Does Tamping pressure cause an affect to coffee strength?

I get asked this question from time to time, so we thought we would answer it in an article. But if you don’t have the attention span. Then I will simply say No it does not! However, for those that have chosen coffee as their career, they may wish to look a little more closely, as it may give some insights into coffee puck dynamics as a whole. Looking at one element of coffee in isolation will teach you very little. But looking at coffee as a whole can. Espresso coffee in particular has multiple principles rising and ceasing to be at any one time. Experiments like this can help to make a “mud map” of puck dynamics. But be very wary of drawing conclusions from one set of results. Instead, do experiments to build on a theme. This is the only way to generate an objective understanding of espresso coffee. This article should therefore be viewed in the context of others to build an understanding. One of them may be of some use. Whilst all combined will draw some definitive conclusions.

But before getting to the bones of the experiment let’s look first at why tamping won’t make your coffee stronger.

Pump Pressure.

This is an article is about Tamping and grooming. It is not about Tamping pressure. (or more accurately, tamping force) This is because the force you tamp is largely irrelevant when discussing Tamping. However, the effects of Tamping pressure, or force, is one of the most common questions that I get asked. So, I feel some answer is necessary. And perhaps for most, a basic understanding is enough. Any in-depth discussion will no doubt lead down the path of pressure and flow. A discussion for another time. I have asked my brother Jake to try and provide a basic understanding of the difference between tamping force and machine extraction pressure.

Jake’s Comments

              “  This is fairly simple once you understand that the terms people use in this question are incorrect. Unfortunately, this leads to incorrect assumptions.

                The application of the tamp to the coffee grounds should be described as a Force and is measured in Newtons. This describes the movement of the tamp to the Coffee grounds which are held stationary by the porta-filter.

                The application of water to the Coffee Grounds by the pump is described as Pressure and is measured in Pascals (pa). Many pumps operate in the vicinity of 9Bar which is 900Kpa. The application of this pressure to the surface of the coffee grounds mistakenly are calculated out to fantastic forces using the formula F=PA ie F=900000pa x 26.42cm^2 = 2377.8N of force or 242Kg’s. The reality is that this formula does not apply because the coffee is simply a permeable material in its relationship to the water and acts as a Restrictor to the flow of water. Its is not a moveable object in this context and cannot be compared to the piston face of a hydraulic cylinder. If 900Kpa of fluid were applied to a cylinder piston of 58 mm dia it could lift 242Kgs. Clearly, there are no similarities and the confusion is derived from the mistaken use of the term “pressure” applied in the context of physics.

                Any comparison between Force applied to a tamp and water Pressure from the pump is illogical and irrelevant. “

Whilst this may be a somewhat complex explanation, it is important to not mislead. However, distilling of this information any further will take us in another direction so we shall leave it there. Needless to say, the 9bar Pressure that the pump is generating is much stronger than the 15kg Force that you are applying through tamping. It is for this reason that your tamping is not a factor in determining your coffee strength. This is determined by Pump pressure and flow to the opposing coffee grinds.

Why Distribute and tamp at all then? Is it important?

Good Question. To answer, we set up a series of experiments, to not only test some of the gadgets in the marketplace. But more importantly to use them to try and gain an understanding of grooming, tamping and puck extraction. We looked for tools that allowed us to break down puck preparation into two separate stages.

1.       Distribution.

2.       Tamping.

In this way, we could try to gain some knowledge of the parts as well as the whole.

Coffee and Equipment.

In this experiment we used a single origin Micro lot Kenyan as our test coffee, roasted to halfway between first and second crack. (Never use a blend when testing)

We used a Mahlkonig peak grinder, with our standard Gino Rossi hopper replacing out the Mahlkonig hopper. We use this hopper for all experiments because when maintained between 750grams and 1000grams it delivers a constant particle size through metered pressure on the burrs.

We strongly recommend against the use of single serve coffee when performing experiments. It promotes popcorning and uneven particle size. We shall look at the crime of single serve coffee in an upcoming article, but in the meantime, for reasons why you should never use single serve coffee when testing please see our article

• know your grinder

A Synesso Hydra was used for this experiment. So much to my brother Jake’s disgust, there were no volumetrics used. As a result, we weighed and cut each shot to its desired length. (there is some overrun.)

