Coming on the heals of my last post/link (4 months ago!) this post may seem somewhat out of place...but pressure profiling and messing about with pre-infusion has peeked my curiosity. I believe that there is definitely evidence to show that a lower pressure period of 'pre-infusion' yields a sweeter more balanced and even extraction.
Infusion begins as soon as a solid is soaked in a liquid so I suppose it stands to reason that pre-infusion is the process by which the solid becomes soaked. As it relates to coffee (not bacon and bourbon...a very good infusion) arguably the most gradual and even soaking method will yield an even and balanced extraction; in combination with a score of other variables including water temp, grind consistency, dose and tamp.
Lever machines are the inspiration for what we are trying to do now with our electric pump powered machines. As the lever is pulled forward water is allowed to pass from the boiler to the brew chamber as the group spring is compressed. The time that it takes for the coffee to fill the small gap between the bed of coffee and the top of the piston would be considered pre-infusion time. The water entering this gap is at a pressure of about 1 to 1.5 bars (depending on boiler set point) but since there is a measurable space or gap there is a period of time that it takes to ramp from 0 to 1.5bar. This gradual increase of pressure gives the coffee grounds time to absorb the water it is being exposed to before the water is forced through the mass of coffee at higher pressures. Once the coffee grounds are fully saturated the lever is released and the tuned spring ramps pressure up to approx. 9 bar before slowly ramping back down as the spring relaxes. What results is (potentially) a really tasty shot of espresso.
What has been on my mind for the past 2 years or so has been how to (simply) do this on some of our current espresso machines. My first concept was a 'pre-portafilter' essentially a spacer that would fit into the group head, but then have a similar bayonet mounted to the bottom so as to receive the traditional portafilter. Essentially this concept adds a small space between the diffuser screen and the mass of coffee. If the machine could 'pre-infuse' at line pressure then there would be a slow ramp up during the time that it took to fill the spacer...and if only pump pressure then maybe an adjustment on the size of the spacer could be made. There may still be something to this idea but I can't say I really have the time or cash to prototype and produce such an idea...maybe someday!
(You can see my original thought was to have three lines...crazy!)
I'm not sure when or what caused my second idea to come together, but I don't really think it is anything new, necessarily. I haven't yet seen it specifically carried out anywhere, but it isn't rocket science so I imagine it probably has been implemented somewhere. It is akin to what Herkimer did here. But it is unclear to me how they are switching the pump bypass on and off...it may be on a timer which would be really cool and repeatable. What I ended up coming up with was installing a switched solenoid valve on a split water line. Mains pressure would go through the switchable (opened or close) valve and then using a pressure regulating valve (PRV) I'd reduce mains pressure down to 2.5bar or whatever one wanted. The two lines would meet at the pump. The pump would be calibrated to take mains pressure up to 9bar. My experiment, masked as a birthday present for Bradbury's (turned 3 earlier this year!), was designed for a Synesso Cyncra. The Synesso has a standard sized rocker switch on the front face panel to operate the hot water valve. I swapped the switch out for a Pavoni double rocker switch and had one side operate the hot water valve and the other one operate the solenoid valve that was installed on the mains pressure line.
With the paddle at stage one (brew valve open/pre-infusion) and the new switch at the off position the pre-infusion pressure would be running through the PRV and would be at the set point, 2.5bar in this case. With the paddle still at stage one the switch would be turned to the on position and the valve holding back mains pressure (5 bar in this case) would be energized. At this point you could add a third stage of pre-infusion with the paddle at stage two (pump pressure) and the new switch off so that the pump was increasing just the pressure regulated line yielding 7bar of pressure. Then you could turn the new switch back on, opening the mains pressure and increasing brew pressure to 9bar. 3 stages of preinfusion, let alone 2, is tough to standardize and repeat...and quite arguably not even necessary...but fun to play with nonetheless.
Josh,Jill and Ruby were kind enough to let me mess around with this concept and hopefully it will yield some fantastic results...so far the only measured result it has yielded for me was a caffeine/espresso induced fitful night of sleep and weird dreams (in which Josh and I were security guard baristas preparing coffee for a female Scott Walker speaking to a crowd of Epic employees...yah weird) I'm sure Josh will have alot of feedback and thoughts on pressure profiling after some 'lab' time with it. Anecdotely the shots we pulled that evening were quite nice.
My next thought would be to install timers on the switches (brew switch and mains line rocker switch) to control pre-infusion times more accurately.
As a side note as well to the Marzocco w/volume control users who don't already know: I think you should at least disconnect the pump lead from the manual switches and use those as your pre-infuse switch then hit the continuous pour button to engage the pump. A really simple modification that in my opinion would impact the flavor of your espresso in a really good way. I did this over at True Coffee in Fitchburg with great results as well. I didn't at the time, but the manual rocker switch could be replaced with the same Pavoni switch I used at Bradbury's to separate control of the brew valve and the pump...this would be great for the semi-auto Marzoccos as well. Linea's by the way....GB5 users are out of luck until they make a programming parameter that can switch the pump at user determined values. Or you could lime-up your GB5 pre-heater....but that is another story!