(Note: this is crossposted on alt.coffee)
RAMBLING INTRODUCTION
I'm (Jim Schulman) visiting Ken Fox in Idaho to do some skiing, wine imbibing, and Cimbali tweaking. We'll be focusing on preinfusion:
On his vibe pump pourover machine (Cimbali Junior S model circa 1996) Ken has modified the vibe pump's OPV so that the brew pressure now ramps up from 0 to 9 bar fairly linearly over a period of 7.5 seconds. With regard to taste, a few preliminary shots confirmed that this system works similarly to the E61's; if the espresso starts flowing before the ramp up is complete, the shot will not be as good.
On the rotary, he's followed the advice of Michael Teahan and installed a delay timer on the pump, and also has a pressure regulator on the inlet mains water. The result is that one can expose the puck to water at pressures adjustable from about 1 bar to full mains pressure (~3.5 bar here), over a period adjustable from 0 to 10 seconds. This is a simple mod that can be done on any plumbed-in machine.
We cannot test the ramp-up preinfusion on the vibe, since the effect depends on the home made soft gasket Ken made for his squeaking OPV. We will test the mains pressure preinfusion on the rotary, since this is a mod that can be applied to any plumbed in rotary machine. We plan to pair shots from the rotary at various settings to the vibe machine's shots using a simple better/worse scheme for about 10 to 15 paired shots. If the scores from a preinfusion setting significantly beat the no preinfusion score, we have a winner.
(Ken Fox here) Those who may remember our earlier "Tale of Two Juniors" post can recall that attempts were made to adjust the two machines to similar extraction temperature and pressure settings, and then paired shots were tasted, attempting to discern if similar machines made different shots based primarily on the type of pump contained within them. Our results showed no definable differences that either of us could taste. These test shots were made with standard, spouted portafilters. Whenever a machine produced an obviously "bad" shot, both of the simultaneously pulled shots were pitched and whichever one of us was serving as "barista" at that time made another paired set. We did not record our bad/sink shots and they were not counted. One of the theoretical benefits of preinfusion on a rotary machine, which would otherwise have a very fast pressure ramp-up, is that shot consistency will improve and sink shots will decrease. Therefore, this time around, we intend to serve the paired shots whether or not they are good, and the sink shots will therefore count against the offending machine.
So, if there IS an improvement in shot consistency with preinfusion in a rotary machine, it SHOULD show up in the measured results when compared to what was obtained without preinfusion.
(if you have any thoughts about the testing protocol, please post immediately and we will consider your suggestions)
Today we decided to get down to preliminary basics, before doing our formal blind taste testing of shots produced. As a start, we wanted to make sure the pressure and timing of the preinfusion were reasonable. We also wanted to make sure that if there was a range of reasonable settings, we would investigate them all.
METHOD FOR TESTING PREINFUSION PARAMETERS
So what's "reasonable" for a preinfusion?
After a lot of arguing, we decided that two criteria were inherently logical:
1. That the preinfusion pressure and length were sufficient to wet the entire puck.
2. That this complete wetting occurred before any liquid was released from puck, i.e no drips falling off the naked PF basket before the rotary pump kicks in.
People may disagree (please post any thoughts), but this seemed about right to us.
We interrupted the wires to the rotary pump, so that when the shot button was pushed the input solenoid opened, but the pump never ran, and timed how long it took for the first darkening to appear at the bottom of basket as viewed with a naked PF. At this point we stopped, knocked out the puck, and examined it. The installed input pressure regulator was capable of producing about three repeatable and unique input water pressures that we could measure; 1.5 bar, about 2-2.5 bar, and about 3 bar. We repeated this three times at each (regulated mains water) pressure level with the grind set so it produces a properly timed shot. The majority of this testing was done with decaf beans due to a temporary excess supply of fresh decaf, but were confirmed with fresh regular coffee blends as well. Since no tasting was done in these shot sequences, we decided to reserve the (better) caffeinated bean stash for shots we will be blind tasting.
RESULTS
First off, there were no significant differences with the caf and decaf; the caf beans required about 1/2 second longer for the bottom of the basket to darken. For all the different pressures, the darkening times were within 3/4 second for all trials
1.5 BAR: It took around 17 seconds to see the bottom of the basket darken. The puck was not saturated at this point. Instead, the water had seeped down the perimeter of the basket and soaked into the puck sideways. Since there was more seepage time at the top, there was a cone of dry grounds at the center of the basket. Different tamps and tampers produced the same result.
2.25 BAR: It took 8 seconds on average to see the bottom of the basket darken. Again the puck was not completely wet and had the cone of dry coffee in the center.
3 BAR: It took 6 seconds on average to see the bottom darken. In most cases, the puck was completely wetted, but we did see a few small scattered dry patches on one shot.
Photography of pucks at each input pressure was attempted but as you can imagine this sort of subject matter is not very photogenic:-) The image following was obtained after a 2.25 bar preinfusion, but looks very similar to those obtained at 1.5 bar. The pucks obtained at 3 bar look similar to the pucks you expel every day after pulling your daily espressos; fully or almost fully wetted and with a wet and soft consistency. (The feel of the wetted puck is different from a fully extracted one, more reminiscent of sponge cake)
A 2.25 bar preinfusion puck
CONCLUSIONS
Times change, grind and shot pulling technologies change. The classic lever preinfusion of 1.25 bar for 10 seconds may produce drips and a completely soaked puck on a lever machine, using coffee ground appropriate to such equipment, but it doesn't do anything except ruin the puck on a pump machine. Our observations make sense of Andy's not being able to get a low pressure preinfuse to make good shots at his recent Gimme stint. In fact, given that the puck is left half dry and half wet, the shot is likely to be worse than a no preinfusion shot. I believe Barry once posted that he was unable to get good shots (on an LM?) using the classic 1.25 bar preinfusion. Again this result explains it.
Going to to 2.25 bar got us a surprise. The timing is close to the 3 bar one, but poor wetting was exactly like the 1.5 bar case. Again, the seepage was down the sides.
At 3 bar, good stuff is happening, the water has enough pressure to go through the whole puck in a reasonable time.
We have no idea whether 6 seconds at 3 bar will work for all machines like it works for the Junior; other design factors, such as the distribution of holes in the machine's diffusion disk, may play a role. However, if you're setting up a preinfusion, you might want to answer this question - is the puck completely wet before the pump kicks in and it starts dripping? This is most likely a prerequisite for a positive impact of preinfusion on the quality of shots in the espresso cup.