The machine has .6 restrictors fitted and although it has pre-infusion timers added, they were not used in this experiment. Only the machines natural pre-infusion was used.

With our base calibration set, we measured the grind, (more on this in future articles) and at the end of each round of testing, the grind was checked to ensure it had not moved with environmental factors.

To calibrate the machine and grinder, we dialled in the coffee to 20.5 grams at a shot length of 42 mls, with a resultant TDS of 10.95% at an extraction yield of 22.4% This became our control to measure subsequent tests against. With the machine calibrated we proceeded to testing by using the OCD distribution tool and the PUQ press.  We used the OCD tool to test distribution, and we used the PUQ press to represent tamping

Note: You can of course use your hand to distribute coffee successfully, however for the sake of continuity and the elimination of personal grooming techniques, we chose to use the OCD distribution tool to this end. 

With this set of results, we can see that none of the test results reached as high as the control. Which was a mix of these techniques put together. The main difference between these results and the control was human interaction. When the base control was set. It was done so with a view to setting any deficiency’s in the puck. Basically, a combination of palm tapping, distribution, tamping and most importantly, gut feel. 

But wait there’s more!! what about basket shape??

For those avid readers of our articles, would know here at the “The truth” and “Bush and Bush” we are big fans of the tapered basket design. Regardless of whether you share our beliefs, we have been consistently referring to them throughout these articles, because they can help us to shape our views on puck dynamics.  So before we move on to a discussion of the results above, we thought we would run the same test again with a tapered basket. In the hope of gleaming even more useful information.

For more information on the differences with extraction in regards to tapered baskets. Check out our two prior articles in this order

• Impacts of Basket (espresso portafilter) shape on extraction

• Fines Migration and it’s effects on temperature in the coffee puck

Besides their shape, these two baskets have the exact same basket volume. This means they are identical for extraction purposes.

For an explanation on how to determine your basket volume see our editorial

• How to determine basket (espresso portafilter Volume

Moving on, we did the same experiments again this time with the 16-18 gram tapered expo bar basket.  (pictured in figure 3) There is always the option of dosing a tapered basket differently. However, for the sake of “apples to apples” we calibrated the tapered basket to be the same dose, and grind as the VST 15gram.

So with the results in, what would I surmise from the above experiments? My take would be…

OCD tool. (or any other distribution tool and hand techniques)

The OCD tool is good for creating a surface that promotes an even extraction. I.e. it’s good for setting up the water delivery. I believe, (though cannot conclusively prove) that it makes the water pool on the top of the puck and most likely creates a layer of water which in turn makes for a uniform delivery of water to the puck, through the pre-infusion stage to when the pump pressure kicks in after pre-infusion. I’m drawing this conclusion by the overall run time being the same as the control. (in the Tapered basket series) This highlights the first rule of puck preparation. Make an even surface. However, the OCD tool did not deliver as high a TDS as the control which was also subjected to tapping and tamping. This highlights to me that the OCD or any other distribution tool or hand technique whilst making a prepared surface, is not be used alone.

PUQ Press

The PUQ press, delivered a consistently higher TDS. Than the OCD through both baskets. Even though run speed was quicker. Because we know through physics that this is not due to tamp pressure, I deducted that the tamping helps to alleviate the airspaces within the puck, and therefore help prevent micro channelling. As expected though, it did not deliver discernible differences in the strength verse kg pressure. This highlights our initial assumption that tamping hard is not going to make a coffee any stronger. It does however make the shot slightly longer on average.

Scott Rao gives a great explanation for why this is the case this in his book “The Professional Barista’s Handbook, espresso page 17 ”

“ one interesting reason many baristas overestimate the impact of harder tamping on flow rate is that, for a given dose and basket, a harder tamp will compact the bed more, leading to more “headspace” between the grounds and the dispersion screen. Because the entire headspace must be filled with water before the water will percolate through the grounds at full pressure, the extra headspace increases the lag time between pump activation and the appearance of extract from the portafilter. The extra lag time might lead a barista to overestimate how much the harder tamp slowed the flow rate”

It does not directly make the shot stronger through tamp pressure but if anything, may indirectly make the bed more consistent through the elimination of air pockets that can cause microchannels. This may in turn result in a higher TDS reading, and in turn, may also affect flavour.

