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#139

Putting it together: Pre-infusion on a low overall brew pressure

We've tested different brewing pressures, we've tested different pre-infusion timings and pressures; now it's time to put them all together! These days, we pull our espressos with a pre-infusion between 9-16s and full brew pressure at 7 Bars at the shop. Does pre-infusion at this lower brew pressure setting make a difference in extraction percentages? Or will it prove to be equally insignificant as compared to the standard brew pressures?
We've tested different brewing pressures, we've tested different pre-infusion timings and pressures; now it's time to put them all together! These days, we pull our espressos with a pre-infusion between 9-16s and full brew pressure at 7 Bars at the shop. Does pre-infusion at this lower brew pressure setting make a difference in extraction percentages? Or will it prove to be equally insignificant as compared to the standard brew pressures from our previous experiments?

For this week's experiment, we will be using the soft infusion method and testing out different amounts of water (x), time (y) and pressure (z) against a shot with zero pre-infusion at a full brew pressure of 7 Bars. Since many schools of thought promote pre-infusion as beneficial to espresso, resulting in more even and consistent extractions aka higher extractions, we will set that as the Null Hypothesis this week.

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 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 : Natural Sundried
Variety: Heirloom
Roast Age: 10 days from roast date 
Agtron : 58.5 (Whole Bean), 78(Ground)

Equipment

Grinder : Nuova Simonelli Mythos One Climapro Grinder
Distribution Technique : Stockfleth Method
Tamper : PergTamp 58.5mm diameter
Portafilter Basket : VST 20g
Espresso Machine : Slayer V3
Brew Pressure : (Controlled Variable) / Initial Calibration with 7 Bars / Measured using Scace II
Grouphead Flow Rate : 117g water / 10s ( Full Brew Pressure ), 28g water / 10s (Pre-infusion Brew Pressure set to maximum 3 bar)
Coffee Refractometer : VST Coffee III
Calibration : 20g Dose, 40g Yield, Shot : 0s Pre-infusion, 24s 7 bar full pressure, Total brew time: 24s
                   Water temperature : 92.3°C. Final EY 21%.
*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:

Soft pre-infusion results in higher extraction percentages than espressos brewed without any pre-infusion.

Methodology :

(Steps)
1. Calibrate the coffee with 7 bars brewing pressure and 0s pre-infusion.
2. Pull 10 espresso shots using the calibrated settings.
3. Calibrate the coffee with 7 bars brewing pressure and 5s pre-infusion.
4. Pull 10 espresso shots with 5s pre-infusion before ramping up to full 7 bar brewing pressure.
5. Calibrate the coffee with 7 bars brewing pressure and 10s pre-infusion.
6. Pull 10 espresso shots with 10s pre-infusion before ramping up to full 7 bar brewing pressure.
7. Calibrate the coffee with 7 bars brewing pressure and 15s pre-infusion.
8. Pull 10 espresso shots with 15s pre-infusion before ramping up to full 7 bar brewing pressure.
9. Calibrate the coffee with 7 bars brewing pressure and 30s pre-infusion.
10. Pull 10 espresso shots with 30s pre-infusion before ramping up to full 7 bar brewing pressure.
11. Wait for espresso shots to cool to room temperature.
12. Measure Total Dissolved Solids (TDS%) and Extraction % for each sample.

Results:

