The Particle Article

Many different Industries exist throughout society these days and generally speaking, the significance of each industry to Society will generally determine the extent to which Scientific Rigour and Standard are applied. The Electrical Industry for example and its significance calls for licensing regulation and legally enforceable standards to be applied The Water reticulation and treatment Industry the same, Transport Industry Etc Etc. The hair dressing Industry? Maybe not. Very important to many, no doubt, but within the scope of a functioning society it is probably down the list, and while some hair styles might represent crimes against us, legislation has not yet been brought. The coffee industry is the same. Very few will deny the importance of the morning brew can make or break the day, and some could argue that “Doing Coffee” underpins societies social mechanisms, but still we do not require a licence to operate an espresso machine.

This can also be an indication of the scientific rigour which is applied to an industry as well. All the latest whiz bang control methods used on the latest espresso machine have been in existence and operating for over 30 years minimum. While Illy, Viani and their Ilk have done significant investigation into the science behind Coffee, because of the unlicensed nature of the industry, much of their good work does not make it to the actual coffee bar in practise every day. In an industry increasingly dominated by “Trends” and “Marketing Hype” using the social media platforms somehow disguised as Scientific fact, it is easy to see that populist hear-say becomes accepted scientific basis.

Particle size, and the buzz term “Particle Distribution”, are stunning examples of Scientific sounding terms automatically convincing us that it must be correct and I should buy whatever is associated with it.

But is Particle size really the Most dominant factor in coffee?


particle size chalk board.jpg

If you were to ask coffee professionals what is the highest principle of coffee. Many would reply particle size.  And I think that is because of the long established three T’s of coffee.

  • Time

  • Temperature

  • Turbulance

Particle size is undoubtable the king of these three principles, because their set points are all resultants of particle size. That is, Particle size determines Brew time, Brew temperature and Brew Turbulence.

But is there a higher principle?

Particle size and roast degree.

To answer this question, we set out to roast and test the same Costa Rican coffee at two different roast degrees.

The first, which we shall call “Costa light”, we roasted to a medium light roast degree between 1st and 2nd crack.

The second, we shall call “Costa Dark”, was roasted to a medium roast degree, just into second crack. 

While the two coffees come from the same green bean, and started with an identical green bean density, because of their varying roast level, they will have different roasted densities. The first crack coffee, with less time exposed to heat, will be harder than the second crack coffee that was roasted for longer. And when the bulk densities of these coffees were tested they proved this with a 10% variance in their respective roast densities.

The Costa Dark had a bulk density of 0.34, and the Costa light had a bulk density of 0.37

These results were far enough away from each other to see differences, but close enough to keep similarities whilst performing our brewing experiments.

The job of a coffee grinder, is to grind to a consistent particle size. So when these two coffees were put through the grinder at the same setting and there particles tested, regardless of their respective densities there particle distribution curve was near identical. In this way we can test the different densities of these coffees influences expressed through the same particle size.

To do this we put them through two tests. A Pour over and an Espresso extraction.

TEST 1: The pour over

To perform the pour over test we ground our two coffees on a Malkonig Ek and brewed a pour over of each coffee, on 4 different grind settings.

The brew recipe that we used for the pour over was as following.  >15grams coffee: 250 ml water.

1st pour to 45 ml. 2nd pour to 150ml. 3rd pour to 250ml.

Before you all start getting on your high horse about pour over recipes, and the pros and cons of what was used here, please reflect on the purpose of this article. My brew recipe is not important. What is important, is that we subject the same coffee particles, to the same recipe. We also had to select a method that would be the “middle ground” for various particle sizes. Because as the particle size changes, so does the drawdown time, which in effect, would change your technique. So, we had to choose a method that is acceptable to all, but favoured none.

In any case, it seemed to work as the results were largely consistent. Below is the grind setting and the resultant pour over yields

pourover table.PNG

The results highlight that even though the particle sizes were the same, as expected the coffee with the lesser density/greater porosity (costa Dark) was on average extracted more

TEST 2: Espresso

For the second Test we put the same coffees through a Synesso Hydra and ground the samples on a mythos Grinder. In this experiment we used the tapered espresso basket. For how tapered baskets make a difference to espresso extraction see the article: Impacts of Basket (Espresso Portafilter) Shape On Extraction (click here).

In this experiment we kept the temperature, grind/particle size the same for both.

