The ultimate aim of the distribution and tamping techniques described in the previous chapter is to create a bed of coffee that is evenly permeable to water throughout, so that there is an equal flow of water — and hence, equal extraction — in all parts of the coffee bed. Evenness of extraction is hard to measure directly, so the approaches we recommend to achieve it are mainly based on the practical experience of numerous baristas, over decades of trial and error.
Research available from other industries, on the subject of powders and the way they pack together, might help us understand what’s happening at the particle level for coffee. This research can explain, for example, why tamping harder doesn’t, but nutation does, slow extraction of a shot. In this chapter, we explore this research and look for clues to how we might use this knowledge to achieve even better distribution.
What Determines Permeability?
Permeability is a measure of the ability of a substance to allow fluid to pass through it. This factor controls the flow rate through the coffee bed in a typical espresso machine: Darcy’s Law tells us that for a given pressure, the flow rate is proportional to the permeability. In other words, double the permeability and you double the flow.
The permeability of a coffee bed depends on two variables: the particle size distribution and the way those particles are packed together. As baristas, we mainly control permeability by changing the grind size.
For a given grind setting, however, the flow rate through the coffee bed will be affected by how densely packed the coffee is — and small changes have a dramatic effect.
For example, an increase of 30% in the density of a coffee bed causes the flow rate to decrease by a factor of three or four (BR Corrochano et al., 2015). This helps explain why good distribution is so important: even the slightest unevenness in density will lead to uneven flow.