Brew pressure and its effects on espresso

Want to talk espresso but not sure which forum? If so, this is the right one.
Anthony

Postby Anthony » Aug 18, 2008, 11:47 am

I am looking for advice on brew pressure and its possible effects on extraction. Here is my set-up: I have recently upgraded to an Andreja Premium and a Macap M4 stepless, bottomless portafilter, and including the family/friends deal, I pull 4-6 shots a day (I prefer ristrettos). Not being a pro-barista, I realize a lot of things can go wrong in tamping, grinder adjustment, etc. Sometimes I am able to pull some sound shots, sometimes not. The "not" part is my concern. Worst case scenario, I have noticed getting a couple to a few stalactites on the bottom of the PF that hang around (and not a consistent cone and gloppy drop formation; sometimes I get drops over the cup). I also notice sometimes little pinholes in the puck (incidentally?). Someone told me that I could be pulling the shot a too high of a brew pressure. When I backflush, the needle reads 10.5 or a little bit higher, and after I release it (end the backflush), it reads 10 exactly. I just pulled a shot to double check and it seemed to brew it at exactly 10.

My question is whether aside from variables in technique, grind, tamp, would I be better off with a slightly lower brew pressure? Perhaps it is just more and more practice!
Thanks,

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HB
Admin

Postby HB » Aug 18, 2008, 12:28 pm

Anthony wrote:I just pulled at shot to double check and it seemed to brew it at exactly 10.

I would lower the brew pressure to 8.5 to 9.0 bar. Lowering the brew pressure will you a wider margin of error and reduce the possibilities of extracting bitters. Crema production drops off below 8.0 bar, but can produce good flavors and more nuanced espressos. On a related note, spring powered lever espresso machines typically start around 6.0 bar and trail off to 4.0 bar; they produce less crema than pump machines, but the shots are still tasty.

It's worth experimenting with brew pressure. Jon's Tweaking the Triple Ristretto suggests higher pressure:

I set my espresso machine's maximum brewing pressure up from 9 bar to 10 bar by adjusting the over-pressure valve (OPV, also sometimes called an expansion valve). Machines with rotary pumps have an adjustment on the pump. Some blends respond well to higher-pressure extraction. I especially love the Espresso Delight Blend from Mitalena Coffee as ristretto (the thicker I can draw a shot, the sweeter and more chocolaty it gets). I find that you can get a syrupy body and sweeter taste that lingers in the middle of the palate when pulling at higher pressure.

Though my experiments for pump espresso machines are all within the 7.5 to 9.5 bar range.
Dan Kehn

Anthony

Postby Anthony » Aug 18, 2008, 4:26 pm

Thanks, Dan, for the reply and advice. I will definitely lower the brew pressure and pull more shots. I remember reading the "Tweaking the Triple Ristretto" article, and while I have a triple basket just for such purposes, I probably need to work on "perfecting" (hmmm) a double basket extraction and varying the grind. I have also gained much from your "Perfecting the Naked Extraction."

I do have a related question, however; if this is a repeated question, perhaps you could point me in the right direction. What more generally is the effect of brew pressure on an extraction and what is it doing to the puck? I am relatively familiar with the differences in taste regarding temperature (say, pulling a shot at 204 and 199), but now I would like to understand more about brew pressure (and its possible relation to the properly (or improperly) tamped puck and brew temperatures).

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cafeIKE

Postby cafeIKE » Aug 18, 2008, 11:49 pm

HB wrote:I would lower the brew pressure to 8.5 to 9.0 bar....Though my experiments for pump espresso machines are all within the 7.5 to 9.5 bar range.

Is this the machine gauge or the gauge on a Scace? 10bar on the Vibiemme gauge is about 9bar on a blind basket and ~8.5bar on a Scace or an actual shot. 8.5bar on the machine gauge would give ~7bar on a shot.

