The pressure applied when tamping has a slight influence on espresso extraction. We wanted to scientifically prove this finding from everyday practice. With the support of a food technologist, we set to work extracting 150 espressos, measuring them, and varying the tamping pressure. In this article, we present the results.
What is tamping?
Tamping is the pressing of the ground coffee powder into the portafilter. Tamping involves using a tamper, which consists of a handle and a metal base that is pressed into the portafilter like a piston. The metal base corresponds to the inner diameter of the portafilter. If the gap between the portafilter wall and the tamper diameter is too large, some of the coffee powder will not be pressed sufficiently.
Pressing the ground material down allows for even water distribution. The brewing water should encounter the same resistance at all contact points on the ground material's surface. If the ground material is not sufficiently compacted in some places, or if there are air pockets below the contact point, there is a risk of channeling . This can be prevented by careful preparation of the ground material, vertical compaction, and even tamping.
Instructions: How to make espresso step by step (articles and videos).
Three classic tampers with a 58mm diameter.
Research question: What influence does contact pressure have when tamping?
Two aspects were investigated. First, we wanted to determine whether changing the pressure during tamping influenced the expiration time. Second, we investigated whether the pressure had an effect on the extraction yield and thus on the concentration of dissolved coffee particles in the cup.
In order to obtain the most precise result possible, work steps and methodology were precisely defined.
material
Rancilio Specialty, a commercial espresso machine with a precise flow meter. However, every espresso was measured with the Acai Luna.
Etzinger etzMax, espresso grinder with integrated scale. However, all grinds were reweighed and repeated if any deviations occurred (see methodology).
Puqpress, automatic tamping machines for the catering industry. Tamping pressure adjustable.
VST refractometer, for measuring TDS in percent, i.e. the concentration of dissolved particles in the beverage in relation to water.
VST syringe filter to filter solids from coffee solution and measure only the dissolved particles with the refractometer.
VST syringe
Etzmax espresso grinder and Puqpress
Methods
The espresso grinder is set to an output of 18.5 g. Grinding results of 18.4 g to 18.6 g are used.
The Rancilio Specialty Espresso machine is programmed to dispense 45 ml of water and brew at a temperature of 93 °C.
The appropriate amount of coffee is ground directly into the portafilter. The coffee is then leveled with the fingers and tamped with the Puqpress. The portafilter is clamped into the machine, and extraction begins.
The amount of espresso in the cup is measured using a scale. After extraction, the portafilter is removed and tapped out.
A rinse shot is taken from the espresso machine.
After extraction, the espressos are set aside to cool. Once cooled, they are stirred and the TDS value is measured with a refractometer.
The espresso is poured into the refractometer using the syringe via the syringe filter.
This cycle is repeated 50 times each for the tamping pressures 10 kg, 15 kg, 20 kg.
The data for the throughput time and the TDS values are evaluated using a one-way ANOVA with a fixed error probability of 5%.
Results throughput time and tamp pressure
No significant difference was found in the variance analysis of the throughput time. In other words, regardless of whether 10 kg, 15 kg, or 20 kg was applied, the results in our measurements did not vary noticeably due to the pressing pressure.
Effect of tamp pressure on extraction time.
| 10 kg pressure | 15 kg pressure | 20 kg pressure |
| 26 | 20 | 27 |
| 21 | 23 | 25 |
| 23 | 26 | 29 |
| 24 | 27 | 28 |
| 23 | 25 | 24 |
| 24 | 26 | 27 |
| 25 | 26 | 23 |
| 24 | 29 | 25 |
| 26 | 20 | 26 |
| 31 | 24 | 27 |
| 25 | 23 | 25 |
| 24 | 23 | 21 |
| 25 | 22 | 26 |
| 27 | 23 | 21 |
| 24 | 30 | 24 |
| 24 | 24 | 21 |
| 25 | 22 | 24 |
| 27 | 21 | 25 |
| 23 | 21 | 28 |
| 31 | 24 | 22 |
| 25 | 24 | 22 |
| 24 | 25 | 24 |
| 22 | 30 | 23 |
| 27 | 20 | 25 |
| 22 | 21 | 22 |
| 25 | 23 | 25 |
| 25 | 21 | 21 |
| 24 | 21 | 22 |
| 24 | 27 | 21 |
| 23 | 22 | 24 |
| 27 | 23 | 22 |
| 21 | 22 | 30 |
| 24 | 24 | 21 |
| 25 | 21 | 25 |
| 24 | 25 | 29 |
| 25 | 24 | 26 |
| 23 | 24 | 23 |
| 23 | 23 | 24 |
| 23 | 23 | 24 |
| 22 | 23 | 23 |
| 21 | 22 | 24 |
| 26 | 24 | 26 |
| 23 | 25 | 24 |
| 25 | 22 | 20 |
| 24 | 23 | 27 |
| 20 | 23 | 22 |
| 24 | 26 | 25 |
| 24 | 22 | 26 |
| 22 | 21 | 22 |
| 23 | 24 | 20 |
| Groups | Count | Sum | Average | Variance |
| Column 1 | 50 | 1212 | 24.24 | 4.51265306 |
| Column 2 | 50 | 1177 | 23.54 | 5.60040816 |
| Column 3 | 50 | 1210 | 24.2 | 6.08163265 |
| Source of Variation | SS | df | MS | F | P-value | F crit |
| Between Groups | 15.4533333 | 2 | 7.72666667 | 1.43133301 | 0.242301 | 3.05762065 |
| Within Groups | 793.54 | 147 | 5.39823129 | |||
| Total | 808.993333 | 149 |
| Fk(0.05) | 3.05762065 |
The test statistic F is smaller than the critical F-value, thus the result is not significant. The actual P-value exceeds the specified error level of 5%. Thus, no significant difference can be determined between the groups.
