One of the first “truths” I learned as I became exposed to the world of better coffee was that omni-grind is inherently a nonsense. This seemed entirely logical, how could one grind fulfill the idiosyncrasies of a multitude of brewing methods. I did not offer much resistance to my acceptance of this conventional wisdom. The dogma of relative grind sizes has been widely propogated in the popular coffee lexicon to the extent that even those with the most basic of understandings of coffee brewing will probably be able to recite the ordered list. From fine to coarse it goes Turkish – Espresso – Moka – Drip – Press – Percolator (or thereabouts). Omni-grind was just one more by-product of commercial coffee roasters that we could bash them over the head with (also not sure why we call THEM commercial coffee – as far as I can see all coffee is commercial – I digress).
I think we were hasty to do so.
I accept that espresso is unique and requires a very specific grind consideration. To be perfectly honest I have next to no interest in Turkish or Percolator, so I will refrain from expending further wordage on their merit. That leaves us with Moka, Drip and Press. In the last week I have brewed to acceptable extraction percentage (and taste) with all three of these brew methods using the same grind setting on my grinder.
To all intents and purposes my grinder had become a producer of omni-grind… an omni-grinder.
The question that presents itself to me is why did we come up with this preconceived notion of relative grind sizes for different brewing methods. Why do we assume for instance that French Press requires a different (read: coarser) grind size to drip?
Another piece of conventional wisdom that I think contributes to this, and that I take umbrage with is the prescription of 4 mintues brew time. I’ve yet to be convinced that a 19% extraction in 2:30mins is inferior to 19% in 4 minutes, though the arguement has been made by some. I’m open at least to at some point in the future being proven wrong on that point. However, trying to reach this 4 minute mark can place certain mechanical restrictions on the grind size that go beyond considerations of extraction (and taste). Such as…
The flow rate (or percolation) of brew water through a bed of coffee during drip brewing is inversely proportional to grind size. In order to get to that 4 minutes, depending on the shape and other physical characeteristics of the brew basket, might require that we make adjustments to the grind.
A mechano-physical consideration with regards to the french press is the permeability of the filter mesh, and the resulting sludge. Logic suggests that a coarser grind will result in less sludge. I wouldn’t question a right-thinking mind would assume that the mesh was designed with a certain size of grind in mind, and that using this combination together couldn’t result in a cup with such an obviously displeasing flaw. Such seemingly reasonable conclusions are unfortunately largely false. While the magnitude of the sludge can be impacted to a degree, short of post-press filtration it is always present (even in my experience with administration of scoopy-scoopy). There are diminishing returns in pursuing the coarse grind route with french press, and it becomes difficult to marry it to the 4 minute dogma short of significant agitation.
Somewhere along the line it also became popularized that so called “full immersion” brewing (steeping) is more efficient at extraction than drip. Grinds in a french press tend to sit for most of the contact time in a static manner (sometimes forming a partially floating layer on top of the brew water). Though the osmotic potential of the brew water is initially quite high it is always declining. The lack of agitation also means that local concentrations of dissolved solute surrouding the grinds is comparably higher than the overall concentration as there is insufficient kinetic energy to rapidly even this out. This adds up to a relatively inefficient system on face value.
Consider a drip brew on the other hand. Fresh brew water is added throughout the brew (save the final drain) and dissolved solute is constantly leaving the system, maintaining osmotic potential. Add to that a small but not insignificant degree of agitation due to the pouring (and the kinetic energy due to the percolation) and in theory at least would suggest that there is the potential for greater efficiency of extraction.
In real terms this concurs with my typical brewing parameters. Not only have I used the same sized grind for drip and press, but my drip brews tend to complete final drain between 2:30mins and 3mins, while I tend not to plunge the french press until 4 minutes. When I drag the V60 brew out to 4minutes using that grind it results in a distinctly overextracted, bitter cup.
I doubt I am the first person to make these observations, but perhaps it is worth suggesting that people avoid being mislead by what amounts to a folklore, that we waste less time trying to fit our brews to what we blindly believe to be “correct” parameters, and perhaps that we pause a moment before the next time we rush to scoff at the mention of omni-grind.