Further Ruminations on Filter Grind

This is a heady time for filter brewing. I don’t think at any time in recent memory has the discourse been as vibrant. Gold Cup, Extract Mojo, TDS, extraction percentage, are terms we are all hearing a lot more of in recent times. They are becoming a greater part of the general coffee lexicon. This is a good thing, I hope this momentum continues. This post is really a continuation of the thought process in my last post on the subject. This post specifically asks the question:

Why don’t we use powder-fine grind for brewing filter coffee?

Before I attempt to answer that, let me start by addressing why I think a powder-fine grind could be a good thing.

With current grinding technology fines are inevitable. At a coarse grind, fines will pretty much fully extract in the time it takes to get a desirable extraction out of the normal grind population. This creates those not so nice flavours discussed previously. The finer your normal population becomes, however, the two populations of grind sizes start to approach unity in terms of speed of extraction, to a point where theoretically you could approach a single peak, of fines.

A lot of consideration has gone into populations of coffee grinds, but a coffee grind as a single entity is also worth considering. It has depth, volume, surface area, all characteristics that effect how it is extracted. Upon brew the outer-most layer of coffee, the surface, will begin to immediately extract. This is true for all grind sizes. Below the surface, however, the layers will not begin to extract until the solvent (water) has permeabilised the outer layers.


We cannot, however, create a grind that is composed of either all exposed or all unexposed layers. The smaller our grinds become, the surface area to volume ratio approaches, but never reaches infinity. The coarser our grinds become the surface are to volume ratio approaches, but never reaches zero. Both scenarios offer routes to near as possible grinds composed of one type of layer vs the other. Of course we are restricted in how coarse we can brew, by bean size, and practical brew times, not to mention that our assumption that an outer though not surface layer of a grind is equaivalent to an inner layer is evidently false.

Fine grinds are far less limiting. The finer we grind, the more surface area is exposed. Theoretically we could reach a point where we are close to, though never arriving at, a population of grinds that are all surface area.


So in terms of even extraction, fine grinds in theory should be better.

Another consideration is of course that fine grinds will extract much more quickly, and I don’t think anyone has ever said, “That’s a great cup of coffee, if only I had to wait longer for it”.

So, why do we not use powder fine grind for filter coffee?

The short answer is that the finer you grind the harder it becomes to separate the insoluble matter from the brew.

Relatively coarse meshes like those on a french press, or swissgold filters, will allow too much insoluble matter through. With paper or cloth filters it merely becomes difficult to seperate the grinds from the brew sufficiently quickly to avoid overextraction.

With that in mind, I picked up a piece of laboratory glassware that I have fond memories of using in my undergraduate chemistry labs:

I’m not advocating this as a filter brewing method going forward, but I hope it might give some food for thought. I suspect the way we think about coffee grinding is too simple, there are too many broad strokes in how we describe it, and how we describe its influence on a brew. I don’t think brewed coffee is a finished product. While I like the French Press, drip filter and vac-pots, I hardly think they can be the pinnacle of technological advancements in coffee brewing. Who is going to fill the void left by Clover? Where is the 21st Century coffee brewer?

PS the coffee from the video had an extraction of 18.5% (measured with a cheapo TDS) and tasted mighty fine.

PPS yes I made a rookie mistake at the end of the vacuum period.


5 thoughts on “Further Ruminations on Filter Grind

  1. I so need time to get my head around this but I hate you in so many ways for making me think about stuff I thought I already knew. Its like telling someone oxygen inst what we breath , discuss.

  2. First up, thats brilliant.
    I recently pondered the same question because my attitude towards fines is “if you can’t beat them,join them”. I was also asked recently why coffee is brewed for 4 minutes. These two questions made me come to the conclusion that;
    Coffee was originally ground coarser because of the sludgy nature of brewed coffee when it was ground finer and a 4 minute brew time was the result of this, not an intended aim.
    Coffee loses more flavour to the burrs and through oxidation once ground. One must bare in mind that the vast majority of places do not grind fresh.
    I think these two factors perhaps set the coarse grind standard.
    Its also interesting to note that the available technology at the moment is pointing everyone away from modern updosing methods and back to a proper extraction at a lower dose. Are we merely rediscovering what others discovered before? (another example of a short brew time and fine grind would be the ibrik, the ooldest and most prevalent brewer perhaps?)
    Great article, great vid, we need more information!

  3. Stribb says:

    I don’t think this is a different technique in principle from aeropress, though it’s got to have a higher pressure differential. There’s no difference to the end product between pushing the coffee through by high pressure and pulling it through with a partial vacuum.

    I was entertained to see the coffee boiling in the low vacuum. Do you know (even approximately) what pressure was in the bottom chamber?

  4. I’m afraid I don’t have even an approximation of the pressure, unfortunately no datasheet accompanied the aspirator.

    I would debate that there is no difference between pushing and pulling.

    In the case of a vacuum the grinds remain under atmospheric pressure.

    With the aeropress they are subjected to an increased pressure – which has an impact on the rate and nature of extraction (obviously join the dots to espresso).

    So I wouldn’t say there is no difference.

  5. Chris says:

    The interesting thing is what goes on inside a particle of coffee. Assuming transport is via small holes or pores that get bigger with extraction you could argue that the center of larger particles dont actually feel pressure effects. Once you get beyond a certain depth (particle size) the most likely scenario is that the only thing which effects extraction is time.

    I actually look at a very similar process in mining as my main job so would love to find out more info on coffee leaching – but the general principle is that there are 2 competing issues on what leaches and how fast.

    The first is chemical based- as in what it takes to liberate each soluble species in the coffee grain (which includes things like solubility and rate of dissolving, and probably pressure too – and crucially this will be species dependent- i.e different species dissolve at different speeds)

    The second is diffusion based – the time taken for water to get into the particle and for the dissolved species to travel out. For small particles diffusion is virtually nothing, for bigger particles it dominates.

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