Inside a Coffee Grinder: Why Grind Size Changes Everything

Same beans, same machine, same recipe. Turn the grinder dial just a little, and the espresso can run fast and thin or slow and heavy. The beans haven't changed. Their particles have. We open the grinder and follow a roasted bean as it breaks apart: how burrs fracture the bean and why the gap only helps shape the sizes; why every grind is a mix of larger particles and tiny fines; the three things grind size changes at once: surface, distance and resistance; what one espresso experiment showed when fines were removed and added back; blade vs burr, cold beans, and static; why brew methods use different grind settings as starting points, not rules. CHAPTERS 0:00 Same beans, different shot 0:24 Inside the burrs 1:08 Three effects of grind size 1:59 Fines 2:37 Blade vs burr 3:02 Cold beans 3:21 Static 3:49 Grind by brew method 4:29 The answer 5:07 Next: why ground coffee loses flavor QUICK TAKEAWAYS • Burrs break beans into larger pieces that keep breaking along the grinding path; the burr geometry and the spacing help shape which sizes come out. The gap is not a precise sieve. • A typical espresso grind has a larger-particle population and a separate population of much smaller fines; many fines are fragments of cell walls, though not all. • Grind size changes three connected things: how much surface the water can reach, how far dissolved material must travel out of a particle, and how easily water flows through the packed bed. • In one espresso study, adding fines back to the grounds lowered bed permeability, slowed the flow and extended extraction time; coarser grounds with extra fines could flow much like a finer grind. • Different grinders can produce different amounts of fines at a similar median size, so dial numbers don't transfer between grinders. • In one study, colder beans gave a smaller modal particle size and a narrower distribution: about 31% smaller modal size at −196 °C than at room temperature (a lab condition, not brewing advice). • Grinding builds static charge, mainly through fracture; in one experiment, a little water added to the beans before grinding reduced charging and clumping, and the tested espresso shots ran slower and came out more concentrated. • Finer is not always better: in one espresso study, extremely fine grinding reduced average extraction and made shots less repeatable. SOURCES 1. Petracco M. (2005). Grinding. In Illy A. & Viani R. (eds.), Espresso Coffee: The Science of Quality, 2nd ed., Elsevier Academic Press, pp. 215–229. 2. Uman E. et al. (2016). The effect of bean origin and temperature on grinding roasted coffee. Sci Rep 6:24483. doi:10.1038/srep24483 3. Smrke S., Eiermann A., Yeretzian C. (2024). The role of fines in espresso extraction dynamics. Sci Rep 14:5612. doi:10.1038/s41598-024-55831-x 4. Cameron M.I. et al. (2020). Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment. Matter 2(3):631–648. doi:10.1016/j.matt.2019.12.019 5. Moroney K.M. et al. (2015). Modelling of coffee extraction during brewing using multiscale methods: An experimentally validated model. Chem Eng Sci 137:216–234. doi:10.1016/j.ces.2015.06.003 — and Moroney K.M. et al. (2016). Coffee extraction kinetics in a well mixed system. J Math Ind 7:3. doi:10.1186/s13362-016-0024-6 6. Garcia E., Quidlat F.G.V. (2022). Effect of Embryo Surface Morphology and Grinder Type on the Particle Size Distribution of Coffee. Philippine J Mater Sci Nanotechnol 8(1). (supplementary) 7. Mo C. et al. (2023). Exploring the link between coffee matrix microstructure and flow properties using combined X-ray microtomography and smoothed particle hydrodynamics simulations. Sci Rep 13:16374. doi:10.1038/s41598-023-42380-y 8. Corrochano B.R. et al. (2015). A new methodology to estimate the steady-state permeability of roast and ground coffee in packed beds. J Food Eng 150:106–116. doi:10.1016/j.jfoodeng.2014.11.006 9. Méndez Harper J. et al. (2024). Moisture-controlled triboelectrification during coffee grinding. Matter 7(1):266–283. doi:10.1016/j.matt.2023.11.005 10. Specialty Coffee Association (2018). Protocols and Best Practices, rev. 2018, sec. 4 Brewing Best Practices. 11. Baggenstoss J. et al. (2010). Aroma Recovery from Roasted Coffee by Wet Grinding. J Food Sci 75(9):C697–C702. doi:10.1111/j.1750-3841.2010.01822.x 12. Smrke S. et al. (2018). Time-Resolved Gravimetric Method To Assess Degassing of Roasted Coffee. J Agric Food Chem 66(21):5293–5300. doi:10.1021/acs.jafc.7b03310