If you ask your local compact barista or even a neophyte home barista what is one of the most critical factors in making a good espresso, they will tell you the grind size. (They may say that the grinder is the most critical thing, but what they mean is that it is the size and consistency of the grind that matters, and the grinder is the tool that makes this possible). This is quite common knowledge in most coffee circles and has been known for decades. However, science is starting to confirm it, with recent research findings, and it may be worth sitting down, that grind size plays a significant role in espresso extraction.
To be fair, a study recently published in a journal never claims to break fresh ground, but merely applies a tough scientific approach to what has long been established by baristas through trial and error. (Although you’d be forgiven for rolling your eyes every time a science news site or pub appears, e.g Phys.org praises this fresh discovery that will allow you to get “the perfect espresso every time.” Whatever that even means.)
In the study, scientists try to determine what factors influence permeability. In other words, how basic or hard it is for water to pass through the coffee puck. To this end, they attempt to “empirically determine permeability as a function of porosity and specific surface area of real coffee deposits.” Therefore, their goal is to create a general model for predicting the effects of factors such as grind size and compaction on permeability.
Using two different coffees roasted by Square Mile, the Tumba variety from Rwanda and the Guayacán variety from Colombia, researchers created 22 samples, 11 each, grinding each coffee using settings 1 to 11 on a Mahlkönig grinder. The samples were mapped in 3D using X-ray micro-computed tomography to allow scientists to see otherwise undetectable paths through which water might flow. Using these 3D models, they were able to perform digital flow tests and exploit percolation theory to determine how mounds of coffee grounds would combine to create water flow paths.
This allowed them to create an equation (as above) that predicts the permeability of a given coffee, without a forceful correlation between factors such as grind size and tamping and overall permeability. By using the equation “to calculate both the permeability of the coffee puck and ultimately the mass flow rate according to Darcy’s Law,” baristas can “make certain choices in grind settings to achieve the desired shot mass at the desired time.” It really is that elementary.
However, the equation must take into account other factors, including changes in permeability as the ground gets damp and therefore swells, and the effect of pressure profiling on the result. The study focused primarily on the preparation of espresso in a specific dose at a specific time, without any reservations as to the quality of taste. This means that it is useful because “all will be all” may be a bit exaggerated. And it’s strenuous to imagine that baristas would accept this equation, especially when the process of getting to know your machine and coffee through the classic dialing process is already faster and easier. Nevertheless, the study offers an intriguing scientific approach to understanding what’s happening inside the espresso puck and could provide the basis for future models offering even deeper and more predictive knowledge of extraction.
