You've forced me to draw a picture! (Well, edit
a picture, shamelessly stolen from here
Below is beginning stage of a "typical" extraction where full brew pressure is reached prior to the basket saturating with brew water.
The small section above the grey line is the "Wet Puck". There is 9 bar on top and a dry puck underneath it. This thin and wet puck sits atop the dry puck below and presses on it with the full brunt of the 9 bar water pressing on it. There is nothing else supporting it other than the dry puck below, so the wet puck atop effectively tamps the dry puck below with a force equal to brew pressure times puck area. This Super-Tamp results in a very difficult life for the water as it tries to pass through the dry puck. I've created 3 fingering flows below the grey line, which can
be thought of as channeling. However, any time you see a spot or two of flow on the bottom of a basket, thisis what you're really seeing; it's the tips of the fingering flows before they spread throughout the rest of the puck, which they generally do. Water desparately seeks a path through the compacted puck and once it establishes flow, the flow through those areas first traveled increases. Generally, there are more than three of these fingers, and they coalesce relatively early in the shot. The thicker the wet puck, the better the extraction of the wet part, because unlike in the dry puck, the flow has no preferred path through the wet puck.
Even if the shot pours well, these early flows still extract first and they are the parts of the puck that blonde first and over-extract when you pull long shots. So, why does blooming "fix" this? Well, here's why. Suppose you limit the available pressure as shown below:
Then lets just wait and let the puck wick up the water above it and bloom with no flow at all. So, the puck soaks up some water and releases some CO2
. The CO2
rises up into the headspace and gently forces more water (from the headspace) through the puck. Additionally, diffusion allows the more dry areas of the puck to equalize with adjacent more wet areas and you end up with a relatively homogenous saturated puck. As long as your initial charge of water was sufficient to saturate the puck, I believe the puck will self-charge during bloom and ensure that at the end of the bloom phase, you will have a fully saturated puck with a relatively dry headspace of air and CO2
Now, you are free to extract your shot, but there are a couple interesting things to observe at this point. First, most of your shot is in the puck
at the end of bloom. Your job now is to extract
your shot from the puck and then dilute it to your desired brew ratio. Second, with a completely saturated puck, you can extract at just about any brew pressure you want to without fear of choking the machine. The water supports the physical structure of the puck and keeps it from collapsing on itself, which causes channels. While there's no such thing as perfection with respect to flow through a puck, starting with a completely saturated puck is a MAJOR benefit when it comes to channeling and obtaining a more even extraction. At this point, you start the extraction in earnest and introduce your solvent (the water) to the top of the puck and push out the filtrate (your espresso) from the filter (the puck, now evenly stripped of it's soluble compounds thanks to the long and arduous procedure of infusing the cake with an initial charge of water and allowing it to bloom and saturate and equalize until we're ready to extract.
This whole process is exactly why I prefer to time my blooms such that the basket wets by the end. If it wets mid bloom, I probably charged with too much water during the infusion process, or my grind was too coarse. If it doesn't wet during my planned bloom time, then the opposite of one of those conditions must be true. If I bloom longer and the basket wets, it means I ground a bit fine, but I had plenty of water in the charge. As long as I wait until the basket bleeds, the flow should not be choked, as the longer you wait, the "softer" the puck gets. While there are natural limitations in play here, I find that I can generally work within whatever the bloom tells me as long as it's 20 seconds or longer. Any shorter and I"m doomed with a gusher, which I can pull slower with reduced pressure and make it ok, but it won't be great... If I wait and wait and the basket never wets, that just means that I didn't get enough water in the initial infusion. No big deal, but not Ideal. Generally speaking this doesn't happen as I have plenty of tactile indicators to guide me in getting enough water in before I bloom. My machine is funny in that I can actually hear the heaspace filling up and there is a corresponding shift in the tone of the water through the needle valve that tells me when it's time to kill the flow.
The next natural progression here is extraction, which I hardly need to paint a picture for, but what the heck
As long as you don't botch the puck prep, you can pull this kind of a shot off with a rediculously fine grind and it will pull evenly at whatever extraction yield you want without getting those nasty over-extracted flavors coming through. Since the whole puck is extracted more evenly, there are no areas in the puck that get taken to that extreme level of -over-extraction that you get when areas of a dry puck fracture and channel. You can basically stop the shot after all the filtrate is stripped from the coffee puck and clear water is coming out and it will still be a good shot. Whether or not the coffee you are pulling tastes best when pulled like this is another question altogether. In general, anything taken near second crack is probably not going to be great when extracted this far, but that's not a hard and fast rule. What I can say is that I haven't found this approach to be preferable when pulling coffees that have any ashy or roasty flavors to them. Those coffees seem to be best pulled fast and cool.