation in the mouth is a result of not only lipids but also the influence of fines.

Mouthfeel refers to all the tactile sensations in the mouth, including weight, viscosity, richness, and friction. It plays a decisive role in the overall impression of the coffee and is known to be primarily associated with lipids. However, other factors influencing mouthfeel include melanoidins, chlorogenic acids, and, importantly, fines.

Fines can be introduced in significant quantities, especially in espresso, while in drip brewing, their amount depends on the filter’s characteristics. Typically, particles between 5 to 30 micrometers (µm) pass through the filter. These suspended fines exist in the final beverage as a colloid, physically increasing viscosity and enhancing the perception of weight and texture. Furthermore, fines help improve the dispersion stability of polysaccharides, melanoidins, and lipid compounds, allowing the body to be perceived as richer. These substances would otherwise tend to clump together coarsely or settle out. Instead, fines bond with them to form a stable colloid, ultimately helping to create a silky or velvety smooth and uniform texture.

Fines not only enhance texture on their own but also rapidly release high-molecular-weight compounds like soluble polysaccharides and chlorogenic acids during extraction, further increasing the coffee’s viscosity.

However, an excess of fines can be detrimental to texture, inducing a fine, gritty residue and a rough sensation that harms the overall flavor quality. Therefore, controlling the amount of fines to enhance mouthfeel is critically important.

Flow Control

In coffee brewing, the coffee bed is not a solid but a porous layer of countless particles. Water passes through this bed via forces like gravity and surface tension, extracting coffee compounds along the way. Ideal extraction occurs when water penetrates all particles uniformly and drains at a consistent rate. In reality, however, the bed is composed of particles of various sizes, creating differing densities and resistances. This is why certain areas become low-resistance pathways, causing water to flow through them preferentially. This is called channeling.

Fines play the role of filling the empty spaces between larger particles. This narrows the channels available for water to pass through, reducing overall permeability. The slowed flow allows water to remain in contact with the coffee for a longer duration, giving it the time needed to penetrate deeper into the particles. Consequently, dissolution and diffusion can occur more stably.

The right amount of fines can actually prevent channeling. If only large particles existed, the gaps between them would be too wide, causing water to rush through those specific paths. This would result in extremely short contact times and an under-extracted brew lacking positive flavors. Fines fill these large gaps, preventing excessive flow rates and encouraging water to disperse through multiple pathways.

In other words, a proper amount of fines creates resistance in the coffee bed, ensuring a more uniform extraction. This extends the contact time between water and coffee, creating an environment where more compounds can be sufficiently extracted.

Increased Risk of Over-Extraction

Coffee is composed of hundreds of compounds with different solubilities and extraction rates. Generally, acids, caffeine, and aromatic compounds extract first, lending bright acidity and aroma. These are followed by chlorogenic acids, their degradation products, phenols, and some bitter compounds, which enhance complexity. Finally, polyphenols, high-molecular-weight compounds, and other aromatics extract last, typically contributing bitter, astringent, and unpleasant flavors. Over-extraction is the phenomenon where an excess of these final-stage compounds is extracted, harming the overall flavor.

The high surface area and short diffusion path of fines, while increasing efficiency, also elevate the risk of over-extraction. Fines release a greater amount of coffee compounds in the same amount of time, reaching saturation quickly and continuously leaching negative flavor compounds. The fact that fines slow the water flow and clog the filter also acts as a factor that increases the risk of over-extraction. This can lead to negative flavors like excessive bitterness, astringency, and a dry, chalky sensation.

Furthermore, over-extraction masks other positive flavors with its powerful bitterness and negative attributes, causing the coffee to lose its inherent character. This ultimately reduces complexity, leaving only a dull and flat flavor profile.

Filter Clogging

Clogging the filter is the most significant problem caused by fines. Paper filters used in drippers are typically porous fiber layers, mainly made of cellulose from wood. The pore size can vary slightly depending on the manufacturing process. On average, paper filters have pores around 15 to 30 micrometers (µm), though some more compressed filters may have an average pore size of 10 to 15 µm.

