@ntd.dillon — Rao raises some very valid points, however his definition of fines being fragments of cell wall isn’t all that easy to quantify. Anyway, regardless of what we call them, particles under 70um can apparently create up to 70% of all surface area on an EK43.
@MWJB is correct — I have made a leap by claiming that 70% of surface area correlates to 70% of the flavour. Thank you for pointing this out. There would certainly be plenty of flavour coming from the inside of larger grinds not accounted for by the 30% surface area contributed by larger grinds.
It would be interesting to quantify this somehow to discover how much more potential surface area is hidden within 80+um particles. If @MWJB is correct that <80um particles make up, say, 5% of the mass of a dose, then that could mean 5% of the dose (by weight) makes up 70% of the surface area. Does that mean we are hiding 95% of the dose (by weight) under 30% of the surface area? That would be pretty inefficient.
Water should easily dissolve compounds from the immediate surface of any particle, but then it has to work its way into and out of larger particles in addition to the time it takes to dissolve compounds inside them. Thus, the water will extract (presumably) more of the same compounds from the inside of a particle as the outside, just later on in a brew. It would be the surface area of all grinds that contributes to the primary & furthest extracted flavours in the brew, as the surfaces are immediately exposed to water. Anything tucked away inside grinds would be under-extracted relative to the outsides of particles of all sizes.
While I agree, @ntd.dillon, that sieving fines and boulders (whatever one’s definition of those are) is probably equally viable for minimizing standard deviation, I think there will always be unevenness in flavour between the inside and outside of large grinds. Imagine over-extracting the outside of a boulder, under-extracting the centre of a boulder, and then getting some great, balanced flavours from somewhere in between. Then amplify that unevenness until you have a full dose of those boulders. Unevenness seems to be an inherent property of large particles.
To eliminate the flavours that fines give off, wouldn’t you have to eliminate surface area altogether? Maybe reducing the number of fines helps to decrease the intensity of these flavours, but as long as there is surface area, there will be some amount of relative over-extraction if the no-fines-no-boulders approach is taken.
It would be interesting to test the idea that fines extract instantly. Surface area does mean more rapid extraction, but isn’t this due to availability and not solubility? (I.e. more rapid relative to the same compound hidden inside a grind). Don’t different compounds still take different amounts of time to dissolve? Won’t a complex acid still take longer to extract than a simple one, even from a fine? And how long do the compounds that give us “over-extracted” flavours take to extract? If it is tenths of a second, wouldn’t this be the upper-limit of brew time, whether brewing fines or boulders — as you will still extract these compounds from the surface of a boulder if not a fine?
I would want to see some more evidence before accepting Rao’s proposition that in “perhaps a few seconds” erosion of compounds from fines is complete. I could see this being true for undissolved solids, but isn’t there more to dissolution than moving particles from one place to another?
Great conversation. Can’t wait to hear back.