BY JAKE BUSH

This, our second article in our roast density series is about particle and puck density. For a clearer understanding and reaffirmation of principles in this article, read our primer The Density article Part 1 Green and Roasted Bean

By breaking Roast Density down into Particle and Puck density, we are moving from roasting the coffee into the realm of extraction. This is where things escalate. The introduction of pressure and flow create new principles. The entire scope of which can’t be expressed in one article. So initially we thought it more appropriate to paint some broad strokes in this article and get down into the fine print as time passes. 

An overview

Many of us that have been drawn into the challenge of coffee extraction have observed the same dynamic. We may be grinding a particular roasters coffee through our grinder, then loading the portafilter and extracting a brilliant shot, with what we perceived to be the best parameters for that coffee! However, if we leave all of our settings untouched, and load up a different coffee roasters beans, or even a different origin from the same roaster, and use the same set parameters, the result will be different, often dramatic, sometimes undrinkable.

So how is it that when all settings remain the same, one roast meets the desired outcome and the other roast doesn’t? And even if you dial in for roast 2 to what we believe to be the best shot, then reload roast 1, it becomes undrinkable. We have all seen this, but why is it so?

Espresso extraction can be summed up simply as perfecting the relationship between the roasted coffee bean and the espresso machine, through the use of the grinder. The talented barista Interprets the roasted bean and manipulates its form to suit the coffee machine they know so well, through the use of the coffee grinder. Most in the industry would agree that this is a very broad-brush statement but it is essentially true.

Another truth about the coffee industry is that there are very few standards. There are almost no rock-solid definitions that every decent Coffee maker would agree as an absolute fact. The reality is that it is an industry that has basically bumped its way through history finding its direction through a mirage of marketing spin and uneducated customer feedback.

At Bush & Bush, we have sought to understand the above dynamic using undeniable measurements based on pure physics. 

In very simple terms, we have sought to establish some basic measuring principles to aid us to diagnose the Roasted Bean, to Characterize the Coffee machine, and then using some simple calculations based on these measurements, manipulate the bean in the grinder to best work with the machine for the most reliable result.

At the core of our process is a very simple Physics Principle called Density. We talked about this in the first density article, but to recap, its mathematical representation is a very simple formula: D=M/V. That is Density = Mass (Weight) Divided by Volume (Space). We use the metric system here so our units for these measurements are Mass in grams (g) and Volume in milliliters (mls). This can be thought of as mass per volume.

All of our work is based around this principle, and an easy way to grasp the concept is to understand the density of water. Pure water has a density of 1. Its mass is equal to its volume in our SI (standard international) units of grams (g) and milliliters (mls). 10g of water will occupy 10ml of volume (space). 123g of water will occupy 123ml of space. If you sat a dish on a scale, tared it to zero and poured 57g of water, that water would be occupying 57mls of volume. It doesn’t matter what shape it is, its still 57mls of space. Its Mass is equal to its Volume. From this point on, it must always be remembered that in our process we are always referring to a density of 1 is Mass in grams is equal to volume in milliliters.

You just can’t get any simpler or undeniable than that!

While there will be people who want to argue about definitions and wording etc, the principle remains. We reference this example so that people are not required to have a doctorate in astrophysics to understand our system. It will also make grasping our more complex process later easier for everyone.

We can characterize a coffee bean in many ways through our 5 senses but every one of those assessments will be subjective, dependent on the observer’s life experience.

Using the basic old physics of Density, our observation can now be paired with an undeniable and factual Truth. How much does the Roasted Bean weigh and how much space does it occupy? If we know both of these attributes, just like water, we can determine its Density. We will call this Roast Density and it can be thought of as the character of the Bean. We can think of it this way because as a Green Bean, prior to Roasting, It would have been very different. A typical very average Green Bean Density is 1.186. This means its mass per volume is heavier than the mass per volume of water, which means if it was put in water, it would sink because it is denser!

When this Green Bean is manipulated by heat in the roaster, it simultaneously decreases in weight and increases in volume. The final density result of the roasted bean is variable and will depend upon the roasting process. A typical Roast density of a second crack Coffee Bean is between 0.53 and 0.59. So the Roasted Coffee beans mass per volume is lighter than the mass per volume of water. Roasted coffee therefore has a lighter density than water. And yes, that means it will Float! Simple eh?

