I’ve realised that, when I talk about coffee, what I say isn’t backed based on a data that’s been proven or experienced myself. This frustration has lead me to actually experiment on few things, rather than pondering and hypnotising myself that my pocket science (very little of it) is correct.
One subject that I always wanted to, but had no commitment to experiment, was the importance (if there is any) of headspace between the shower screen of espresso machines and the coffee bed.
In my perfect little world, I always believed that it did, that having some head space produced higher percentage of consistent shots, without any data to back my opinion on. I started to have firm believe because of several reasons, firstly, I got so frustrated when, even though I have the same dose, grind setting, extraction time and temperature of the machine in each shot, sometimes the results fluctuated quite abit. I started to dissect and see what variables I could control and those I couldn’t, I didn’t even bother about the uncontrollable variables, but I’ve realised that I’ve never accounted head space as a variable.
Those of you who use the Nuova Simoneli Aurelia ,would know that each group head has a brass dispersion block. Which disperses the hot water into 8 different holes for even distribution of water from the group head to the coffee bed.
Now this brass block comes in 2 different thickness, 3mm and 5mm ones. I first thought 2mm difference would only make a minute difference but the results below proved me wrong.
To compare and note the difference between the 2 different sizes of the dispersion block, I wanted to gather 2 different sets of data. First being consistency, and second, rate of flow/rate of espresso extraction.
So firstly, consistency:
I wanted to see the consistency of espresso produced in grams, with different size of dispersion block, when other variables become constant. (Impossible but I’ve tried my best!)
Machine used: Aurelia T3
Time: 28 seconds
Basket: 18g VST
Coffee used: Jirmiwachu, Ethiopia SOE
If we ignore the fact that, 3mm block produced big shots that wouldn’t normally cut the mark, it is apparent from the data that, 3mm block produces fairly consistent shot in terms of weight , you have to remember that I’m just looking for consistency in weight of espresso produced.
In the other hand, the 5mm block shows that the shots produced are not even, it fluctuates rapidly after shot 4, it produces some values which could be considered as an outlier, and its hard to produce a mean value.
I’m no scientist, and I think people cleverer than me could give me a better answer, but here’s an educated guess. If we think of the shower screen as a perfume, and surface of our clothes as the coffee bed, applying perfume from a distance will cover broader surface area of your clothes evenly. On the other hand, spraying your perfume in close range will highly saturate a specific area. If we want the water to be evenly distributed amongst the coffee bed, it makes sense to me to have a gap between the surface of the coffee bed. I mean, we are talking about 93~94C water under 9bars of pressure, 2mm difference in my opinion, gives the water some space to actually distribute the water evenly.
*Note that spent coffee bed from 5mm leaves clear mark of the screw, where 3mm does not.
Lastly, I wanted to record the rate which the espresso was being extracted, by plotting a time/espresso weight graph. The reason why I wanted to obtain this data was to observe the pattern in which the flow of water through coffee bed was affected by the head space.
The result is quite interesting:
Now this graph was plotted using average values from 5 different set of shots, you can see from the result that, 5mm block produces espresso faster than the 3mm block; and that flow is maintained until 17 seconds, where the 3mm block actually overtakes 5mm block and produces more espresso at the end.
Now why does this happen? why does the flow rate gain momentum from 17 seconds on the 3mm block? Here’s my guess, the reason why 3mm block took longer to produce any espresso initially, is because the water starts to fill the basket and slowly, and saturates most of the coffee bed before extraction occurs.
Where as the 5mm block (this is just my opinion), the coffee bed is mostly pushed up against the shower screen, and when the extraction occurs, the water isn’t able to fully saturate the the coffee bed evenly and finds a path which allows extraction to occur without full saturation.
With my shallow knowledge, I wanted to know why there is an increase in flow from 17 seconds with 3mm block, and I came up with this.
Darcy’s law states that, flow rate of fluid increases as pressure increases, until it reaches a certain pressure (correct me if I’m wrong), so it would mean that there are less resistance created by the coffee bed than the 5mm block, allowing the water to flow through the coffee bed at an increasing rate, which means full saturation at the start of the extraction loosens up the coffee bed, and allows the water to flow through evenly, which also benefits even extraction throughout the coffee bed?
I really enjoyed the shots coming out form the 3mm block, not saying the 5mm block didn’t produce any good shots, but 3mm block hands down delivered the consistency, and I think we all need that in our lives.
The pocket science rant above could be all wrong, but having the data gives me boost of confidence in what I believe in, and I would love to hear other peoples thought on this. I would also love to be proven wrong, if anyone has better explanation please please comment!!
** For those of you without an Aurelia where you don’t have any interchangeable dispersion screen maybe try playing with different basket size, but keeping the dose same.