marlap Senior Member Joined: 5 May 2012 Posts: 48 Location: Montreal Expertise: I love coffee
Espresso: Starbucks Barista Athena Grinder: Braun KM 30 (modded)
Posted Sat Sep 15, 2012, 8:04pm Subject: extraction pressure
I'm trying to understand what effect the extraction pressure has on the flavor of espresso.
I read that the ideal espresso extraction pressure should be around 8.5 to 9 bars. If so, why do so many manufacturers boast that their machines deliver 15 bars of pressure? If they indeed are 15 bar, does that mean that they must use an OPV valve to lower the extraction pressure in order to obtain a proper extraction? Why don't they just use a pump that delivers 9 bars of pressure, and brag that their machines deliver an optimal extraction pressure instead?
Posted Sat Sep 15, 2012, 8:48pm Subject: Re: extraction pressure
Manufacturers tend to use the rating of the pump to characterize the machine. There are several factors at work here that keep that "15 bar" pump from delivering its full rated pressure. One is that pumps are generally rated at their peak pressure, not their constant pressure. Another is that the portafilter full of packed coffee grounds is, in fact, permeable, and hence there's a built-in 'leak' that keeps the pump from building to its full pressure (unless, of course, you choke the machine). Third is, yes, the pressure relief or OPV.
Having that extra capacity in the pump is also like having sufficient headroom in an audio amp. Most modern speakers are efficient enough that you can get seriously loud on a single watt of power (or less), but to keep the amp in a linear mode (ie. what goes in is what comes out, only louder) you want a lot of headroom so that sudden low frequency transient that needs a lot of power (comparitively) to move the mass of the speaker cone doesn't drain all the power from the rest of the system and turn it into mud.
NobbyR Senior Member Joined: 10 Jul 2011 Posts: 1,969 Location: Germany Expertise: I love coffee
Espresso: Poccino Opus One, Ariete Grinder: Eureka Mignon Istantaneo Vac Pot: N/A Drip: Melitta Linea Unica de Luxe Roaster: N/A
Posted Sat Sep 15, 2012, 9:07pm Subject: Re: extraction pressure
Advertising (theoretical) maximum pump pressure simply has become a marketing instrument to lull uninformed buyers. All you need to properly extract an espresso is around 9 bar, but that doesn't sound as cool as 15 or 19 bar. It's somewhat like the pixelmania we see in digital cameras: more pixels don't necessarily mean better photos.
*** "This drink of the Satan is so delicious that it would be a shame to leave it to the infidels." (Pope Clement VIII on coffee, when he was urged to ban the beverage)
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