Results TDS values and tamp pressure
The variance analysis of the TDS values also revealed no significant difference. The concentration of dissolved coffee particles was not significantly affected by varying contact pressure.
Effect of tamp pressure on dissolved particle concentration.
| 10 kg pressing | 15 kg press | 20 kg pressing |
| 8.53 | 7.77 | 8.15 |
| 7.99 | 7.94 | 8.15 |
| 8.03 | 8.02 | 8.16 |
| 8.13 | 8.03 | 8.27 |
| 8.01 | 8.08 | 8.2 |
| 8.01 | 8.06 | 8.08 |
| 8.1 | 8.07 | 8.08 |
| 8.05 | 8.26 | 8.01 |
| 8.2 | 8.12 | 7.96 |
| 8.24 | 7.96 | 8.03 |
| 8.17 | 7.98 | 7.99 |
| 8.15 | 7.99 | 8.03 |
| 7.91 | 7.85 | 8.11 |
| 8.09 | 8.02 | 8.02 |
| 8.09 | 8.88 | 8.02 |
| 8.09 | 8.12 | 8.01 |
| 8.11 | 8.11 | 7.93 |
| 8.15 | 7.99 | 8.04 |
| 7.96 | 7.99 | 8.04 |
| 8.3 | 8.05 | 7.99 |
| 8.25 | 8.15 | 7.99 |
| 7.99 | 8.2 | 8 |
| 8.07 | 8.28 | 7.99 |
| 7.99 | 7.81 | 8 |
| 7.81 | 7.84 | 7.97 |
| 7.8 | 7.92 | 8.01 |
| 7.93 | 7.9 | 8.01 |
| 7.94 | 7.87 | 7.66 |
| 7.86 | 7.97 | 7.8 |
| 7.93 | 7.88 | 7.89 |
| 8.06 | 8.02 | 7.99 |
| 8.08 | 8.02 | 8.02 |
| 7.99 | 7.95 | 7.92 |
| 8.04 | 7.7 | 7.99 |
| 8.05 | 7.89 | * |
| 8.13 | 8.01 | * |
| 8.2 | 8.11 | * |
| 8.19 | 7.95 | 7.91 |
| 7.98 | 8.02 | 7.96 |
| 7.94 | 8.07 | 7.7 |
| 7.82 | 7.94 | 8.01 |
| 8.08 | 8.04 | 8.08 |
| 8.08 | 8.05 | 7.92 |
| 8.07 | 7.96 | 7.83 |
| 8.06 | 7.97 | 8.03 |
| 7.88 | 7.78 | 7.97 |
| 7.88 | 8.01 | 7.97 |
| 7.93 | 7.81 | 7.91 |
| 7.97 | 7.73 | 7.65 |
| 7.96 | 7.85 | 7.69 |
TDS values. *Measurements are missing in these fields
| Groups | Count | Sum | Average | Variance |
| Column 1 | 50 | 402.27 | 8.0454 | 0.01824167 |
| Column 2 | 50 | 399.99 | 7.9998 | 0.03163465 |
| Column 3 | 47 | 375.14 | 7.98170213 | 0.01635356 |
| Source of Variation | SS | df | MS | F | P-value | F crit |
| Between Groups | 0.10546284 | 2 | 0.05273142 | 2.37573217 | 0.0965805 | 3.058928 |
| Within Groups | 3.19620383 | 144 | 0.02219586 | |||
| Total | 3.30166667 | 146 |
| Fk 0.05 | 3.058928 |
| Fk 0.1 | 2.33979948 |
The test statistic F is smaller than the critical F-value, thus the result is not significant. The actual P-value exceeds the specified error level of 5%. Thus, no significant difference can be determined between the groups.
Raw data and measured values of the tamping test
The raw data and further information can be found in the Excel spreadsheet. If you have any further interesting insights, we'd love to hear the results.
Download the results of the tamping project
When analyzing the data, a linear correlation was found between the extraction time and the determined TDS values between the following data sets, which was determined using the Bravais-Pearson correlation coefficient:
- Tamp pressure 10 kg: Extraction time and TDS: weak positive correlation -> r=0.5068
- Tamp pressure 15 kg: Extraction time and TDS: strong positive correlation -> r= 0.6854
- Tamp pressure 20 kg: Extraction time and TDS: weak positive correlation -> r= 0.5423
where: |r|= 1 -> perfectly linear correlation
|r|= 0 -> no linear correlation
Contact pressure and taste and additional information
The taste of the espresso was not examined in this test, so no conclusions can be drawn. However, based on the test findings, we interpret that varying the pressure in the range we examined has no, or only a marginal, influence on the taste.
Based on our test, we also can't comment on how pressure less than 10 kilograms affects extraction. Based on research, partner reviews, and our own experience, we recommend tamping with at least 6 kilograms of pressure.
discussion
When investigating whether tamping pressure affects the extraction time of espressos, no effect was found. However, this does not mean that no effect exists; it simply means that no effect was detected in the test model used. Confounding factors include, for example, inaccuracies in preparation and the insufficient precision of the testing machines used.
The equipment used is designed for professional use in the catering industry and not precise laboratory equipment.
However, in practice, there are far greater interference factors in the preparation of an espresso than in the test procedure used.
In practice, tamp pressure is unlikely to have a significant impact on extraction, as there are numerous inconsistent parameters influencing the extraction process. Tamp pressure is merely one of many parameters.
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The tamping test was conducted by Michel Aeschbacher in collaboration with a food technologist who wishes to remain anonymous. You deserve our thanks!
We are open to further research partnerships and are happy to use our materials and expertise to develop insights into the topic of coffee.