While the average pore size is 15-30 µm, particles commonly referred to as fines are under 100 µm, with experiments showing that particles between 20-80 µm are most frequently detected. Although filter pores are smaller than many fines, fines still manage to pass through. This is because the fiber network isn’t composed of uniform pores; fines escape through a few irregularly large openings. Some also suggest that the pores can momentarily expand under pressure, but the former’s influence is far greater.

During extraction, fines migrate downwards and block the microscopic pores of the paper filter. Additionally, paper fibers tend to swell when wet, causing the pores to shrink slightly. As a result, the filter’s performance degrades sharply. This effect is maximized in light roast coffee. Light roast particles of the same size tend to be denser and heavier. In other words, light roast fines sink more easily than dark roast fines. This is why drip-based extraction of light roast coffee tends to be much slower than that of dark roast coffee.

Generally, the human tongue and oral mucosa can detect particles of 15 to 30 µm and larger, with some individual variation. Fines larger than 30 µm, if present in large quantities, can cause a rough or sandy sensation. On the other hand, fines under 10 µm can be observed by the naked eye under proper lighting if they are clumped together, but are difficult to perceive by physical touch. In coffee, they actually contribute to an increase in weight and create a smooth tactile sensation.

Inducing Channeling

When the bottom of the filter or the middle of the coffee bed becomes clogged, water reroutes to find the path of least resistance. In this process, water flows intensively through a specific path, a phenomenon called channeling. This causes the areas where water passes to be over-extracted, while areas it doesn’t touch are under-extracted. The result is a disastrous cup that is simultaneously sour and sharp from under-extraction, and bitter and astringent from over-extraction.

The process of channeling occurs as follows: As water begins to flow, light fines travel with it to the bottom of the coffee bed. These fines either block the filter pores or accumulate at the bottom of the bed, forming a dense, impermeable layer. When water can no longer pass through this high-resistance layer, it seeks a new path of least resistance. Thereafter, most of the water will flow through this newly formed low-resistance channel.

Harming Consistency

One of the biggest problems fines present in a professional setting is that we cannot perfectly control them. Every grinder produces a wide distribution curve of particle sizes, not just one. The portion of particles under 100 µm can vary with every grind.

Furthermore, the amount and distribution of fines can change with the season and weather, largely due to the effects of static electricity on such small particles. Inside the ground coffee, fines are either stuck to larger particles by static or clumped together with other fines. When you pour water, these randomly arranged fines migrate randomly, meaning a path that was clear yesterday might be blocked today, and vice versa. This causes inconsistencies in brew time and results.

On humid days, like in summer or when it’s raining, coffee beans absorb moisture from the air, becoming heavier and denser. These moist beans require more energy to grind, leading to more shearing (tearing) than fracturing, which results in a decrease in the production of fines. Conversely, on dry days, like in winter, the beans become more brittle and prone to shattering, which can generate a larger quantity of fines. While the absolute production of fines might be higher in winter, the opposite is often observed in practice. This is due to static electricity.

A massive amount of static is generated from the rapid rotation and friction inside a grinder. Dry air fails to dissipate this static, so it is fully transferred to the coffee particles in winter. These charged fines cling to the grinder’s burrs, chute, and container. If not properly cleared, the coffee that goes into the filter is in a state where a significant amount of fines has already been removed. Whether this removal has a positive or negative impact is unknown. What is clear, however, is that the distribution of fines can change dramatically with the season or weather. To solve this consistency issue, some baristas will spray a small amount of water on their beans before grinding.

Furthermore, even if the same amount of fines is produced each time, variables like the subtle strength of the water stream and the presence of agitation will affect the migration patterns of the fines, making it impossible to maintain a consistent final product. This is why, even with the same amount of coffee, the same pouring technique, and the same temperature, the flavor quality of the final cup can inevitably vary.