Or is it better just to leave them in, when brewing filter coffee?
Back in 2012, our very own Matt Perger won the World Brewers Cup, using a 250μm sieve to remove fines. This created a more even extraction, which allowed him to extract higher without getting dry and bitter over-extraction flavours. However, since then he’s come out and claimed that ‘fines are fine’. In fact our last post discussed the idea that, in espresso at least, the very idea of over extraction is under threat.
If you’re finding this development confusing, you’re not the only one! However, the apparent contradiction is resolved when instead of thinking about ‘fines’ and ‘boulders’ we go back and look closely at what it is we’re trying to achieve — an even extraction. For a quick reminder on why this is so important to flavour, take a look at this post from the archives.
Why Are We Sieving at All?
Any grinder produces a range of particle sizes. These extract at different speeds, because of their different surface areas relative to their size. The reason the surface area is so important is that diffusion is very slow – so much so that in espresso, diffusion has almost no effect on extraction, and more or less all the extraction occurs at the particle surface (M Petracco, 2005). Even in slower filter coffee brewing, under normal conditions extraction through diffusion will probably never reach completion.
This means that no matter how well dialed in your brew, some of the smallest particles might contribute over extraction flavours, and some of the larger particles will be under extracted. Removing particles from either end by sieving (or by any other method, such as using a grinder with large, sharp, and well aligned burrs) will improve the brew.
The Limitations of Sieving
Sieving out the smallest particles can definitely improve some brewed coffees. However, sieving out fines by hand is a time consuming and inefficient process. Small particles tend to stick together, or to larger particles, making it hard to remove them all when sieving by hand.
A typical scientific protocol for sieving soil, for example, might involve sieving in an automated shaker for at least 10 minutes, then weighing the particles sieved out and shaking for another minute, until further shaking produces a less than 1% increase of the small particles. This protocol would impractical for most coffee brewers!
However, it’s possible to fully remove the larger particles rather quickly. Anything that passes through the sieve will be below the target size threshold, so even if some of the smaller particles are left behind by inefficient sieving, the sieved grounds will have a smaller range of grind sizes than they did before sieving. This potentially makes boulders a more useful target for baristas than fines.
The Extraction Ceiling – The Extraction Floor
When we sieve ground coffee to remove fines, what we’re doing is trying to raise the extraction ceiling — the highest point that we can extract the coffee to before the over extraction flavours become apparent. The higher we can extract without getting these flavours, the more even the overall extraction must be.
However, by removing boulders instead, we can lower what you might call the extraction floor — in other words, the coarsest size that you can grind, before the brew is ruined by under extraction. This is effectively the opposite idea to the extraction ceiling, and again makes the brew more even.
There’s one neat twist to this, which is that setting the grinder to a coarser grind size also reduces the amount of fines it produces. Fines are produced by the bean shattering each time it’s crushed in a grinder. At a coarser grind setting, each bean is crushed fewer times to reach the desired grind size — so fewer fines result. This means that by sieving out boulders, you can also reduce the amount of fines without having to sieve a second time.
So to return to the initial question — yes, it may improve your brew to remove the smallest particles with a sieve. However, we at Barista Hustle suspect that it’s going to prove much more effective to remove large particles instead. The point we are really wanting to double down on here is that sieving boulders allows you to grind much coarser — a fines reduction double-whammy.
You can hear more about Matt’s thinking on this subject in person or online, at his lecture at the Barn during World of Coffee in Berlin next Friday (7/6/19) — it’s a fundraiser for World Coffee Research. If you can’t make it, we’ll be uploading it to the the new video archive. BH Unlimited subscribers will have permanent access but for other readers we open it up to everyone for a few weeks.
We are extremely excited to also welcome special guest Tim Wendelboe at this event. We will be filming an interview with him, live at the event which will form part of our chapter on agronomy in our latest work in progress: a course called ‘Terroir’. Due for general release in six weeks time.
M Petracco, 2005. Percolation. In: A Illy and R Viana (Eds), Espresso Coffee: The Science of Quality, second edition (pp 259–287)