Polishing

To polish or not to polish? This is one of the questions that your everyday barista may never ask themselves.  Why? Like so many things in the coffee industry, it seems to be an un-spoken rule that you polish an espresso puck as a final step in preparation before extraction.  By polishing, I am defining it as "To make the surface of an espresso puck smooth and (possibly)flawless by using a 360 degree spin and the pressure of gravity on the tamper (after tamping) to lock the top-most layer of grounds in a more ridged refined surface."  Most commonly when I ask the question to unsuspecting baristas, I get a rather blank stare, with responses varying from "Um... I don't know?" to "It prepares the espresso puck for extraction."  Yet, when I probe a little further, the reason baristas polish is because they were trained to do so.  Polishing then shows itself to be a dogmatic practice passed down from one generation of baristas to the next without providing any solid reasoning as to how espresso is improved by this process. The big questions then become: Why do baristas polish their espresso pucks? and is this a practice that should be perpetuated?

I have a very similar story; I am a wholesale account trainer for Anodyne Coffee Roasters in Milwaukee, WI. One day a few months back, I was conducting a training session on all things espresso for a new account, and in the middle of a tamp I was about to lead into the old "...and after you tamp, spin the tamper 360 degrees on the top of the puck without adding any additional pressure." The next words out of my mouth were "Because... um, it prepares the puck for extraction."  If I were the people being trained, I would not have been sure whether or not to believe me. Being the inquisitive person that I am, I started asking questions of other baristas, calling friends for information, and running tests.  I  took my question and ventured to the north side of Milwaukee to Roast Coffee Company, and began running tests with my good friend Brett Felchner, as well as being in constant contact over the phone with Josh Longsdorf at Ritual Coffee Roasters in Napa CA.

Here are the different things that we looked at:

* Pre-weighing the coffee, 18 grams (double basket) and 20 grams in the Ritual tests
* Tamp the puck, checking the weight after tamping
* Tamp the puck, polishing by not adding any additional force, but spinning the tamper 360 degrees
* Extraction of the shot at desired temperature, time, and weight (201°F, 26 seconds, and roughly 28 grams polished and 34 grams unpolished, respectively.)
* Weight of the extracted puck
* Weight of the liquid of the pulled shot
* Rate the taste of the pulled shot
* Watching the shots extract on the bottom of the naked filter (we found spiraling in the polished, and no spiraling in the un-polished, could be because of the twisted compression from the polish?)

From the testing at Roast, Brett and I found, on average a lighter post extraction puck and a heavier liquid shot weight with the un-polished pucks.  We also noticed a huge difference in flavor, with an average of 5 grams heavier of liquid shot weight in the unpolished tests.   We found the un-polished shots to have a fuller taste, better body, and a more desirable flavor.  Whereas the polished shots all seemed more harsh and slightly thinner when pulled using the same parameters.  Conversely, in the Ritual tests although they were pulling 20 grams of espresso while we were pulling 18 grams, they were finding similar results with heavier liquid shot weights of almost 5 grams and lighter post extraction pucks on the unpolished pucks.  The big thing though is that everyone at the ritual tests found the unpolished shots to taste "a bit over-extracted."  I should note that none of our tests were as scientific as humanly possible, taking into account the gram size differences, different  used coffees used in our two tests and as we both were testing on machines and equipment that gets used and abused regularly.  I personally do not have access to science labs and uber-precise testing equipment but tried instead to monitor and be as precise as possible with the controls we have. Our tests were more or less aimed at seeing what the average everyday barista would experience with slight modifications to their everyday routines of just polishing or not. But even with all that, the consistency of differences found between polishing and not polishing in between the Ritual tests and the Roast tests brought me to consider the differences in the espresso machines being used: the machine we were testing on here in Milwaukee was a PID modified La Marzocco Linea, whereas at Ritual they were using a La Marzocco GB/5 with pre-infusion. 

The reality, it seems, was not as obvious; or maybe it was so obvious that we had not considered it before.  It had to do with that magical bar pressure that is often talked about.  The bar pressure on an average espresso machine is around 9 bars, which roughly relates to about 130 pounds of pressure per square inch.  That is a lot of pressure coming out of an espresso machine, almost unimaginable to a human, but a look at the physics can begin to show us a different picture.  A search around the Internet and a few physics books showed me that the real pressure on the coffee is a total of around 540 pounds in every direction, not just downward pressure.  The most specific information on this topic was found in the comments on the coffee blog 'Coffeeaspirations' by Owain: "If you have a container under a pressure of 9 bars - every part of the container is under 900,000 Newtons per square meter - in every direction. 9 bars is equivalent to 130 pounds per square inch. A standard filter basket is about 2.3 inches in diameter. Pi x radius squared gives an area of 4.15 inches squared. 130 x 4.15 ~ 540 pounds of pressure covering the 'top of the puck' - minus the minimal pressure relief due to the release of the extracted coffee from the system." What does this mean in terms of the espresso puck? Well, as it happens, the pressure pancakes the puck, and what our testing results are really showing is the difference in headroom between the top of the espresso puck and the screen filter.  This polish essentially evens out the top of the puck, locking the grounds into a tighter formation, even though no visible downward force is applied.  It compacts the puck down just a hair, and that minuscule amount produces a far greater difference with 540 pounds of pressure being applied to the puck.  This slightly larger amount of headroom, and therefore larger amount of brewing water on top of the polished shot, which will be relatively quite a bit larger than the unpolished shot.  This difference in the amount of brewing water above the puck could account for our weight difference in the pucks and shots, based on the amount of water being held back and sent through the pressure relief valve on the espresso machine when the pancaked puck bounces back from its pressurized form.