We said that the roast density is how we can characterize the roasted coffee bean because it has been the efforts of the coffee roaster that has taken the green bean from something which would sink in water, to something that would float. They have used their craft and knowledge to change its form radically, and there are many other attributes that can characterize that transformation, but none more undeniable or reliable other than density. In fact, there are many attributes that can be associated to the roasted bean by its roast density figure, but we will cover these later.

So now we can characterize a green bean by its green bean density figure and a roasted bean by its roasted density figure. We now have a number to associate with the bean which links it to its current physical state. 

Of course, we don’t extract from one bean at a time so we must be able to measure multiple beans at a time and our new measurement tool does this, in fact, it measures them in the same quantities that we would use for espresso extraction so that it is more representative of our real-world circumstances. We simply do this by measuring the mass of the beans and the volume of the beans by displacement. Displacement is the use of water (our known density of 1) to measure the free space between the beans and take that away from the known total volume, which leaves us with the volume of the beans. Essentially taking many beans and reducing it to, in theory, one large bean. This means that we can also get a better-averaged representation of a blend of coffee beans which might all have different independent roast densities.

Which brings us to Particle Density. Up to this point, we talked about whole beans, but we all know that we can’t use espresso extraction on whole beans. We must use the grinder to reformat the physical shape of the beans so that they can be deposited into the porta-filter for extraction.

This is a very important point, so think very carefully about this. 

When the roasted coffee bean has been ground up in the coffee grinder it becomes a collection of particles, known as ground coffee. The density of each particle remains the same as the roast density. (Assuming of course that the coffee bean has been evenly roasted in the first place.) Each particle is simply a fragment of the original bean and just it’s size has been changed by the grinder. If you could weigh 1 particle and measure its volume you would find that its density is still equal to its original roast density, it has simply been cut into much smaller pieces so that it can be packed into the porta-filter. The character that has been implied on the bean by the coffee roaster remains embedded in each particle as it’s roast density, which can only be changed by the Roasting Process. Simple Eh?

Stay with me, it’s all about to come together.

When we put a collection of these particles into a porta-filter and tamp it down, we call this puck density. Puck density becomes a whole new entity. The coffee roaster controlled roast density as their craft. The barista controls puck density. Remember that density is mass divided by volume. Now we have the mass ( How many grams of ground coffee in the portafilter) divided by the volume it occupies (how much space, measured in mls, it takes up in the portafilter).

As baristas, we all know the age-old dilemma of “how much coffee should go in the portafilter, at what grind setting?” Now it has a definitive name and a way to measure it. It is called puck density. Puck density is different from roast density in that roast density was a “true density” because we used displacement to measure the volume occupied by the beans very accurately. Puck density is what is known as a bulk density figure of all the particles, condensed into a puck. We only know the outside volume of the collection of particles, which collectively after being tamped comprise the puck. We have not subtracted all the little nooks and crannies in between the particles of free space, space which we call galleries from the overall volume. These galleries are the tunnels which water will later be forced through as we attempt extraction.

At this point in our assessments, the crudeness of puck density is enough to review our first question.

Why did roast 1 and roast 2 extract so differently when we left all our other adjustments the same?

It is easy to see now that they were different roast densities. They could have started as identical green beans, but as the coffee roasters implied their craft on them, and they were roasted differently, and they gained distinctly different characteristics. We can now measure and refer to this as roast density.

If the role of the barista is to interpret the roasted bean and manipulate it to satisfy the relationship between the bean and the machine, then these are the linkages by which it can be done. It is not magic, it is simply Density.

Soon it will be commonplace for baristas to be able to work collaboratively together using a common and undeniable measurement system that is wholly relative to the extraction process.

Comments such as extracting a 20% solubles yield from a RD (Roast Density) of 0.63 with a PD (Puck Density) of 0.58 will be commonplace, and they will have very definite meaning amongst those in the industry. They also help the roaster to identify issues for their customers in cafes when the café owner can report “we are Running Batch 2122 with RD 0.58 at PD 0.52 but it’s not playing the game!”

At Bush & Bush, we have been using these measurements for a number of years and this is literally the very tip of the process. In the next edition, we will continue our journey on how this very simple principle of everyday physics, helps us to diagnose not just the character of the roasted bean, but also the very distinctive character of individual coffee grinders and their burrs.

See you then 

Cheers 

Bush & Bush Coffee Systems