thoughts

Bruperstitions: In defense of omni-grind

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.

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17 thoughts on “Bruperstitions: In defense of omni-grind

  1. Emily says:

    Forgive my ignorance, but when you talk about ‘efficiency’ in extraction are you talking about achieving a goal % in a set time or am I missing the point? I’m very new to brewing ratio’s….

  2. Rob says:

    The guiding principle of the SCAE Brewmaster course was ‘all roads lead to Taste’ and I couldn’t agree more. If it tastes under or over extracted then it probably is. Somewhere during the brewing process a variable has not been judged correctly and the optimum brew has not been achieved. Grind size is just one of the variables, so there’s no reason why it can’t remain constant so one of the other variables such as temperature, turbulence, contact time or coffee/water ratio can be changed. It would interesting to know what variables you might have had to change to ensure you achieved an optimum extraction for each method with the same grind.

  3. richa says:

    Great post! I’ve also migrated to one grind for press and drip. Over the years I’ve gone progressively finer on my press grind to get the extraction I want at 3 minutes. It works, which makes sense: finer grind=more surface area=faster extraction.

    Also, I don’t let the grounds layer float. A floating bed of grounds is not really helping in the extraction department. After filling the carafe I do an initial plunge (halfway down and back up) to introduce turbulence and thoroughly mix the grounds and water again. When I bring the plunger back up, I stop the screen just below the surface to keep the grounds submerged and steeping.

    No matter what grind I use, I get about the same sediment in the cup. People forget that it’s the fines, not the grounds that pass through the screen. It seems the main reason coarse grind has been recommended for press is safety. Especially in larger presses, fine grinds tend to damn up and make disasters more likely.

    A coarse grind at 4 minutes makes sense. A finer grind at 3 minutes (or whatever time it takes to taste right) also makes sense. Tasting will usually tell you which way to go.

  4. ziad says:

    David,I know you didn’t want to talk about it, because you have no interest in it,but can’t help myself not asking you ,why is it an x for Turkish?

    • Ziad, I just don’t think it’s a particularly relevant brewing method, at least not to my taste. Overextracted, silty coffee isn’t my thing. It’s certainly interesting from a cultural point of view though.

  5. Interesting post David. Can you detect any difference in taste between a 2:30min and 4min brew on the French Press?

    By grinding finer, this should narrow the range in size of the grounds produced. So basically what I’m asking is: does the shorter brew time/finer grind method produce greater clarity in the cup?

    • James, I get what you are saying. However, the physics of bean breaking dictates that although you are narrowing the range between fines and the desired peak by grinding finer overall, you are also increasing the proportion of fines.

    • Joe Griffin says:

      So I relies I’m a little late to the conversation. But this is the same question I’ve been asking my self after watching the Tamper Tantrum from the Uber factory. I would narrow the question slightly thou.
      Is there a taste difference between a perfectly extracted grind (mojo tested the same) for 10 sec, and a perfectly extracted grind for 4 min? To me this question is asking specifically can we tell the difference in the diluted solids that we are tasting if they are the first part of the extraction or the something that has slowly been extracted.

  6. ziad says:

    Yes David,I understand,it may be the oldest brewing method ever,and it is cultural more than anything else.
    Overextracted and silty coffee wouldn’t be my thing either,certainly if it’s brought to the boil several times like many do,that’s more likely what we would get,but I got your point.

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  8. I have been pondering these same questions and locally I find a growing number of people that think a pour over (Chemex namely) needs a four minute extraction time regardless of final volume… It is all folklore proving to me that the tongue is still not as important to people as it should be.

  9. Well if you are going to defend the omnigrind then I will have to go on the attack.

    First off – its preground. Doesn’t matter how it was preground, it is still a stale, undesirable food product that isn’t fresh or delicious and never can be. I know that wasn’t the point of your post but I reserve the right to despise omnigrind for this alone.

    The balance of grind size, contact time, flow rate, agitation and water extraction efficiency can most likely be balanced out if you choose to keep the grind constant. But this can’t be done easily, or in a convenient way. Problems will arise (Paul highlights some in his post), and allowing the grind size to take some of the burden is no bad thing.

    My only other thought on this is pairing a grind size and a brew method with a view to the margin of error that is easily controlled via that brew method. A coarse grind, and a longer extraction time has the benefit of a wider window of time in which the coffee is correctly extracted (in a french press as an example). Not everyone at home (for reasons I will never understand) is brewing on scales or timing their brews but a press pot won’t suddenly become all kinds of evil if you miss the brew time by 20s or even 30s. As a goal is surely people drinking more delicious, sustainable, quality coffee at home then brew methods/grind sizes that don’t make them frustrated on the first few goes are probably a good thing.

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  12. I have the opinion – i.e. it is not a proven fact that large grind coffee always leads to underextraction because the water never penetrates properly to the center of the particle. This certainly happens with large leaf tea where it can be shown that over 50% of the flvour is thrown into the trash.

    I believe thata espresso grind with 30 seconds extraction gives a very good cup – better than the same quantity of a large grind brewed for four minutes

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