A discussion on this subject with my math and physics-whiz of a sister, Megan, who does all sorts of scientific testing for a living, brought even more insight on what is possibly happening in the polishing and extraction processes.  The general principle can be explained using the following example: when using a garden hose (as I am sure all of you have), there is a certain amount of water that is flowing through the hose, under very little pressure.  When the flow of water is inhibited by placing a finger over the end of the hose, the back pressure on the water in the hose increases.  To equalize that pressure, water must flow through the smaller opening at a much higher rate.  Relating this to extraction of espresso: although perceptually very minor, a polish on a surface of an espresso puck may create smaller openings in the surface of the puck for the water to flow through.  This would increase the flow rate to try to equalize the back pressure in the system, as in the example of the garden hose, but also magnifying any channeling that could occur to allow the water to flow through more easily.  This is counter intuitive in that one would think that a higher flow rate should create a larger amount of water in the pulled shot. However, the water would be able to flow more freely through an un-polished shot, reducing back pressure and allowing more water through the grounds, even at the slightly lower flow rate.  This lower flow rate would also allow the brewing water to be in contact with the espresso grounds for a slightly longer period,  extracting more evenly and producing a richer flavor, while also contributing to the differences in the final weights of the puck and pulled shot.  This idea would also follow through to differences in how much pressure a barista uses when performing the initial tamp of the espresso puck.

Let's not forget that though the tests performed at Ritual showed similar results in the outcome of the final weights, there was a definite difference in the rating of the taste of the shots that were pulled.  Through even more research, many emails with industry professionals, and even more experimentation, the big game changer in these tests happened to be the pre-infusion. Although the same idea applies to espresso extraction with pre-infused as a non-pre-infused shot , the difference comes in the pressure control during the saturation and extraction of the espresso.  The basic idea behind pre-infusion is to saturate the espresso puck to potentially eliminate the chance for channeling and prepare the puck for extraction, as is similarly done when letting coffee grounds 'bloom'.  In turn, pre-infusion is put on espresso machines to get rid of any imperfections in the puck, saturate the puck and gradually introduce the puck to water, not just blasting it with nine bars of boiling water.  What does pre-infusion mean to polishing?  Specifically, polishing with a pre-infusion capable machine would be unnecessary and even potentially detrimental, as the pre-infusion on a polished puck could be inhibited or at least causing a delayed saturation and extraction by the polish as indicated above.  So why the marked difference in taste?  The most likely answer is that on a pre-infusion capable machine, the espresso is extracting differently and possibly over extracting with an unpolished surface, as the unpolished grounds are in contact with the water used to pre-infuse the puck, as well as being in longer contact with the brewing water while pulling the shot. Whereas the polished puck holds back more water during the process and slows down the overall extraction.  This is not to say though that on all pre-infusion machines that you will have over-extracted unpolished pucks, but that there is a certain amount of compensation taking place on the part of the barista in the method of preparation of the shot with the pre-infusion.  More information can be found about pressure control and pre-infusion at 'Home-Barista.com' or 'SlayerEspresso.com,'

To be honest, my first thoughts in all this leaned towards the idea that the un-polished puck has a more porous surface allowing the full puck extraction to start sooner, as opposed to the polished puck causing more resistance and holding back water during the extraction, while also over extracting the top portion of the puck. What are my actual conclusions? I would say confidently now that my preference is for un-polished shots.  But the reasons for the difference are a little more complex than simply being my taste preference.  There is so much going on in espresso preparation, and the average everyday barista does not really have the time to sit and analyze the physics and science behind what is happening.  The concentration for the everyday barista is to make drinks that taste good, in a timely manner.  Using all of this gathered information, and through personal observations, I have come to conclude that the average barista may be compensating with the polish, slowing down the process with the inhibited water flow, usually by pulling smaller volume and longer timed shots (as in case of some ristretto shots I have had), with pre-infusion having a much larger influence than previously thought.

So, does polishing have an effect?  I would venture to say YES, and a rather large one at that.  As a barista, should we be adding extra steps to an already imprecise routine and possibly over-tamp a beautifully prepared shot?  I would say no.  Some industry experts believe that polishing is an unnecessary and detrimental step in an already convoluted process.  Considering all the other controllable factors the everyday barista has to deal with, it would seem very logical that the focus should be on perfecting those steps in espresso preparation that can actually help to produce a beneficial outcome in the cup.  It should be noted that like all things in the world of coffee, there are a million different methods and a million different things to always learn and tweak.  Adressing polishing without an understanding of what goes into making espresso I strongly recommend that baristas take the time to duplicate our experiments, and I hope that you will taste a difference.  And as any inquisitive person would, I ask my self: if I were to go into a reputable, well-trained coffee shop without watching the barista who is pulling the shots of espresso, could I identify whether or not the puck was polished? Could you?





Note: I wrote this with some help from quite a few people, but the idea is no where done or finished, I just have not had time to play anymore with the idea.  Maybe someday I will revisit this and try adjusting, tweaking and trying new things with polishing and tamping and such... one day.