Sometimes grinding finer doesn’t increase overall extraction yield. There is a nonlinear relationship between grinding finer and increasing extraction yields with espresso and drip coffee, usually experienced as a slight bell curve. As we grind finer over a range of shots starting from 15 seconds, up to 60 seconds, extraction usually peaks around 40 seconds and then starts to reduce after this.
It is clear that increasing surface area accounts for the increase in extraction from 15–40 seconds. What is more theoretical is why extraction drops after this high point. We think the reason is that of ‘ineffective voids’. As an easy way to visualise this, think of a pour-over where you have ground your coffee too fine. With a paper filter, water can escape out of every pore in the paper, so where the coffee bed puts up too much resistance, the brew water can just channel out the holes above the coffee bed. If this were not possible, then the very fine grind would result in a very long contact time and we would expect this scenario to yield a very high extraction percentage.
Espresso baskets only have holes in the bottom, not the sides like paper filters, reducing the formation of ineffective voids compared to filter brews. They do still persist, however, with a possible explanation due to the smooth stainless steel edges of the baskets. This presents an opportunity for channelling down the sides of the basket. This phenomenon can be observed if you prepare a shot in a naked portafilter and insert the portafilter into the group head in an aggressive way that unseals the puck from the edges of the basket.
In this video, Matt shows you some diagrams of ineffective voids in action
The Extraction and Shot Time Survey
If you have access to a coffee refractometer, to add some data to our theories on this subject,