7 Bar + 0s pre-infusion7 Bar + 5s pre-infusion7 Bar + 10s pre-infusion7 Bar + 15s pre-infusion7 Bar + 30s pre-infusion
YieldTDSEYYieldTDSEYYieldTDSEYYieldTDSEYYieldTDSEY
139.709.74%20.04%39.609.94%20.40%39.8010.05%20.72%39.909.42%19.47%40.5011.06%23.21%
239.809.60%19.80%40.009.72%20.15%40.009.93%20.58%39.9010.39%21.48%40.2010.73%22.35%
340.109.70%20.15%40.009.63%19.96%39.909.55%19.74%39.709.84%20.24%40.2011.35%23.64%
440.1010.00%20.78%39.709.54%19.62%40.009.60%19.90%39.609.81%20.13%39.8010.93%22.54%
539.809.78%20.17%40.309.49%19.82%40.009.54%19.77%40.2010.17%21.18%40.209.80%20.41%
640.109.41%19.55%40.309.71%20.28%40.009.91%20.54%40.409.84%20.60%40.009.90%20.52%
740.1010.00%20.78%40.209.60%20.00%39.909.64%19.93%39.9010.26%21.21%40.1011.10%23.06%
840.209.84%20.50%39.709.58%19.71%39.9010.70%22.12%40.0011.00%22.80%40.0010.35%21.45%
940.109.75%20.26%40.009.68%20.06%39.909.69%20.03%40.5010.67%22.39%39.709.75%20.06%
1040.309.91%20.69%40.109.69%20.13%39.609.95%20.42%39.709.91%20.38%39.5011.78%24.11%
Average EY%20.27%20.01%20.38%20.99%22.14%
EY% Range1.23%0.78%2.38%3.33%4.05%
Standard Deviation0.0041715440.0024544970.0071080940.0103801950.014432159
Variance1.74018E-056.02456E-060.0000505250.0001077480.000208287
P-Value10.1118862690.6984318660.0661568330.002608626
10.1558656030.0160860010.001141429
10.1429980480.00417604
10.057816059



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.

In our previous experiment with pre-infusion, we pin-pointed a number of areas for improvement in our experiment. 
1. There should be individual calibrations for each pre-infusion timing to allow for better optimisation of the results of each individual method.
2. Testing with tasting notes to check for taste differences between the different pre-infusion timings.

For this round of testing, we calibrated each round of pre-infusion methods and wrote down the tasting notes for each method before proceeding with puling the 10 shots and measuring extractions.

The tasting notes are as follows:

0s pre-infusion: Bright Acidity, Sweet, Fruity, Balanced.
5s pre-infusion: Bright Acidity, Stronger Aromas, Sweet and Fruity, Balanced.
10s pre-infusion: Intense Sweetness, Stronger Aromas, slight Dryness in the finish
15s pre-infusion: Muted Acidity, Slightly Flat, Less Flavour Clarity Balanced,
30s pre-infusion: Heavy Body, Strong Intensity, Muted Acidity and Flavour Clarity, Slightly Dull

Based on the data observed, it seems like lowering the full brew pressure to 7 Bars has indeed affected the results quite a bit. The EY % Range has actually increased with longer timings 10s and above whereas in our previous experiment here with 9 Bars it had a declining trend instead. The same can be said for the variance and standard deviation. Whereas pre-infusions of 10s and more in the previous experiment showed significant differences (less extraction), this time pre-infusions of 10s or more have not shown any significant dfferences up till 30s since the P Value is more than 0.05. With that in mind, we cannot reject the Null Hypothesis. There is no significant evidence to show that pre-infusion leads to more even (higher) extraction percentages. 

The only method with any significant difference is brewing with 30s of pre-infusion. However, as we have seen in both experiments, it does not leave you with a very desirable tasting cup. In this particular experiment, there is in fact, even higher variability and inconsistency within the EY%, proving a detrimental effect. From a theoretical standpoint, it is quite difficult to provide an explanation for the difference in results because of the full brew pressure. A pure guess may be that pre-infusion was designed to be helpful in reducing inconsistency only at 9 Bars which is the pressure most machines come with and not for lower pressures which might cause less turbulence in the puck, reducing the need for pre-infusion.

There is definitely still much room for exploration within this topic as we can hardly begin to understand the physics and interaction of the coffee with the differing pressures without procuring some special equipment like transparent portafilters and the like. Hopefully future experiments may shed more light on the matter. For the time being, we will stick with a lower brewing pressure of 7 Bars since that has shown to provide significantly better results. As for pre-infusion, we will keep the timing to a maximum of 10s to prevent excessive variability and maintain a consistently good tasting shot.

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