In the past few years, several well-known coffee experts, scientists, and grinder manufacturers have measured and publicly discussed grind particle-size distributions. Some have even rented or borrowed expensive machines that offer relatively precise measurements. Despite the increased interest in PSDs, our understanding of how burr geometry affects PSD is rather crude, and I don’t believe any of us have a clear idea of what an optimal PSD looks like. We may be vaguely aware of wanting fewer fines for filter coffee than espresso, and there is general agreement that “boulders” (particles much larger than the average) are undesirable, but to my knowledge, no one has quantified optimal fines levels with any precision, nor does anyone know much about the impact on the cup of “micro fines” vs larger fines.

Along the way, thanks mostly to the work of Jonathan Gagné, author of The Physics of Filter Coffee, we have come to understand more about how fines affect channeling and liquid flow in percolation brewing. According to Illy in Espresso Coffee: The Science of Quality, “finer particles enhance the exposed extraction surface and the coarser ones allow the water flow.” In The Physics of Filter Coffee and on his blog Coffee Ad Astra, Jonathan elaborated on the role of fines in percolation, including dynamics related to fines migration and channeling. I recently asked Jonathan for his current thoughts on fines and he said: “When there is an insufficient proportion of fines, it causes larger, uneven gaps between particles.” Jonathan agrees that having too many fines can cause channeling, and having too few fines leaves too much large, open flow paths for astringent particles to travel out of the coffee bed. Fines fill the gaps between larger particles and help limit the amount, and perhaps size, of low-density flow paths within the coffee bed.

I’m not a scientist, but I have 30 years of experience in coffee and have personally brewed more than one million cups, which has given me some insights. What I discuss in this post is a speculation I call the “happy medium theory of fines.”

In my experience:

When there is an excessive proportion of fines, many flow paths clog easily, and water must find channels through which to bypass the clogged areas. These channels result in undesired astringency.

When there is an insufficient proportion of fines, large flow paths form in the coffee bed, and a disproportionate amount of water exits the bed through those paths, lowering brew strength, and carrying larger, astringent compounds into the cup.

The “happy medium” amount of fines provides enough fines to even out the size of gaps between larger coffee particles, but not so many fines that excessive, localized clogging occurs.

The happy medium amount of fines for a given brewing situation is the amount that minimizes astringency.
Optimal espresso percolation requires a greater proportion of fines than filter-coffee percolation does.

Burr types
Coffee grinders almost always use one of three types of burrs: flat (disc), conical, or roller. Flat burrs usually produce a modest amount of fines. Exceptions include “Turkish” burrs, and other burr sets designed for espresso and Turkish coffee; for the rest of this article, “flat burrs” will refer to the more common flat burrs designed for filter coffee. Conical burrs generally produce more fines than flat burrs do. Roller mills produce an average amount of fines, and the lowest proportion of boulders, assuming the beans are passed through more than one set of rollers.

In Everything But Espresso, I proposed the idea that flavor clarity and body are always in opposition. That idea has stood the test of time. Whether one changes a brewing method, filtration type, or grinder, gains in clarity always coincide with loss of body, and vice versa.

This diagram first appeared in Everything But Espresso, 2009. These days, there is a greater variety of paper filters than when I created the diagram, so the “paper filter drip” circle would be much larger. I probably messed up the positions of Clover and Chemex relative to each other, but hopefully the reader gets the gist of the diagram.
Note: when I wrote Everything But Espresso, the Vac Pot, Clover, and Eva Solo were quite popular; if I were to update the diagram, I would delete those brew methods and add the Aeropress and the forthcoming NextLevel Pulsar.

Coffee professionals often recommend flat burrs for enhanced flavor clarity, and conical burrs for increased body. However, in the past year, I’ve come to believe there is a third aspect of grind quality to consider: astringency. Astringency is the puckery or dry sensation one often experiences in the mouth when drinking wine, tea, coffee, and consuming many other foods or beverages. While astringency is welcome in wine, it is less desirable in tea, and very undesirable in coffee. Personally, any detectable astringency ruins my enjoyment of coffee.

The consensus beliefs that 1) flat burrs are good for filter and conicals are bad and 2) that low-fines flat burrs are better for filter than high-fines flat burrs, are too simplistic. While I wouldn’t sacrifice clarity for body, I’m willing to give up a little flavor clarity if it means decreasing or eliminating astringency. For example, I have never enjoyed percolation brews from “ultra-low-fines burrs” because every cup I have had from them was too astringent for my taste. Likewise, while many have criticized a grinder like the Niche Zero as having too little clarity for filter coffee, I find the Niche’s balance of clarity, body, and astringency acceptable with some brewing methods because it tends to produce very little astringency.

Note that having an adequately deep and intact coffee bed, which is rare in pourover brewing, helps to enhance clarity and limit astringency. For example, on the recent Decent Espresso Tour in Europe, I made Filter3 with coffee ground in the Niche. I polled dozens of attendees, and everyone agreed the flavor clarity was exceptional. I wouldn’t have used the Niche to brew V60s, but I am happy with it as a Filter3 grinder.

The Bottom Line

I’m not proposing baristi use conical burrs for hand pours or never use ultra-low-fines burrs. To determine what type of grinder and burrs to use for a brewing method, one must consider the proportion of fines, the brewing method, and the effectiveness of the coffee bed as a filtering device. An informed barista must consider his or her desired balance of clarity, body, and astringency, and choose the appropriate burrs and brewing method to achieve that balance.

Personal preference will vary, and of course all of this may be too tedious or technical for many. But for some of us, the endless complexity of coffee is what makes it so much fun.

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The happy medium amount of fines