There’s one quote from American astrophysicist Carl Sagan we at Socratic Coffee really appreciate: “What counts is not what sounds plausible, not what we would like to believe, not what one or two witnesses claim, but only what is supported by hard evidence rigorously and skeptically examined.”

With that, we feel an apt topic for us to tackle is particle sizing: why particle size is important for coffee preparation, and the challenges faced using various sizing techniques. To do this, we’d like to provide you with some background and empirical data to help you make better coffee-related purchasing and preparation decisions.

What is particle sizing?
Particle sizing is a key property of particulate materials for many applications. At its core, particle size is simply a way to describe some quality about the material we are interested in. There is vast literature on describing the size and shape of particles for many materials. One thing worth noting is the diversity of methodologies, which is largely due to the many different disciplines that approach the problem. Though, on the surface, it may seem simple, measuring and sizing particles is not trivial. In a perfect world, particles would all be completely spherical and smooth. See figure 1.

Describing a circle or sphere is rather simple — we just need to know the radius. From that, we can calculate the object’s volume and surface area. However, three-dimensional objects that stray from this shape become much more complex and difficult to describe. Take, for example, Figure 2.

Is there a single metric that best represents the size of the particle in Figure 2? What happens if we view the particle from a variety of perspectives? See figure 3.

These particles outlined are traces from an actual coffee particle. The task of assigning a single metric to describe the size of this particle is becoming increasingly difficult. Now, add to this picture the true surface topography of a ground coffee particle (see Figure 4), and you can start to see what a difficult task sizing particles quickly becomes.

Why is particle size important to coffee?
In coffee, particle size influences several properties, including extraction rate and resistance the grounds create as water passes through. Because particles of comparable size extract at a reasonably equal rate, it is thought that the more uniform the particle distribution, the easier it is to achieve an even extraction.

Note, we intentionally did not say “better extraction”. Better is a subjective quality relating to taste. In a later write-up on taste physiology and flavour perception we can discuss why “even extraction” does not necessarily equal “best extraction”.

This article features in the June 2016 edition of BeanScene Magazine.

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Joe Chalhoub and Jeremy Nelson are Co-Founders of Socratic Coffee (SC), a small team of coffee and research professionals from around the world. Through its mutual affection for coffee and science, SC teams up to provide objective and honest data. SC is a hobby lab and all of its experiments are self-funded.

For more information about Socratic Coffee visit socraticcoffee.com