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.