Boulders, Fines and Extraction Evenness – Thoughts.

A wide grind distribution gives us lots of boulders & fines for a given average grind size. The disparity between these oversized & undersized particles is currently thought to lead to less even extractions because the large boulders under-extract (causing sourness) and the smaller fines over-extract (causing bitterness).

So, we reduce boulders, to prevent the proportion of under-extracted particles contributing sourness, reducing the average grind size…and we get? A less sour & a less bitter result, or we find we need to push extraction on, in terms of numbers, to achieve a balanced taste, to prevent the coffee from tasting under-extracted (not over-extracted).

Hang on, if we are just removing the under-extracted particles from a nominal brew we should be left with the desirable ‘fillet’ of the extraction, plus the over-extracted components?

We know if we grind finer extraction yield rises, but we also know that this doesn’t continue indefinitely…after a point, grinding finer reduces extraction yield as the water cannot penetrate the puck/bed evenly.

Is it then possible that the boulders are contributing to both bitter over-extraction, as well as sour under-extraction? Brew water may have better access to these larger particles & simultaneously be stripping dissolved solids too aggressively from the outer layers, whilst still under-extracting the inner core?

What if the fines themselves are not directly the root cause of over-extraction, what if they slow the flow to facilitate over-extraction of larger particles? The fines might have lots of surface area, but when tightly packed into an espresso puck, or wedged together in a bed, or clogging a filter, is that surface area still accessible to the brew water, or does it act like a plug?