[Edit : HB pressure measurements are with a Scace II which simulates puck back pressure.
Quoting Greg Scace, developer of the Scace Devices, from Digital Pressure Adapter :
"Yeah, the pressure difference between pumping against a blind filter and pumping thru a coffee cake is often very large. It's also machine specific, varying between types of machines, and sizes of gicleurs. I tested a machine that had a 4 bar difference between static pressure and dynamic pressure. Measurements were made using the same gauge, so gauge calibration wasn't an issue. "]

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malachi

Postby malachi » Aug 19, 2008, 1:20 am

It really depends on the coffee - and on the extraction profile you're shooting for.

That being said - to use as a simple example a double shot of Hairbender updosed to 19grams, temp at 198F pulled a little short in around 27 seconds from a temp stable machine...

While pressure has definite impact on flavour profile, the easiest way to describe the difference is by focusing on texture. Higher pressure shots are "lighter" on the palate and seem to coat less, while lower pressure shots can have a "gritty" texture and are often very syrupy.

At a flavour level, with the above profile the shots tend to be more transparent but also less rich and dominated by the high end of the midrange flavours when pulled at a higher pressure. When pulled at a lower pressure the shots are "rounder" with less high end but better aromatics.
"Taste is the only morality." -- John Ruskin

Anthony

Postby Anthony » Aug 20, 2008, 10:20 am

Let's see if I have it by way of summary. Please correct me if I am wrong. There seem to be two sets of issues here, (1) the one concerning the mechanics of measuring brew pressure, and (2) the closely related issue concerning the experience of what's happening with brew pressure on espresso.

1. Mechanical Concerns. Thanks for your replies and references Dan and Ian. Reading more of the threads you have highlighted, I understand there to be a difference in brew pressure between the brew pressure measured with a blind PF, which is "static," and the brew pressure that is "dynamic," that is, taken with the puck that itself can vary according to grind, amount, and tamp. We are concerned with dynamic pressure ultimately, and when setting the expansion valve, for example, we are really after the dynamic brew pressure even if we do it (or have to do it) according to a static read.

There may also be important differences between the Group Brew Pressure and the Machine Brew Pressure. The former, the Group Brew Pressure, can be measured by the Scace 2 device, and might be called (for lack of a better term) an "absolute" measurement because it delivers a reading right there on the group head. So now a question: In the case of an HX machine like my own, an Andreja Premium, will the Machine Brew Pressure (for lack of a better term) be more "relative" because it is gauged just before it goes into the heat exchange system? In any case, the Machine Brew Pressure might be called "relative" because the readings can vary from machine to machine (say, an HX and double boiler) and may not correspond to the reading from the Group Brew Pressure.

As I understand it, the actual numbers of brew pressure from the gauges are not important in themselves, but are significant insofar as they give us a consistent point of reference. Assuming that one's own machine is consistent—and speaking of one machine only—the reading of the brew pressure is arbitrary but functional so long as it can serve as a stable point of reference. But if we want to speak "across" machines, and as Ian remarks, help another diagnose a problem or give a "social" reference point for dialing in a shot with a specific blend, then the matter becomes more difficult. We would want a consistent way of measuring brew pressure across machines, and it seems that the Scace device could serve this purpose—assuming they are calibrated the same way (of course, we would all have to own one :D ).

So, I take from this a few things: First, when I am speaking with others about brew pressure, it is important to know how they are measuring it. Second, it may be possible that my 10 or 10.5 bar might be a "group" 9.0 or 9.5 bar brew pressure. Again, this only matters as a point of reference. Third, once we have this straight, it can be meaningful to speak of optimal dynamic brew pressures for such and such a blend, or perhaps to help diagnose an extraction problem as a "brew pressure" problem. Does this sound right?

2. Experiential Concerns. I realize that none of this can replace an experienced barista and a trained palate—it's all about the taste!--and that's the point of the brew pressure measurements. Chris's comments about texture and flavor were really helpful.

