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Thread: So why 9 bar?

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    Senior Member gonzob's Avatar
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    So why 9 bar?

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    After an hour-or-sos searching I couldnt find a specific reference to this on the forum, so here goes...

    Why do you need such high pressure for espresso?

    The model in my head goes something like this:
    • To get as much of the coffee flavour as possible out of the beans, you need to grind them finely.
    • When you grind them finely you need more pressure to get the water through
    • Empirical work has shown that the grind you need for optimum extraction means you need about 9 bar

    Is it as simple as that?

    Gonzo

  2. #2
    KJM
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    Re: So why 9 bar?

    I believe the research done by Illy (and others) shows the maximum/optimum oil extraction happens at those pressures...

    I did actually go to that source once, but I cant actually recall the exact reference right now (it is Christmas eve, and red alcoholic substances might have been consumed....)..

    /Kevin

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    Re: So why 9 bar?

    Indeed, there was some research done, as indicated above, that indicated 9 bar produced good results. There are now some who are using lever machines that dont necessarily produce 9bar, or a constant 9bar brewing pressure, and some of these machines are producing quite nice espresso. My Lusso is closer to 6 bar (from memory?), and it may not produce quite as much crema s my Silvia used to, but it does produce nice sweet shots.

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    Senior Member gonzob's Avatar
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    Re: So why 9 bar?

    Any more thoughts on this?

    I had a look for the Illy reference, found it, but couldnt quite convince myself to spend 87USD for the second edition (Espresso coffee - the science of quality), let alone the 484USD for the first edition (Espresso coffee - the chemistry of quality).

    This sounds like a chicken/egg situation. is it:
    a. You want high pressure, so you grind finely so its difficult to pump water through the puck and the pressure goes up,
    or
    b. You want a fine grind to get max surface area, but in doing so you make it difficult to pump through the puck, so the pressure goes up.


    Note in case (a), you want high pressure everywhere, but what you actually get is a gradation of pressure through the puck. The coffee at the top surface gets 9 bar, but the coffee at the bottom surface gets zero. So, if the pressures the thing (rather than the grind size), then a pressurised portafilter would seem like a good idea (you get high pressure in the basket irrespective of the grind size, up to a point).


    Ill go and have an espresso and think about it....

    Gonzo

  5. #5
    Senior Member Vinitasse's Avatar
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    Re: So why 9 bar?

    Or you could design a grouphead/portafilter combo approximately 180mm in diameter and only 1mm deep (give that the volume of a cylinder = Pi x radius squared x height) thus allowing every ground to get the benfit of a full 9 bar extraction. ;D

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    Re: So why 9 bar?

    im getting pretty doubtful over the idea of a perfect espresso and its affiliation with 9 bars. Not that i own an espresso machine but i can say that out of all the espressos i have drunk the machine getting 9 bars of pressure consistently and evenly through the shot was the least of my worries. I would actually throw a wrench into traditional coffee theory and say that people like the "controlled" uneven extraction of coffee that current coffee machines provide. As gonzob said its a chicken and egg. Grind size and surface area. If we all only wanted coffee that was extracted at a perfectly consistent rate every single time we would probably drink something radically different to what espresso machines give today (ESPRESSO SYPHON!) But as it is now its a bit of a mix between high pressure at the top and not so much through the rest so we get the oils and the liquids in our cup.

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    Senior Member Dennis's Avatar
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    Re: So why 9 bar?

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    Its my understanding that the notion of 9 bar/92 degrees as the ideal pressure and temperature for the extraction of espresso was decided upon in the late 40s or early 50s.

    The decision was based on the preferences of a sample group. I dont imagine that this sample group was great in number, and the way coffee was roasted then, is certainly likely to be different to how it is commonly roasted today.

    In other words, we still use measures that were determined to be ideal more than half a century ago despite using a different roasted product. It makes me very pleased to hear of people experimenting with different pressure and temperature profiles, though I am not a fan of over-complicating the process. In fact, I cringe if I see a barista with their ear glued to the brew chamber of a Strada waiting to hear that moment in time they consider it ideal to increase or reduce the pressure coming through the coffee. :P ;D



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