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Espresso Brewing Pressure: Is 9 Bars really optimal?

Almost all semi-automatic espresso machines are calibrated and set by default to brew at roughly 9 bars of pressure. In many machines, adjusting the brew pressure is either not possible or a very difficult task that should only be performed by a technician familiar with the machine's inner workings. Lucky for us, we use the Slayer espresso machine, one of the few espresso machines out there that allows for easy tweaking of the brew pressure. This week, we will be setting out to test the impact of brewing pressure on the resulting espresso and it's extraction percentage.

Brew pressure is a relatively advanced topic, hence before we begin it's probably best to clear up some ideas with regards to brew pressure.

1. Many espresso machines have a gauge that shows the pressure of the machine. This is the pressure of the water pump in the machine, it is not an exact representation of the brew pressure that happens in the grouphead.

2. To measure the pressure at the grouphead, we need to use a tool called the Scace. It works similar to a regular portafilter but instead of a regular hole for the basket, there is a blind basket with a tiny hole for the water to pass through. The sensors are then able to pick up how hard the water is being forced through the hole (aka pressure).


(The aforementioned Scace Tool)

3. The resistance provided by the blind portafilter in the scace is not representative of the resistance provided by the coffee puck in different brewing conditions. Diffferent Doses, basket sizes, grind size, distribution and quality are all factors contributing to the coffee puck resistance. As a result, the calibrated pressure from the scace may not give the same brewing pressure as when brewing with the actual coffee. However, this is probably the closest we can achieve to getting a good and consistent read on the pressure inside the grouphead. Most espresso machines also have gauges to compensate for difference in resistance and pressures to reach the calibrated pressure so it is relatively accurate.

4. Pressure is NOT the same as flow rate. The flow rate of a grouphead can vary at different speeds but still result in the same brew pressure, the reasons can vary from pipe/tubing sizes to showerhead resistance to water chemistry etc etc. A quote from Prima Coffee "Simply put, pressure is the continuous application of physical force--in this case the pump--and flow is the actual movement of water". Anyone looking to re-create this experiment or put into practice different brewing pressures at their shop must measure and account for their water flow rate as it will have an impact on the results of the experiment/espresso. Coffee Puck resistance reduces over time as some of the coffee gets dissolved over time during brewing. Many machiens will increase the flow rate during this time to compensate for the decreasing pressure due to reducind resistance. The measured flow rate is the maximum flow rate of the machine up to a calibrated pressure.

If you are unfamiliar with the workings of expresso extraction percentages and the use of the coffee refractometer, you can always read our previous article over here : http://compoundcoffee.com/experiments/9_King-of-Distribution-Techniques to get a quick overview and understanding of espresso extraction percentages.

Now let's dive straight into the experiment!

Disclaimer : We are by no means statistical / math / science experts. This is just a simple experiment carried out to give us an idea of the performance of each method only. People are more than welcome to give their feedback on how we can improve and carry out a more academically approved experiment / report.

Coffee 

Ethiopia, Sidamo Guji
Process : Sundried Naturals
Variety: Ethiopian Heirloom
Roast Age: 14 days from roast date 
Agtron : 73 (Whole Bean), 92(Ground)

Equipment

Grinder : Nuova Simonelli Mythos One Climapro Grinder
Distribution Technique : Stockfleth Method
Tamper : Levy Tamp 58.5mm diameter
Portafilter Basket : VST 20g
Espresso Machine : Slayer V3
Brew Pressure : (Controlled Variable) / Initial Calibration with 9 Bars / Measured using Scace II
Grouphead Flow Rate : 117g water / 10s
Coffee Refractometer : VST Coffee III
Calibration : 20g Dose, 40g Yield, Shot : 0s Pre-infusion, 28s 9 bar full pressure, Total brew time: 28s
                   Water temperature : 91.3°C. Final EY 19.75%.
*Optimal Calibration is set at the furthest possible point in extraction whereby minimal to zero astringency or notes of over-extraction is detected in the cup.
Dose Tolerance : 20g ± 0.1g
Shot Tolerance : 40.0g ±  0.5g

Null Hypothesis 1:

Brewing at 9 Bars of pressure should provide the the best results in optimal extraction percentage.

Methodology :

(Steps)
1. Calibrate the coffee with 9 bars brewing pressure.
2. Pull 10 espresso shots using the calibrated settings.
3. Calibrate the coffee with 8 bars brewing pressure.
4. Pull 10 espresso shots using the calibrated settings.
5. Calibrate the coffee with 7 bars brewing pressure.
6. Pull 10 espresso shots using the calibrated settings.
7. Calibrate the coffee with 6 bars brewing pressure.
8. Pull 10 espresso shots using the calibrated settings.
9. Calibrate the coffee with 5 bars brewing pressure.
10. Pull 10 espresso shots using the calibrated settings.
11. Wait for espresso shots to cool to room temperature.
12. Measure Total Dissolved Solids (TDS%) and Extraction % for each sample.

