Talking About Pressure Profiling

January 17th, 2010


Is it me or is everyone else a little surprised at the lack of discussion of pressure profiling?

Most of the interesting discussion, if not just about all of it, has been over at home barista where various people are building pressure profiling units for their one group machines.  Still much of that discussion remains more about how to build it, than about desirable profiles.

Pressure profiling is undoubtedly going to become more readily available.  Though the capacities of the Slayer, the Strada and Cimbali’s new machine are all different, they are all chasing the ability to manipulate pump pressure to improve espresso.  A lot of this desire comes from the profile of espresso produced by lever machines, which have a very different pressure profile compared to a pump driven machine.

Perhaps I shouldn’t be surprised, given my own experiences with pressure profiling.  I was asked, a little while back, about why I hadn’t posted anything about it and I thought I would take this opportunity to explain:

John Ermacoff very kindly leant me a pressure profiler that he had built for a while.  I am not going to go into exactly how it worked, but we hooked it up to one of the groups in my 2 group Synesso.  This meant that I could only use one group at a time because then I had a Cyncra, not a Hydra, so the changes in pressure would influence both groups if they were open.

The machine allowed for almost any profile, and included a manual and automatic mode.  I played with the manual mode for a while, but prefered the automatic mode because it was repeatable and allowed me to make specific adjustments.

The entire time I used it I felt lost.  Juggling dose, grind size, flow rate, volume of espresso and time is enough.  Suddenly introducing pressure variations meant that it was a lot harder to isolate the problem.  If the shot was underextracted then keeping the profile but changing one variable often resulted in chasing my own tale and very quickly burning out my palate.

Changing the pressure even slightly undoubtedly changed the cup.  The impact was always obvious, but rarely did I ever feel as if I had genuinely improved the cup.  By giving me so much control I ended up lost in a sea of variables from which I never really found my way out.

I confess that I didn’t approach this as clinically as I should have.  I didn’t record enough data and often ended up defaulting back to a very simple profile – which seems to be quite a popular one online:

  • - A relatively lower pressure preinfusion period, lasting long enough to soak the cake (5-8s depending on dose).
  • - A smooth, relatively quick rise to 9 bars over 2-3s.
  • - A period of 9 bars lasting 7-10s
  • - A declining pressure in the remaining section, from 9 bars down to 8 or perhaps lower.

This is by no means what I would claim to be the perfect profile.  It is one, however, that I could rationalise in my head.  The period of preinfusion at the start would increase the likelihood of an even extraction.  The period of declining pressure in the final third meant that we were extracting the nearly-spent coffee less aggressively, yet still adding volume to the shot – aiding balance, and reducing negative flavours.  Of course you could easily tear this theory to pieces – I just couldn’t find a better way to explain it. 1

One thing that surprised me was how a lever machine acts, as I had initially thought this profile would be quite close to it.  I was quite wrong – though my experiences comes from the use of a Victoria Arduino Adonis and a Scace 2.

In terms of pressure the lever allowed line pressure through when pulled down, as the machine has a heat exchanger for brew water.  Upon releasing the lever the pressure shot up to around 12 bars before almost immediately starting to drop down towards 0 in a fairly linear fashion.  I only found this out after I had returned the profiler, and I wish I could have tried it on the Synesso.  That way I could have dose two identical baskets had a better idea of the influence of temperature on our love affair with lever espresso.  The Scace 2 showed that initially the brew water was very stable – in the first 12-15 second after pulling the lever the temperature barely moved – maybe 0.2/0.3°C.  Then the water gradually began to decrease in temperature, losing 4 or 5°C by the end of the shot.  This makes sense.  When the lever is pulled water fills the chamber above the coffee.  Only once it starts to pass through the coffee, and lose thermal mass, dose it really start to cool off. 2

This profile is a very long way away from what most pressure profiling machines out there are doing – or at least what people have said they are doing.  Then again, as I said at the start, I can’t find many people talking much about it.  I guess I had hoped to see more on places like coffeed, coffeegeek, twitter or on various blogs.  Personally I’d be as interested in finding out what isn’t working for people, as much as what is.  When I asked the Cimbali engineer which profiles he liked best, he was a little evasive – in fact very few people are willing to really get behind any one profile.  I know these may change coffee to coffee, but with enough data out there I hope we’ll eventually be able to make a pretty good educated guess, the way we can with things like temperature and dose, based on the density of a particular coffee/blend.

I do have great hopes for pressure profiling, but at the same time realistic ones.  I don’t expect pressure profiling to discover sweetness, or flavours that have previously remained undiscovered in coffees.  I hope they can improve the clarity and presentation of them and help reduce detracting tastes and flavours.  It won’t make average greens taste great, or bad roasts acceptable.

Right now it seems that engineering is, for once, way ahead of the barista.  The pressure profiling machines coming can do almost anything we want – we just need to work out what that is…

I will close this post by saying thank you again to John Ermacoff – I learned a great deal through his incredibly generosity, and can’t wait to see what he comes up with next.

  1. If anyone has any info on the effect of pressure at that stage in the shot – on caffeine extraction, oil emulsion etc etc then I would love to see it!  ↩︎
  2. Again, correct me if I am wrong here  ↩︎

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