Uneven Extraction in Coffee Brewing

TL;DR

This paper investigates why finer coffee grinding can lead to lower extraction, proposing a model that explains uneven extraction due to flow and dissolution interactions in the coffee bed.

Contribution

It introduces a low-dimensional model analyzing flow pathways and their interaction with extraction, explaining experimental observations of uneven extraction at fine grind sizes.

Findings

Finer grind size can decrease extraction due to flow pathway imbalances.

Flow and extraction interactions can lead to complete extraction in one pathway.

Model aligns with experimental results showing reduced extraction at very fine grinds.

Abstract

A recent experiment showed that, contrary to theoretical predictions, beyond a cutoff point grinding coffee more finely results in lower extraction. One potential explanation for this is that fine grinding promotes non-uniform extraction in the coffee bed. We investigate the possibility that this could occur due the interaction between dissolution and flow promoting uneven extraction. A low dimensional model in which there are two possible pathways for flow is derived and analysed. This model shows that, below a critical grind size, there is a decreasing extraction with decreasing grind size as is seen experimentally. In the model this is due to a complicated interplay between an initial imbalance in the porosities and permeabilities of the two pathways which is increased by flow and extraction, leading to the complete extraction of all soluble coffee from one pathway.