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Pre-infusion and it's impact on espresso

This week we will take a look at pre-infusion (pre-brew) and it's result in the espresso. Pre-infusion can come in many forms, from soft infusion to hard infusion to variable pressures and different lengths of time. We will be testing multiple pre-infusion techniques over the coming weeks but first, let's define some of the different pre-infusion variables that are commonly used in the coffee industry.
Last week we talked about brewing espresso at different pressures apart from the default 9 bars that most machines come with. This time we will take a look at pre-infusion (pre-brew) and it's result in the espresso. Pre-infusion can come in many forms, from soft infusion to hard infusion to variable pressures and different lengths of time. We will be testing multiple pre-infusion techniques over the coming weeks but first, let's define some of the different pre-infusion variables that are commonly used in the coffee industry. If you need some clarification on brew pressures and water flow rates, head on over here :

- Soft/Progressive Infusion : A method where x amount of water is slowly and steadily introduced to the coffee puck over y length of time to build up to and maintained at z calibrated pressure for y length of time before brewing at full pressure.
- Hard Infusion : A method where amount of water is quickly dumped into the coffee puck at full brew pressure and then stopped for y length of time before brewing at full pressure.
- Line Pressure Infusion : A method where x amount of water is introduced up to the same pressure as the water inlet pipe and maintained for y length of time before brewing at full pressure.

As can be inferred from the above definitions, there are 3 main variables regardless of the pre-infusion technique. This would be the amount of water introduced to the puck(x), the pressure of the water in the grouphead(z) and the length of time of the pre-infusion technique(y). All these variables along with the different pre-infusion method may have different effects on the resulting espresso.

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. Since many schools of thought promote pre-infusion as beneficial to espresso, resulting in more even and consistent 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.


Colombia, Neiver Samboni
Process : Fully Washed
Variety: Caturra
Roast Age: 12 days from roast date 
Agtron : 63 (Whole Bean), 79(Ground)


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 ( 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, 32s 9 bar full pressure, Total brew time: 32s
                   Water temperature : 90.3°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 :

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


9 Bar + 0s pre-infusion9 Bar + 5s pre-infusion
14g water / 0.5 bar
9 Bar + 10s pre-infusion
28g water / 2 bar
9 Bar + 15s pre-infusion
42g water / 3 bar
9 Bar + 30s pre-infusion
84g water / 3 bar
Average EY%21.85%20.87%20.26%20.39%20.88%
EY% Range5.48%3.33%3.67%1.48%0.50%
Standard Deviation0.0154349570.0110293750.0102973350.004455820.001548332


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.

Based on the data that we obtained, it appears that brewing at 9 bars wthout any pre-infusion yields a higher extraction percentage than with any form of pre-infusion. In fact, since the P-Value of the extractions at 10s and 15s pre-infusion are less than 0.05, we can reject the null hypothesis for these two scenarios. Espressos brewed with soft pre-infusion methods for 10s or 15s actually lead to lower extraction percentages. Espressos brewed with 5s of soft pre-infusion or 30s of soft pre-infusion however, have P-values above 0.05 and we cannot reject the null hypothesis. There is no signficant evidence to show that pre-infusion leads to higher extraction percentages.

Although the numbers from the results of the experiment do not support pre-infusion, we have to interject with a different opinion. As we tasted the different espressos that came out of each pre-infusion method, there were significant differences in taste as compared to the original espresso brewed with zero pre-infusion.

5s pre-infusion : brighter acidity and smoother mouthfeel, better structure and integration of flavours.
10s pre-infusion : bright acidity, heavier body with tamed flavours and a shorter finish
15s pre-infusion : muted acidity, rounded body with increased sweetness but less flavour clarity.
30s pre-infusion : sharp acidity, heavy body, astringent and dry finish

Although the numbers tell a different story, we can pin-point a number of flaws within the experiment itself. First of all, the calibration was based off the initial brewig with zero pre-infusion. The subseuent shots with pre-infusion probably deserve to be re-calibrated to allow for better optimisation of the results for each indiviual pre-infusion method. Secondly, a noticable impact of the pre-infusion is the decreasing EY% range when longer pre-infusions are used. This could mean that pre-infusion techniques may be not produce higher extraction percentages but may help with consistency and reducing margin of error. As shown from our taste observations, pre-infusion may also be offsetting bad/improper espresso extraction technique as the shots sem to be tasting better even though the extraction percentages are lower. This may mean that the original calibration may not be entirely optimal or that there was overextraction from channelling which might have resulted in the higher numbers but less delicious taste. This is also supported by the results table which contain quite a number of outliers within the data and a very big EY% range. This could be a result of improper espresso preparation and brewing technique from shot-to-shot. Therefore, it may be entirely possible that the pre-infusion techniques are in fact helping to minimise the margin of error and off-set any prior inconsistencies or mistakes during the shot preparation. This is in line with the teachings of many other schools of thought within the coffee industry.

There is definitely still much room for exploration and improvement within this topic and we will be carrying out further experiments and research to improve upon and clarify some of the issues and flaws brought up from this round of experiments. Future experiments will include individual calibrations for each pre-infusion method, testing out with different coffees and with different full brewing pressures!

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