Again, like the pour over comparison above, we had to find a dose and shot length that would be beneficial to both coffees to give a fair comparison. The machine was initially set up for the Costa light. The shot length TDS and Extraction Yield was recorded. The exact same particle size/grinder setting was then used for the Costa Dark. The results were tabulated below

Espresso table.PNG

In the above comparison, the signs of bean density, and its affect are much the same as in the pour over. That is, the Dark sample was over extracted more because of lower bean density/greater porosity. However, in this set of results the brew time has been added. Brew time in the pour over experiment was omitted because draw down times were sacrificed for the consistency of process in the experiment. However here shot time is shown as a variable that has a direct relationship to density. This should be a sign to you that particle size is not what is affecting these results. Density is. For higher density coffees, a finer grind is needed. For lower density coffees a courser grind. Clearly the darker coffee here is over extracted and needs a courser grind setting.

Temperature and density in the puck

For those who are avid readers of this editorial series, in the last article we discussed using the double probe thermocouple portafilter to illustrate changing temperatures in the puck. For those unfamiliar with this concept, you will need to read it here in order to understand the next two graphs. Off you go and read, see you soon!

Graph 1. is the Costa Light.

Graph 2, is the Costa Dark.

Graph 1  :  Costa light Espresso. 20.5 gram, 31 second shot time, 48 ml yield, 8.45 TDS, 19.75% extraction yield

Graph 1:  Costa light Espresso. 20.5 gram, 31 second shot time, 48 ml yield, 8.45 TDS, 19.75% extraction yield

Graph 2  :  Costa Dark Espresso. 20.5 gram, 43 second shot time, 48 ml yield, 10.13, TDS, 23.7% extraction yield

Graph 2:  Costa Dark Espresso. 20.5 gram, 43 second shot time, 48 ml yield, 10.13, TDS, 23.7% extraction yield

Comments on the Graphs:

You will notice that on the Costa Dark graph the temperature difference at its closest point is 12 degrees, and on the Costa light graph it’s only 9 degrees. Despite the fact that these coffees were both run at the same particle size, because of the lower density of the Costa Dark coffee, in comparison, it needs a courser grind setting for espresso. Once we deliver a courser grind setting the extraction time will shorten, the TDS and yield will lower and the temperature will stabilise, and become closer together. Funnily enough even though the overall temperature of the costa dark was lower it extracted higher. This should tell you that density determines extraction temperature.

There is much more to discuss when it comes to density in the espresso puck. And there is much more information in these two graphs but it’s scope starts to delve into density, a topic we shall look more at in up-coming articles.



Particle Size is often the focus when brewing. When we look at grinders, we look at that which gives the narrowest particle distribution curve. But at no point do we look at density of the particles, and in reality, this is what we are brewing.

Roasters know the importance of density when roasting, but strangely enough, abandon all acknowledgement of it when it comes out of the drum into the cooling pan.

Particle size is not the highest truth in Coffee. Particle size only starts when coffee is ground. Density however starts at the green bean stage and continues it’s influence all the way to the finished coffee. Density is the highest principle in coffee. Its influence determines time, temperature and turbulence through the expression of particle size. 

This is a relatively simple article, and too be honest there are many practitioners who will read this article and simply say Of course density affects particle size. But if that’s the case why not test it?

If you spend all your efforts to roast a coffee to perfection, but don’t test its density, how do you know what to expect when it comes to the espresso basket? Perhaps it’s time to move from the most fundamental of espresso analogy the old “pull a shot and see what happens” and toward more scientific analysis of density.

In order to make our two Costa Rican coffees run time the same for an espresso shot. The grind setting was made 2 notches courser for the Costa dark. When this was done the shot speed and volume was the same. However because of it’s lower density, the Costa Dark still extracted more coffee. Resulting in a higher TDS and Yield.

If you are to roast two or more coffees and put them together in a blend, unless they are of the same density they will not extract at the same time or temperature. And all that flavour you have spent so much time cultivating will be lost. With the resulting coffees extracting as an average of the two rather than a peak of both. Yet even with this, most roasters I know still don’t measure roast density.

For us at Bush and Bush Coffee Systems and the God's Honest Truth, Density is the highest principle and it must be respected as much as flavour, because without respecting density there will be no flavour.

Not only is particle sieve analysis impractical with regards to coffee service, but just because you have sieved coffee is not to say that the particles in any one distribution curve are identical. Because no two particles are actually the same, they just happened to fall through the sieve at the right angle and be caught there. But more importantly sieves don’t represent density.

The simple fact is that you can’t make a coffee from 1 particle. A known distribution of particles is also unhelpful to us. To achieve anything, we must treat all of the particles together, and it is their interaction with each other as an evolving mass which is important to espresso extraction.




Bush and Bush Coffee Systems.

Eric Chung