Pressure Profiling Update 3
I think I'm homing in on some concrete information about pressure profiling that is worth blabbing on about, so here goes:
First thing is that there seems to be a practical maximum pressure value beyond which things go to hell in a handbasket tastebudswise. I found this out when I was experimenting with pressure profiles that included a high-pressure hump at the beginning of the extraction. Values for the hump were as high as 165 psi (11.3 bar). My thinking was that if sweetness was extracted early in the brewing process, then maybe I could emphasize it by increasing pressure to a high value early on, reducing to more conventional values later. The result was that I extracted crema that was markedly bitter. Bitterness was muted by reducing the magnitude of the hump. Bitterness was removed once the max pressure value was reduced to 9.5 bars at the coffee cake, which adds credence to the conventional 9-bar wisdom. I learned later that Jim Schulman has also observed this effect. I also successfully replicated the effect for Peter Lynagh, of Terroir Coffee, when he came down to visit.
Once I learned that there was a maximum practical brewing pressure, I began to think of pressure profiling in terms of the minimization of undesirable tastes, rather than in terms of super-extracting desired compounds. Taste tests with Nick Cho demonstrated to me that the sweet tastes and mouthfeel are developed early in the brew process, with dilution occurring later. Unfortunately, in the tests mit Cho (gesundheit!) extraction of bitter compounds also occurred during the dilution phase, meaning that a balance needed to be struck between dilution of the drink to a desired volume and introduction of negative taste components. Reducing the brewing pressure as brewing progresses seems to be beneficial in reducing bitter tastes extracted during the dilution phase. Tests with Nick pointed to accentuated sweetness in Counter Culture Toscana, which in retrospect was really a subtraction of bitter components added in the last third of the brew process.
The most successful general profile that I have come up with to date (I've now got an excel file with a bazillion profiles mapped out in it) is a pressure profile in which the pressure rapidly increases from some nominal start value to 9 bars at the group (elapsed time of 1 second from start to full pressure) with a short 9 bar soak, then ramps downward in a slow linear fashion (straight line degradation) to around 7 ½ bar, with a more rapid, second-order (curved downward) slope over the last few seconds to around 6 bar, arriving at 6 bar 30 seconds after initiation of the brew cycle. This general shape can be used with or without a pre-infusion step. The pre-infusion that I've been using is to soak the cake at around 2 - 3 bar for 3 seconds, then quickly ramping to maximum value with an exponentially increasing slope. Full pressure is attained 3 seconds after the end of the low-pressure soak. This combination produces liquid evenly over the bottom of the filter basket almost as soon as the pressure begins its rapid increase. Regardless of whether or not pre-infusion is used, I've been using a very similar profile toward the end of the brewing period. If pre-infusion is not used, I increase the time of the high-pressure soak by a few seconds.
It seems to me that the efficacy of preinfusion is coffee specific. There are three basic coffees that I've inflicted pressure profiling upon and with which I can comment. I've been drinking dry-processed Ethiopian SOs, and Ethiopian-based simple blends that have a lot of mouthfeel and body. My tests with Nick used Counterculture Toscano. I don't really know what is in it, and for the purposes of this discussion I don't think I really need to know. And last weekend, Peter Lynagh and I concentrated on a very lightly roasted Brazil. Here's a link to Terroir's information about the coffee:
http://info.terroircoffee.com/content/view/17/2/
For brewing temps, the Ethipian DP and Toscano seem to like around 201F. The SO Brazil was brewed at 195 F. The blends seem at first blush to benefit from preinfusion. I think that the soak and subsequent rapid pressure ramp may produce more mouthfeel, but I need to revisit this as I haven't been systematic enough, particularly in light of our results with the Brazil. We found that pre-infusion was a waste of extraction time when brewing the SO Brazil. Unlike the blends, the SO Brazil produced two predominate tastes with great clarity - nuts and sweetness reminiscent of dried figs. Both nuttiness and sweetness were enhanced when the pre-infusion step was removed. The clarity of the Brazil made differences between brewing at constant pressure and brewing with profiled pressure easy to discern. Bitter tastes were evident with constant-pressure brewing and demonstrably removed by profiling.
Recently folks have come up to me on the street, shoving their pudgy fingers in my puny chest, demanding to know that if pressure profiling meant deemphasizing the last portion of the brewing process, why not just stop brewing early? After I imagine breaking their finger with a deft marital arts-type motion, I counter with the argument that early brew termination is different. Compare two shots of the same volume, but with one brewed in the style of brewtus interruptus, and the other brewed to satisfying completion with pressure profiling. If one terminates early, for example at 18 secs in a 27 sec extraction, the volumetric flow rate during the 18 second period is much faster than the flow rate for an extraction taking place over the entire period. The extractions that we are observing with profiled pressure have more or less constant flow rate throughout the entire time period, which means that the volumetric flow rate is less by 50%, but over a longer time period. This changes the taste.
As I try different coffees and gain more experience I'm getting more confident that pressure is worth exploring as a brew parameter. There is still a lot to learn here, but the tests with Terroir indicate that variable pressure is useful when one is brewing clean SO espressos in which one or two tastes are showcased. I'm not sure which cart drives which horse when it comes to blends. I don't know if various widely used pre-infusion schemes were developed because they work with traditional blends, or if blends end up being developed to mask machine deficiencies. My cynical self thinks it's the latter. I have a lot more to learn about this, and it gets more difficult when I'm using coffees I've munged together. I don't feel like I'm a good enough roaster or blending dude to come up with confident conclusions, other than to go back with what I learned about the Brazil SO and see if I can make any hay with the stuff I usually drink. That means I'm not close to closure, which means that if you all don't behave, rather than dislocate your digits I'll spring Pressure Profiling Update 4 on you.
. I think I get what you're saying, though... Good thing I had already tasted the Reserve Sixtine before deciding to serve it. So, we'll see 
