Over the last couple weeks, we have covered questions of blends and solubility. Talking about how a tasty cup requires roast development because it ensures solubility and proper extraction. We are going to work backwards today and talk about grinders to answer: why do we have the Ek-43 as our surrogate measurement on our parameters sheet?
Let us start simple with Scott Rao, who writes, “grinding is the fracturing of coffee beans into many smaller particles” creating “more specific surface area” to interact with water. Despite appearance, beans are complex. Rao describes them as a “matrix of cellulose strands coated with oils and other solids”. Through roasting, we make the cellulose brittle. By grinding, we shatter these intricate structures. Each woven strand creates a negative space called a cell. If you are having trouble visualizing, think of the syrup collecting gaps on a waffle, those would be the cells.
For Rao, one key challenge in extraction is that “particles with intact cells extract different than particles without any intact cells”. Returning to the breakfast metaphor, by stacking two waffles, you create internally shielded and sadly syrup-less void spaces. The ‘cells’ which usually pool maple sweetness actually do the reverse; they create a buffered dome that requires saturation for access. In coffee, those particles without intact cells are known as ‘fines’ and extract rapidly. Conversely, the ones with intact cells extract slowly (or not at all) because “some of the cellulose surface is … not directly exposed to water”.
Alright! With some background in place, we can turn to what actually happens when we add water (insert dated Gremlins joke here). Nearly half of the extraction occurs immediately through ‘surface erosion’. Hot water washes across the grounds and immediately pulls the ‘coffee solids’ from exposed surfaces. The second stage occurs much more slowly, as “bean fibers absorb water and swell” and the remaining coffee solids “dissolve and migrate into the surrounding liquid by diffusion”. The reason we pre-infuse a brew or allow for a ‘bloom’ on a pour-over is to shift the extraction erosion towards the sluggish work of diffusion. Given the difficulty in reaching those intact cells, we also do things like stir or percolate.
You can probably start piecing together what separates a good grinder, like an EK-43, from a bad grinder, say the blade spice grinder you begrudgingly use while visiting family. The more uniform the size of grind, the better the cup. You will not have huge deviations from large under-extracted chunks to smaller over-extracted bits. The crucial error in blade grinder design is the random assortment of results. On burr grinders, coffee passes through a slight gap between two interlocking sets of blades. We see much more uniformity. There is a myth that EKs create fewer fines, which Matt Perger proves is simply not true. Their true strength is burr size and motor power which closes that overall gap in size variations.
Something that is easily lost in all these conversations on solubility or grinders is the roof for extraction. A meager 35% of the mass of a bean is actually water soluble. As Scott Rao muses, “without extreme temperatures, unrealistically long extraction times or additional solvents, it’s a fair to assume that you’ll never extract more than 35% of the weight of a pile of coffee grounds”. And then we have this internal problem with so much of that potential 35% occurring instantly and then the rest dragging on and on. It’s a mess. Hence, we try to provide some baselines for shops to dial-in because knowing finer or coarser is a good start. And to finally answer the original question, we use the EK it has the most control in this chaotic jumble of grinds.