Results:


9 Bar8 Bar7 Bar6 Bar5 Bar
YieldTDSEYYieldTDSEYYieldTDSEYYieldTDSEYYieldTDSEY
139.909.52%19.68%39.709.73%20.01%40.2010.36%21.58%39.509.85%20.16%40.009.36%19.40%
239.709.69%19.93%39.609.32%19.12%40.5010.16%21.32%40.209.77%20.35%40.308.78%18.33%
339.909.19%19.00%40.509.67%20.29%40.509.78%20.52%40.309.65%20.15%39.609.54%19.57%
440.009.61%19.92%40.509.51%19.96%40.109.73%20.22%40.309.71%20.28%40.009.83%20.37%
540.509.83%20.63%40.109.44%19.61%40.109.72%20.20%39.809.72%20.04%40.509.38%19.68%
639.909.22%19.06%39.909.89%20.45%39.709.97%20.51%40.009.24%19.15%40.209.53%19.85%
740.008.67%17.97%40.5010.20%21.40%40.109.88%20.53%40.209.57%19.93%40.409.53%19.95%
840.008.10%16.79%40.309.63%20.11%40.109.57%19.88%39.909.16%18.94%40.509.37%19.66%
940.109.76%20.28%40.3010.22%21.34%39.609.46%19.41%40.0010.20%21.14%40.509.16%19.22%
1040.209.18%19.12%40.3010.25%21.40%39.809.81%20.23%40.309.54%19.92%39.809.68%19.96%
Average EY%19.24%20.37%20.44%20.01%19.60%
EY% Range3.842.282.172.202.04
Standard Deviation1.1482140720.7874635790.6340346990.6152903560.549311691
Variance1.3183955560.6200988890.4020.3785822220.301743333
P-Value10.0206535980.0116742230.0837243710.386200229
10.8268637390.2665955910.021952717
10.1377512290.005399478
10.136294184


Conclusion:


For those who might be unfamiliar with statistics, you can read the conclusion of our previous article to get a quick explanation on statistical significance here: http://compoundcoffee.com/experiments/9_King-of-Distribution-Techniques

Based on the data that we found, brewing the espresso at 8 bars and 7 bars of pressure both yielded significantly higher extraction percentages than the default 9 bars. Since both P-values when compared to 9 bars are less than 0.05, we can safely reject the null hypotheses and conclude that brewing at 8 and 7 bars returns siginificantly higher extraction percentages as compared to brewing at 9 bars.

Another trend we noticed is a steady decrease in the difference in range when reducing the brewing pressures from 9 bars. This could mean that lower brewing pressures could possibly lower the variance in coffee extractions which would also mean better shot consistency.

Looking into the future, this experiment would ideally be carried out across different grinders and coffee origins/roast levels to determine if the effect of lower brew pressures are carried over across different situations or only particular to certain variables. As a note of reference, a previous experiment carried out by us showed that 6 bars was in fact, the more preferrable brewing pressure in terms of final extraction% and consistency. This was done however, with the EK43 Grinder on the Slayer Espresso Machine. Therefore, it is highly likely that different grinders will have different relationships with brewing pressures and their resulting extractions. We have no doubt coffees of different roast levels would have an equal impact on the experiment results.

Next week, we will look at the effects on pre-infusion / pre-brew within the different brewing pressures and how they impact the final brewing as well.

Last but not least, if you like the content that we are giving out don't forget to subscribe and support us by buying some awesome coffee here

7 Comments

    • Avatar
      Chas Kane
      Jul 5, 2018

      I love that you posted the stats. Your conclusion is fascinating, as every coffee shop I've been to uses 9 bars. Are you planning on expanding this work at all? Small sample size and diversity may mean that 7-8 bars is best for this coffee, or for this machine, grind size, etc.However, this could be big -- it's easy for people to simply copy what's already in place when changing is costly (few know how to change the pressure in their machines).

      • Avatar
        Kenneth Ong
        Jul 7, 2018

        Hey Chas,I'm glad you enjoyed the article :) There is loads more to research in this area of course. We've run a number of other experiments but haven't published just because it is extremely time-consuming to write about them.You are correct to say that there are other variables that contribute to the optimal brew pressure, the key variable among them being distribution of grinds in the portafilter. However, as the majority of us do not usually distribute 100% evenly, I would say this findings are applicable to many cases regardless of coffee, machine etc.. The reason why I say this is because we are not the only ones who have discovered that the optimal espresso brew pressure lies below 9 bars, where optimal is defined as ideal for even and higher extractions instead of speed.Other coffee professionals like Scott Rao, Matt Perger from St. Ali, Socratic Coffee and numerous others have had similar findings within their own research. My motto is always to try it for yourself and you can decide from your own tests.To greater coffee experiences, Kenneth

    • Avatar
      Cristian
      Nov 10, 2018

      Hi! Very interesting and nice work! I have a question and I am sorry if the answer is in the text. Did you adjust the grind setting when lowering the pressure? I.e., adjust the grinder such that the extraction takes 28 s as for 9 bars. Thanks!

      • Avatar
        Kenneth
        Nov 15, 2018

        Hi Cristian, No we did not change the grind setting throughout the experiment, only the brew pressure.Cheers, Kenneth

        • Avatar
          Michael Betz
          Dec 6, 2018

          Thanks for the excellent experimental work!Provided you did not adjust the grind, how did you manage to keep the extraction times constant?

    • Avatar
      Sherman
      Dec 6, 2018

      This is a flawed series of tests on so many levels and there is absolutely no value that anyone can extract from these results.

    • Avatar
      Ian
      Dec 8, 2018

      The authors clearly gave a disclaimer about the validity of the experiment, at the beginning of their report, and stated that they are prepared to receive criticism aimed at improving the methodology and validity of the results.That having been said, and without any comment on the methodology, I am not aware that there is a direct correlation between higher TDS and greater espresso quality. The most significant deficiency in the experiment is that there was no tasting component (which they admit is subjective, but that is the nature of all taste related experiences). I am not concerned about the TDS if the flavor at 9 bar is better than the flavor at 6 or 7 bar. A previous experiment about ‘DIstribution’, with a link on this page, mentions that, of all of the soluble components in ground coffee, only about 2/3 is desirable. If I understand this statement, which is unreferenced, higher TDS may, in fact, produce a less desirable coffee. Without being able to correlate TDS with flavor, the experiment has little value, regardless of the quality of the results. Why adjust your brew pressure to 6 bar, if it could result in an unpleasantly bitter, over-extracted brew? If, however, this page results in productive discussion, there may be value there.

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