Grinding Espresso Finer Won’t Always Make It Stronger

TL;DR: A landmark 2020 study in the journal Matter put espresso grind size under a microscope and found that grinding finer doesn’t keep raising extraction. Past a point, the fine grounds make water channel unevenly through the puck, so yield falls and shots get harder to repeat. The researchers’ fix was to back the grind off a touch, use less coffee, and pull a shorter shot: roughly a quarter less coffee for a similar extraction, more consistently.

What the researchers tested

Michael Cameron, Christopher Hendon, Jamie Foster and colleagues built a mathematical model of how water flows through and dissolves a bed of coffee, then checked it against real shots. They also ran the idea in a working café (Tailored Coffee in Eugene, Oregon) across roughly a year and tens of thousands of drinks, which is rare in coffee research. The variable in focus was espresso grind size and its effect on extraction yield (EY), the share of the ground coffee that actually dissolves into the cup.

What they found

• The model, which assumed water flows evenly through the puck, predicted that EY would climb steadily as the grind got finer.
• The real shots disagreed. Measured yield rose, peaked at a slightly coarser setting, then dropped again at the very finest grinds.
• The cause is channeling: very fine grounds pack unevenly, water races through the low-resistance gaps, and much of the bed stays under-extracted. Two “identical” fine shots also vary more from each other.
• Their practical recipe was to coarsen the grind a little, cut the dose (they went from about 20g to 15g), and pull faster, in the region of 7 to 15 seconds. That gave about 25% less coffee per shot for a comparable extraction, more repeatably. This work helped popularise the “turbo shot”.

How confident should we be?

Fairly solid, at least on the parts that matter most. The paper is peer-reviewed, it pairs a physical model with experiment, and it’s backed by café-scale reproducibility and cost data. The best-supported claims are about consistency and using less coffee. Whether a leaner, faster shot tastes as good is a separate and more subjective question that the study doesn’t settle with a blind taste panel, and plenty of skilled baristas still prefer longer, more traditional ratios for certain coffees. So read the headline as “more consistent and more efficient”, with taste left to your own palate.

Where it might not hold

• Your grinder. The benefit assumes your burrs produce a reasonably even grind. A cheap or worn grinder can channel at almost any setting.
• Your coffee. Roast level, freshness and dose all shift the sweet spot. Fresh, gassy beans behave differently from older ones.
• Your kit. Basket size, pressure and puck prep all change how the water flows. Finer-and-slower can still be right for a particular coffee.

At espresso pressure, going finer can easily mean less extraction rather than more, which is the opposite of what most of us were taught.

If you want to try it at home

Change one thing at a time. Nudge the grind a step coarser, drop your dose a gram or two, and see whether the shot runs a little faster and tastes cleaner. If it turns sour or thin, you’ve gone too far and can come back. For the fuller walkthrough on getting a home machine dialled in, we wrote about that over in dialling in home espresso, and it’s worth remembering that fresh coffee and a decent grinder do more for a shot than any single recipe tweak.

Sources

• M. Cameron, D. Morisco, D. Hofstetter, E. Uman, J. Wilkinson, Z. Kennedy, S. Fontenot, W. Lee, C. Hendon, J. Foster, “Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment”, Matter, vol. 2, no. 3, pp. 631–648 (2020).
• Daily Coffee News summary of the study and its reception.
• James Hoffmann, whose videos on shorter shots and grind size cover this territory for a general audience.