The tamping pressure when tamping has little influence on the extraction of espresso. We wanted to scientifically prove this insight from daily practice. To this end, with the support of a food technologist, we set to work and extracted 150 espressos, measured them, and varied the tamping pressure. In this article, we present the results.
What is Tamping?
Tamping is the process of pressing the ground coffee powder in the portafilter. For "tamping," a tamper is used, consisting of a handle and a metal base, which is pressed like a piston into the portafilter. The metal base matches the inner diameter of the sieve. If the gap between the sieve wall and the tamper diameter is too large, part of the coffee powder will not be sufficiently compressed.
Pressing the ground coffee allows for even water distribution. The brewing water should encounter the same resistance from the surface of the ground coffee at all points of contact. If the ground coffee is insufficiently compacted in certain areas or if there are air pockets below the tamping point, there is a risk of channeling. This can be prevented by careful preparation of the ground coffee, vertical compaction, and even tamping.
Instructions: Prepare espresso step by step (article and videos).
Three classic tampers with a 58mm diameter.
Research Question: What influence does tamping pressure have?
Two sub-aspects were investigated. Firstly, we wanted to find out whether a change in tamping pressure affects the extraction time. Secondly, it was investigated whether the tamping pressure has an effect on the extraction yield and thus on the concentration of dissolved coffee particles in the cup.
To obtain the most precise result possible, the work steps and methodology were precisely defined.
Materials
Rancilio Specialty, commercial espresso machine with a precise flowmeter. However, each espresso shot was re-measured with the Acaia Luna.
Etzinger etzMax, espresso grinder with integrated scale. However, all grindings were re-weighed and repeated if there was a deviation (see Methodology).
Puqpress, automatic tamping machines for commercial use. Tamping pressure variable.
VST Refractometer, for measuring TDS in percent, i.e., the concentration of dissolved particles in the beverage relative 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 dispense 18.5 g. Grinding results from 18.4 g to 18.6 g are used.
The Rancilio Specialty espresso machine is programmed for a dispense water volume of 45 ml and a brewing temperature of 93 °C.
The corresponding 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 the extraction is started.
The amount in the espresso cup is measured with the scale. After extraction, the portafilter is unclamped and knocked out.
A flush shot is drawn from the espresso machine.
After extraction, the espressos are set aside to cool. After cooling, they are stirred, and the TDS value is measured with the refractometer.
The espresso is put into the refractometer with the syringe via the syringe filter.
This cycle is repeated 50 times for tamping pressures of 10 kg, 15 kg, and 20 kg, respectively.
The data are analyzed for extraction time and TDS values using a one-way ANOVA with a specified error probability of 5%.
Results Extraction Time and Tamping Pressure
In the variance analysis of the extraction time, no significant difference could be found. In other words, regardless of whether 10 kg, 15 kg, or 20 kg was pressed, our measurements showed no noticeable variation in the results due to pressing pressure.
Effect of tamping pressure on extraction time.
| 10 kg Druck | 15 kg Druck | 20 kg Druck |
| 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, so no significant result was found. The actual P-value exceeds the predetermined error probability of 5%. Thus, no significant difference can be determined between the groups.
Results TDS Values and Tamping Pressure
Even in the variance analysis of the TDS values, no significant difference could be found. The concentration of dissolved coffee particles was not significantly affected by different tamping pressures.
Effect of tamping pressure on the concentration of dissolved particles.
| 10 kg Pressing | 15 kg Pressing | 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. *missing measurements 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, so no significant result was found. The actual P-value exceeds the predetermined error probability of 5%. Thus, no significant difference can be determined between the groups.
Raw Data and Measurements of the Tamping Test
The raw data and further information can be found in the Excel Sheet. Should you be able to extract any further interesting insights, we look forward to the results.
Download Results-Tampingproject
In the analysis of the data, a linear correlation between the extraction time and the determined TDS values was found between the following data sets, which was determined with the Bravais-Pearson correlation coefficient:
- Tamping pressure 10 kg: Extraction time and TDS: weakly positive correlation -> r=0.5068
- Tamping pressure 15 kg: Extraction time and TDS: strongly positive correlation -> r= 0.6854
- Tamping pressure 20 kg: Extraction time and TDS: weakly positive correlation -> r= 0.5423
where: |r|= 1 -> perfectly linear correlation
|r|= 0 -> no linear correlation
Tamping Pressure and Taste and Additional Information
Within the scope of this test, the taste of the espresso was not investigated. Therefore, no statement can be made about it. However, based on the findings of the test, we interpret that a varying tamping pressure within the range we investigated has no or only marginal influence on the taste.
Also, based on our test, we cannot make a statement about how a pressure lower than 10 kilograms affects the extraction. Based on studies, partner recommendations, and our own experience, we recommend tamping with at least 6 kilograms of pressure.
Discussion
In the investigation of whether tamping pressure influences the extraction time of espressos, no influence could be proven. However, this does not mean that no influence exists, but only that no influence was proven in the test model used. Disturbances include, for example, inaccuracies in preparation and the insufficient precision of the examination machines used.
The devices used are equipment designed for professional use in gastronomy and not precise laboratory equipment.
However, in practice, much larger confounding factors are present in the preparation of an espresso than in the experimental procedure used.
Thus, in practice, a large influence of tamping pressure on extraction is not to be expected, as there are a multitude of inconsistent parameters influencing the extraction process. In this, tamping pressure is merely one parameter among many.
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The tamping test was conducted by Michel Aeschbacher in collaboration with a food technologist who wishes to remain anonymous. Our thanks go out to you!
We are open to further research partnerships and, with our materials and expertise, are happy to contribute to gaining insights into all aspects of coffee.