Basket shape.

Perhaps the biggest result here is basket shape. The tapered basket is seen to extract a higher average TDS on all accounts. (Though the results are negligible and hardly a smoking gun.) but it shows the distinct extraction MO of the tapered basket. A tapered basket forces pressure and water into the centre of the basket before exiting. The VST because of its straight sides does not do this. If you go back and look at the run speed comparisons you will find that the straight-sided basket will deviate from the control more in its run speed than a tapered basket if not groomed and tamped consistently. Also, the tamping and grooming have less effect in the straight-sided basket. We see this because the run speeds are very similar across all tests in the straight-sided basket. Whereas the run speeds with the tapered basket vary more greatly, across different tests.

This can be viewed as straight-sided baskets as being either more consistent or as having their own “flow and extraction” personality that overpowers other constraints like tamping and distribution. I’ll let you be the judge of that.

From the outset, it appears yet again that the tapered basket is more consistent than that of a straight-sided basket. But why does everybody use them? Is it just a triumph of marketing, or is there a circumstance where these baskets actually have a place? This we shall look at more as we start to go inside the puck with a series of 4 upcoming articles. Articles that not only serve to give more insight into the coffee puck but highlight a shift that has been taking place in the specialty industry over the last 10 years. Perhaps straight-walled baskets are a sign of that shift….

Conclusion.

Jake’s Commentary

Espresso Extraction is all about Maillard reaction generation. Tamping’s role is solely to perpetuate a balanced commencement of water infiltration through the puck. A flat and level surface that promotes an even pressure across the puck for that commencement is all that is required. Tamping in itself is only a minor contributor, a 5%er, puck integrity from correct dose and grind, pressure and flow, is the core of the espresso extraction process.

                If one finds it necessary to apply all of one’s body weight to each tamp, one can assume that their anger issues may need addressing in some form.

Clay’s Commentary

I urge you to study these results more closely as there is much to discover. Singling out these results one after the other will make for a very long article.  So, we have left these results for you to gather your own conclusions, and think of tools, or even baskets that can help your espresso preparation. However, as a basic premise, there is nothing new here. A well-prepared espresso puck must-have elements of both distribution, and tamp. Both are combined to give a consistent puck presentation. Using only one of these methods will not deliver the same results. Using both can elevate your TDS by 20%.

This should however also draw your attention to the fact that the other 80% of your TDS is coming from other factors. Namely grind, mass and pressure/flow.

I draw this conclusion from the comparison of results to test 1 in each series. There was no distribution or tamping involved in this sequence. So we can say the resultant TDS was generated by pressure/flow and puck density alone.

This test alone should make you put things in perspective. If you’re just tamping, or just using a distribution tool. Then you may not be adding any advantage to your puck preparation. Both however in tandem can elevate your TDS by 20%

But hold on, this is not an excuse to hit up your coffee roaster for a bench full of gadgets may give you an extra 20% on your TDS, because there may be a hidden expense in doing so.  

If there is one thing that the Speciality coffee industry suffers from greatly. It’s a lack of process. The more gadgets you put in place, the slower the process of making coffee, which has flow-on effects on profits. If you can dose and tamp effectively without the use of these tools then I say don’t use them. If however your skills are not up to scratch or fail under the pressure of rush hour then maybe introduce some of them into your flow. Just remember the product you’re selling is only $3.50-$4, you may be losing money by adding unnecessary steps to your process that translate to slow service time.  

So in closing, you would be wise to build a sequence to ensure that both your tamp and distribution are on point. Then drill it so it doesn’t change, whilst becoming repeatable under pressure. After doing this you can set and forget, and never bring the topic up again. You should then draw your attention to the other 80% of the puck, for this is far more important. And that’s exactly what we shall start doing. 

In our next articles we start focusing on puck dynamics, all as a lead-up to the release of our calibration tool later this year.

Cheers

Bush and Bush coffee systems.