- a low pressure preinfusion for some amount of time
- a ramp to 9 bar pressure (or somewhere close)
- a slowly consistently declining pressure throughout the rest of the shot.
I'll lay out some aspects for discussion about pressure profiling in general and then describe some techniques I have been using / experimenting with to play around with different profiles. I am not claiming anything here is undeniable truth, this article is what I have gathered as a combination of things I have read, artisanal data (read: anecdotal evidence), and my own experiments and observations.
Pressure profiling observable effects:
As already described in Pressure profiling, flow profiling, and a new rule of thirds a fast pressure ramp will slow down the flow during extraction, a slow pressure ramp will cause the extraction to flow faster. This is easily observable with any machine that allows control of the initial pressure ramp, such as levers. Essentially performing a slow pressure ramp allows you to grind finer.
Pressure and flow are related: Increase the pressure and you will see an increase in flow. Drop the pressure and you will see a decrease in flow, or at least a decrease in the acceleration of the flow.
Pressure Profiling Proposed Effects (by machine manufacturers at least)
These proposed effects come from two main sources: the Slayer Instruction Manual and a paper I have called Variable Pressure Infusion Modification Results: Joint project between Mark Barnett, Synesso, and Herkimer Coffee. The Slayer effects are detailed here assuming a relationship between flow and pressure within the brew chamber, so take that as you may.
Long, soft pressure preinfusion: proposed by Slayer to increase sweetness. Synesso claims a soft preinfusion for about 4 seconds before moving to a mid range preinfusion creates the best acidity balance, but having a soft preinfusion for too long lends to an ashy taste due to more phenol extraction.
Declining pressure through the extraction: proposed by Slayer to mute acidity. Observed elsewhere to minimize bitterness as well. I find it counter-intuitive that the same declining pressure profile would both mute unwanted acidity and unwanted bitterness, but I have noted that it certainly serves to improve the flavor of the shot.
One-second initial pressure spike before dropping back down to soft preinfusion: Proposed by Slayer to enhance acidity.
Slow ramp preinfusion combined with pressure decline during extraction: Proposed by Synesso to provide the best increased texture / body and best balance between sweetness and clarity. However too slow of a ramp, according to Synesso, makes a shot where the sweetness overpowers everything else and clarity suffers.
Techniques for lever pressure profiling:
Lever preinfusion is rather flexible in that you can preinfuse as short or long as you want. You can also choose whether or not to quickly ramp to full pressure or to slowly ramp to full pressure, and then how slowly this ramp occurs.
Standard Preinfusion: Pull the lever down and preinfuse for the desired time. My personal experience is that darker coffees desire a shorter preinfusion and lighter coffees desire a longer one. I have noted though that too long of a preinfusion though can cause coffee to lose some distinctiveness. Depending on your setup you may have control over this phase. Does your group feed from boiler pressure? Does it feed from line pressure? What sort of control do you have over that? These variables might be worth playing with.
Slow ramp vs fast ramp: How quickly do you lift the lever back up? This determines the speed of the pressure ramp. A quick lift with cause a quick pressure ramp, a slow controlled lift of the lever will cause a slower pressure ramp. Observe how this affects the coffee under the same grind and dose.
Poor man's 2 stage preinfusion: The Synesso Hydra is known for its "2 stage preinfusion" concept, where there is a low pressure stage 1 preinfusion followed by a mid pressure stage 2 preinfusion before going to full pressure. This can technically be done on a lever as well. While lifting the lever stop at some point before it catches and hold it there for a few seconds. What you are really doing is another preinfusion at a pressure that is higher than your machine's standard preinfusion but lower than the full pressure of the spring. This has the very noticeable affect of increasing the body of the shot, especially for coffees with a notoriously weaker body. I have noted that for some coffees this increases the sweetness of the coffee as well. The disadvantage here is that unless you have a portafilter pressure gauge that works with a lever (i.e. needle valve to simulate water flow) you are kind of flying blind as to what this pressure really is. The advantage here however is that it is relatively repeatable. It is not difficult to hold the lever at the same angle once you find an angle that works well for the "stage 2" preinfusion.
During the shot:
Pull back: While the lever normally has a gradually declining pressure profile during extraction due to the spring, you can reduce the pressure through the shot by pulling back on the lever. This will slow down the shot if it is flowing too fast or if you want to increase the time of the extraction.
Push through: On some levers there is a direct connection between the lever and the piston, this allows you to increase the pressure beyond the spring's capacity during the extraction by pushing up on the lever. I have found that this is almost never desirable and leads to ashy tasting espresso with more developed roasts. It does however allow you to recover from a choker by helping it along at least until a stream is established. I will do it if the grind is just slightly too fine to help the flow along, then once there is a constant trickle let the spring finish what it was doing.
Lately I have been playing with a "two stage" preinfusion, inspired by Synesso, which is usually composed of a low pressure "stage 1" and a higher pressure "stage 2" before finally ramping to full pressure. This usually involves pulling the lever down and counting to 5 or 8, lifting the lever up slowly between 5 to 8 seconds to then hold it steady at about 20 degrees above horizontal. I then either hold that for 5 to 8 seconds, or until a gram of coffee has dropped into the cup, depending on how it is pouring, then I let it all the way up and let the spring take over from there. This is similar to Synesso's suggested profile.
Synesso's suggested profile
- Stage 1 preinfusion: 4 seconds at 3 bar
- Stage 2 preinfusion: 3 seconds at 6.2 bar
- Full pressure (8.5 bar for them) for 8 to 11 seconds
- Drop-back to 6 bar for 3 to 5 seconds (simulation of a spring lever profile)
- Traditional espresso blends: 5 second preinfusion then quick ramp, pull back if the flow starts going a little too quickly.
- Medium roasted SO's: 15 second standard preinfusion then quick ramp, pull back if the flow starts going a little too quickly.
- Light roasted SO's: the Synesso Hydra style profile as described above - 5 second standard preinfusion, slow ramp to about a 20 degree angle above horizontal, holding for 5 to 8 seconds, then full pressure.
My typical dial in procedure goes in this order:
- Find the right dose / grind and use my best guesstimate of pressure profile based on the coffee and roast level.
- Adjust the temperature to balance acidity / bitterness
- Adjust the profile to attempt to bring out desired traits of the coffee.