Advection is the movement of a substance, brought about by the flow of a fluid. In the context of coffee, ‘erosion’ describes the advection of solute (dissolved substances) and undissolved solid materials from the coffee bed. The grinding process for most food powders, including cocoa and coffee, produces a bimodal grind size distribution (Popplewell and Peleg 1988). ‘Bimodal’ means two values occur with high frequency. When a coffee bean breaks under the forces involved in grinding, some large fragments (boulders) and some small fragments (fines) are produced by the combined cutting and crushing action of the burrs. Cutting is expected to cleave through beans without producing many fines; most of the fines production takes place when beans are shattered between grinder burrs. A further separation of fines and builders occurs when fragments of coffee cells and droplets of coffee oils are separated from the boulders by the process of erosion.
This image of a pencil tip breaking shows an example of how when something shatters, it can produce large fragments and fines as well.
In brew methods such as espresso that involve a very short contact time, proper extraction of the coffee relies on erosion. A very high surface area is required, and it’s important to understand that fines behave quite differently from larger particles. In the smallest particles, all the cells are broken open, which means all of the soluble material is in direct contact with water and can potentially be eroded. Larger particles, on the other hand, still contain some intact cells, and the water can reach the centre of these only by diffusing across the cell walls. As we will explain further in Chapter 5, when you prepare cezve coffee you get around this issue by eliminating boulders from the grinds.