I am familiar with Hairbender (originally from Portland), and when I was in Portland last month on my migratory trip to the Division St. Stumptown, I had a couple ristrettos. I sensed (in my newbie way) what was probably more on the higher brew pressure side (though I realize there are a lot of variables)—actually it seemed a little nuttier (almost almond-like). In any case, I have been fixed on B and B's Dancing Goats, and have a couple more lbs coming in shortly. I will try to experiment with brew pressure with the newly arrived beans. Do you have any suggestions concerning what I should be attentive to with this blend?
I am eager to learn as an amateur more about this variable, brew pressure, general telltale signs of high or low brew pressure, and from those more experienced than I, what brew pressures do with different origins and blends.

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erics

Postby erics » Aug 20, 2008, 1:22 pm

Anthony -

Some more light reading:

Experiments in programmable, variable brew pressure profiling
Brew pressure profiling update 2
Brew pressure profiling update 3
Playing with Pump Pressure: Part Uno
Playing with Pump Pressure: Parte Due

Your Andreja measures brew pressure at the hx outlet and the difference between what your gage reads and the pressure at the puck is 0.3 to 0.4 bar. Adjusting your OPV such that the gage reads 9.0 with a blind filter would work well. Most home users (me included) adjust their OPV as such and rarely change same. The OPV serves to regulate the maximum pressure on the gage. However, from your well written posts, I sense you want to play :)

Trying to discern the difference that a variation in brew pressure causes is an arduous task. For a specific bean, I would suggest you obtain a 0.1 gram scale and use the same amount of grinds for each pressure variation and extract the same amount of espresso for each trial. If you adjust the OPV in, say, 0.5 bar increments, you should find that you will be making grinder adjustments to keep the extraction volume constant (for the same extraction time). It gets complicated :) but keep us posted on your findings.
Skål,

Eric S.
http://users.rcn.com/erics/
E-mail: erics at erols dot com

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cafeIKE

Postby cafeIKE » Aug 20, 2008, 3:19 pm

Anthony wrote:I am eager to learn as an amateur more about this variable, brew pressure, general telltale signs of high or low brew pressure, and from those more experienced than I, what brew pressures do with different origins and blends.

Over the course of the first year I owned the Vibiemme, I frequently adjusted the brew pressure to learn the effect. By that time, it was pretty obvious that ±0.5bar is insignificant relative to freshness, dose and temperature. Eric's thermocouple adapter prompted me to cobble up the Digital Pressure Gauge, but mainly for academic interest to see where I'd settled by taste over the course of the previous year's fiddling. Other than resetting it after a lube job, I haven't changed the OPV setting in more than a year.

Today I mainly use the DPG to set / reset a machine in the 8.5-9.0 bar range on the puck, regardless of machine gauge reading.

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another_jim
Team HB

Postby another_jim » Aug 20, 2008, 5:14 pm

Andueza and Maeztu, at the food science department at the University of Navarra, did a series of studies on espresso taste, including one that taste tested at various pressures. There wasn't a huge difference, but 11 bar scored lower than 9 bar or 7 bar due to added bitterness, while the 9 and 7 bar shots were almost too close to call. The rough grain of these results make people's claims of big differences for 1/4 bar pressure adjustments somewhat suspect.
Jim Schulman

Anthony

Postby Anthony » Aug 20, 2008, 5:43 pm

erics wrote:Anthony -

Some more light reading:



Your Andreja measures brew pressure at the hx outlet and the difference between what your gage reads and the pressure at the puck is 0.3 to 0.4 bar. Adjusting your OPV such that the gage reads 9.0 with a blind filter would work well. Most home users (me included) adjust their OPV as such and rarely change same. The OPV serves to regulate the maximum pressure on the gage. However, from your well written posts, I sense you want to play :)

Trying to discern the difference that a variation in brew pressure causes is an arduous task. For a specific bean, I would suggest you obtain a 0.1 gram scale and use the same amount of grinds for each pressure variation and extract the same amount of espresso for each trial. If you adjust the OPV in, say, 0.5 bar increments, you should find that you will be making grinder adjustments to keep the extraction volume constant (for the same extraction time). It gets complicated :) but keep us posted on your findings.


Thanks for the references, Eric, the info, and the encouragement!