The coffee particle size debate
Over the past few years, several well-known specialty coffee experts, scientists, and grinder manufacturers have measured and publicly discussed particle size distributions (PSDs) in coffee grounds. Several have even rented or borrowed expensive machines that offer relatively accurate measurements.
Despite the growing interest in PSD, our understanding of how it affects wheel geometry is quite limited, and I don't think any of us have a clear idea of what an optimal PSD looks like.
We may be aware that we want less fines for filter coffee than for espresso, and there is general agreement to reject larger-than-average particles (“boulders”), but as far as I know, no one has quantified what optimal fine levels are precisely, nor does anyone know much about the impact of “microfines” on the cup relative to larger fines.
Some definitions
Nominal peak: Nominal peakDensity of particle volume: Density of particle volumeFines: FinesBoulders: Particles larger than averageFines peak: Peak of finesParticle diameter (μm): Diameter of particles (μm)
Meanwhile, thanks largely to the work of Jonathan Gagné, author of The Physics of Filter Coffee , we've come to understand more about how fines affect channeling and liquid flow in percolation methods.
According to Illy in Espresso Coffee: The Science of Quality, “Finer particles improve the exposed extraction surface area, while coarser particles allow water to flow through.” In The Physics of Filter Coffee and on his blog Coffee Ad Astra, Jonathan delved into the role of fines in percolation, including dynamics related to fine migration and channeling. I recently asked Jonathan what he thinks about fines right now, and he replied, “When the proportion of fines is insufficient, larger, more irregular voids are created between the particles.”
Jonathan acknowledges that too much fines can cause channeling, and that too little leaves open flow paths too large for astringent particles to leave the coffee bed. Fines fill the gaps between larger particles and help limit the number and possibly the size of low-density flow paths in the coffee bed.
I'm no scientist, but I have over 30 years of coffee experience and have brewed over a million cups, so I've acquired some knowledge.
In this post I speculate on what I call “the theory of the ideal mean of fines.”
In my experience
- When the proportion of fines is excessive, many flow paths easily become clogged, and water must find channels to bypass the clogged areas. These channels cause unwanted astringency.
- If the proportion of fines is insufficient, large flow paths form in the coffee bed, and a disproportionate amount of water leaves the bed through these paths, reducing the intensity of the extraction and transferring larger, more astringent components to the cup.
- The “ideal medium” fineness provides enough fineness to even out the hole size between the larger coffee particles without causing excessive, localized clogging.
- The ideal average amount of fines for a specific recipe is that which minimizes astringency.
- Optimal espresso extraction requires a higher proportion of fines than with filter percolation.
Types of molars
Coffee grinders almost always use one of three types of burrs: flat (disc), conical, or roller.
Flat burrs typically produce a small amount of fines. Exceptions include "Turkish" burrs and other burr types designed for espresso and Turkish coffee; for the remainder of this article, "flat burrs" will refer to the more common flat burrs designed for filter coffee. Conical burrs typically produce more fines than flat burrs. Roller mills generate a medium amount of fines and the lowest proportion of boulders, assuming the beans pass through more than one set of rollers.
In Everything But Espresso, I propose the idea that clarity of flavor and body are always in contrast. That statement has stood the test of time. Whether you change the brewing method, the type of filtration, or the grinder, an increase in clarity always coincides with a loss of body, and vice versa.
This diagram was first published in Everything But Espresso, 2009. There are now a wider variety of paper filters than when I prepared this diagram, so the "paper filter drip" circle would be much larger. I've likely swapped the positions of Clover and Chemex, but I hope the reader gets the gist of the diagram.
Note: When I wrote Everything But Espresso, the Vac Pot, Clover, and Eva Solo were very popular; if I were to update the chart, I would remove those brewing methods and add Aeropress and NextLevel Pulsar.
Conclusion
I don't recommend baristas use conical burrs for manual filters or never use burrs that produce very little fineness. To determine which type of grinder and burrs to use with a particular method, the fineness ratio, brewing method, and the effectiveness of the coffee bed as a filtering device should be taken into account.
Any knowledgeable barista should consider what they are looking for in terms of balance of clarity, body, and astringency, and choose the right grinds and brewing method to achieve that balance.
Personal preferences will vary, and of course, many people will find all of this too tedious or technical. But for some of us, the endless complexity of coffee is what makes it so much fun.
These images represent subjective estimates; they are not based on irrefutable data, as there are no standards yet for objectively measuring the three qualities.
Conical: conicalULF: very low production of finesHU/flat burrs: flat burrsclarity / body / astringency
About Scott Rao
Scott Rao is one of the leading figures in the world of specialty coffee. He is the author of several books, including "The Professional Barista's Handbook," "Everything but Espresso," and "The Coffee Roaster's Companion," all written with the aim of helping the industry pivot toward a more educated and scientific understanding of coffee. He devotes much of his time to consulting coffee roasters around the world, teaching coffee roasting classes, and designing cutting-edge coffee equipment.
At Ineffable Coffee, we've been working with Scott Rao since 2018. His advice and experience over the years have been invaluable. As a result of this collaboration, we're delighted to be able to offer this content in Spanish.
Article written by Scott Rao , originally published in English on his blog and translated by Ana Rubio Ramírez of coffeeandtranslation.com
