You asked us to look into this topic, “In espresso, fines migration may not be as much of a thing as we thought.” Here’s what we found.
الطحن الناعم are the smallest particles created when you grind coffee. Depending on who you’re talking to, they might be defined as any particles less than 100 microns in diameter, or any particles smaller than the bulk of the ground particles, or as any particles consisting only of cell fragments.
However they’re defined, الطحن الناعم are extremely important to espresso extraction. Espresso extraction relies on التآكل from the surface of particles (M Petracco, 2005), which means the high surface area of الطحن الناعم is essential for good extraction in espresso.
'Fines migration’ refers to the idea that these small particles move downwards through the bed of coffee — whether during preparation of a shot (which I am going to call ‘dry’ fines migration) and also while the shot is running (‘wet’ fines migration). The importance of fines migration can be seen in our recent post on using filter papers, where using a filter paper to prevent الطحن الناعم from reaching the basket holes allows much higher extractions. For this reason, and because of an assumed effect on evenness of extraction within the القُرص, minimising fines migration is often seen as desirable.
Dry Fines Migration: Granular Convection
When baristas worry about fines migration, we are typically thinking about what happens during preparation of the shot — after all, that’s the part we can most easily control.
When a granular material (whether coffee grounds, sand, or a bag of nuts) is subjected to الرجّ or vibration, it starts to behave more like a liquid. The material will start to ‘flow’, and will show patterns of movement similar to convection in a liquid — known as granular convection, or the ‘Brazil nut effect’. As the particles move, the smallest particles tend to sink to the bottom, while the largest particles ‘float’ to the top.
This suggests that distribution techniques that involve tapping or otherwise agitating the coffee bed would increase the movement of الطحن الناعم to the bottom of the القُرص, forcing the barista to use a coarser grind to achieve an acceptable flow rate. However, there’s actually scant evidence for this having much effect.
Part of the reason for this is that ground coffee is loaded with static charge, created when the coffee particles are broken apart. The finest particles tend to stick to larger particles and are thus not readily removed by sieving — even when following a rigorous sieving protocol.
“Sieving … has a low separation efficiency; this is because coffee particles show strong interparticle adhesion.” (Kuhn et al, 2017)
One experiment by Socratic Coffee (2016) compared the flow and extraction in pucks with grounds separated into different sizes by sifting. They found that layering fine particles on top of larger ones or vice versa had no measurable effect on flow or extraction, compared to the same particles mixed together. This suggests that the rearrangement of different particle sizes due to the flow of water during extraction outweighs any possible effect of particle migration in dry coffee.
Wet Fines Migration
During a shot, الطحن الناعم ‘migrate’ through the القُرص, carried along by the flow of water, until they reach the bottom of the القُرص, where they tend to accumulate and form a densely packed layer. This layer traps further الطحن الناعم, and becomes a major barrier to flow during the first few seconds of extraction.
The evidence for this comes from the way the flow through a bed of coffee changes over time, as shown in the following image from Fasano et al. (2000). When the pump starts, the flow is initially quick, but quickly slows down to reach a lower rate (a). If the pump is stopped then restarted, the flow continues at the lower rate (b), which shows this is not due to الضغط الخاص القُرص. If however, the direction of the pump is reversed (c), then the القُرص shows the same pattern of fast flow at the start, rapidly slowing to a lower rate.
“This effect can be explained by assuming that the fine particles can be removed and transported by the flow” …
… the authors write. As the الطحن الناعم move to the bottom of the القُرص, they begin to obstruct the flow. When the pump is reversed, the الطحن الناعم are now on the ‘top’ and so the flow is fast again, until they reach the bottom of the القُرص and again obstruct flow.
An image from Fasano et al. (2000)
A recently published mathematical model simulating extraction and flow, including fines migration (Ellero and Navarini, 2019) confirms this, showing “remarkably good agreement” with experimental results.
Controlling Fines Migration
So as we have seen, القُرص prep doesn’t seem to have much effect on fines migration. However, there are some factors that might reduce fines migration during extraction. At the extreme end, using a paper filter underneath might not prevent migration through the القُرص, but it does seem to stop the formation of the compact layer of الطحن الناعم that limits flow through the bed.
Preinfusion in espresso machines also seems to limit fines migration (Rao). Allowing water to penetrate the coffee bed before لضغط الضخ is applied causes the larger particles to swell as they become saturated, making it harder for the الطحن الناعم to move through the gaps between them.
However, in both cases, fines migration still occurs. All these techniques are doing is limiting it in some way, in order to allow us to grind finer. The key point is that الطحن الناعم, and wet fines migration, are very important to flow and extraction – so don’t stress about dry fines migration. As Matt himself put it: “Stop worrying … If you want to avoid fines migration then you probably shouldn’t be pumping water through the grinds:”
M Ellero and L Navarini, 2019. Mesoscopic modelling and simulation of espresso coffee extraction. Journal of Food Engineering doi: 10.1016/j.jfoodeng.2019.05.038
A Fasano, F Talamucci, and M Petracco, (2000). The Espresso Coffee Problem. In: Modeling and Simulation in Science, Engineering and Technology, pp 241–280. doi:10.1007/978-1-4612-1348-2_8
M Kuhn, S Lang, F Bezold, M Minceva, and H Briesen, 2017. Time-resolved extraction of caffeine and trigonelline from finely-ground espresso coffee with varying particle sizes and tamping pressures. Journal of Food Engineering doi:10.1016/j.jfoodeng.2017.03.002
M Petracco, 2005. Percolation. In: A Illy and R Viana (Eds), Espresso Coffee: The Science of Quality, second edition (pp 259-287)
M Petracco, M., Liverani, F.S., 1993. Dynamics of fluid percolation through a bed of particles subject to physico-chemical evolution, and its mathematical modelization. In: ASIC (Ed.), 15th International Conference on Coffee Science, pp. 702–711
Socratic Coffee, 2016. Exploring the Impact of Particles on Espresso Extraction (Blog post). Available online at http://socraticcoffee.com/2016/06/exploring-the-impact-of-particles-on-espresso-extraction/