You asked us to look into this topic, “In espresso, миграция штрафов may not be as much of a thing as we thought.” Here’s what we found.
Штрафы 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, мелочь are extremely important to espresso extraction. Espresso extraction relies on эрозия from the surface of particles (M Petracco, 2005), which means the high surface area of мелочь is essential for good extraction in espresso.
'Перенос штрафов’ 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’ миграция штрафов) and also while the shot is running (‘wet’ миграция штрафов). The importance of миграция штрафов can be seen in our recent post on using filter papers, where using a filter paper to prevent мелочь from reaching the basket holes allows much higher extractions. For this reason, and because of an assumed effect on evenness of extraction within the шайбу, minimising миграция штрафов is often seen as desirable.
Dry Fines Migration: Granular Convection
When baristas worry about миграция штрафов, 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 тряска 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 мелочь to the bottom of the шайбу, 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.
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, мелочь ‘migrate’ through the шайбу, carried along by the flow of water, until they reach the bottom of the шайбу, where they tend to accumulate and form a densely packed layer. This layer traps further мелочь, 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 сжатие шайбу. If however, the direction of the pump is reversed (c), then the шайбу 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 мелочь move to the bottom of the шайбу, they begin to obstruct the flow. When the pump is reversed, the мелочь are now on the ‘top’ and so the flow is fast again, until they reach the bottom of the шайбу and again obstruct flow.
A recently published mathematical model simulating extraction and flow, including миграция штрафов (Ellero and Navarini, 2019) confirms this, showing “remarkably good agreement” with experimental results.
Controlling Fines Migration
So as we have seen, шайбу prep doesn’t seem to have much effect on миграция штрафов. However, there are some factors that might reduce миграция штрафов during extraction. At the extreme end, using a paper filter underneath might not prevent migration through the шайбу, but it does seem to stop the formation of the compact layer of мелочь that limits flow through the bed.
Preinfusion in espresso machines also seems to limit миграция штрафов (Rao). Allowing water to penetrate the coffee bed before давление насоса is applied causes the larger particles to swell as they become saturated, making it harder for the мелочь to move through the gaps between them.
However, in both cases, миграция штрафов 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 мелочь, and wet миграция штрафов, are very important to flow and extraction – so don’t stress about dry миграция штрафов. As Matt himself put it: “Stop worrying … If you want to avoid миграция штрафов 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
М. Петракко, 2005. Перколяция. В: A Illy and R Viana (Eds), Espresso Coffee: The Science of Quality, второе издание (стр. 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/