Andy, somehow I didn't see your post til now. Glad you chimed in again.
Andy Schecter wrote:What's interesting about this passage is that the extraction pattern outlined in the book for espresso is the opposite of the extraction pattern you say that the Chemex produces.
Thinking about this, it seems possible that when a Chemex pot leaves the grounds near the top high and dry, it's a good thing: these grounds don't get overextracted as the upper layers might be in a typical espresso extraction.
Andy, I think it is quite unlikely the high-and-dry grounds is a good thing. The difference is that espresso is a top-down-only extraction. The pressure gradient ensures that the flow of water is very much top-to-bottom (for those of you not familiar with this concept, instead of an espresso machine pumping pressurized water onto the grounds, imagine there is an incredibly strong vacuum sucking the espresso from the bottom of a naked portafilter. That is essentially what the grounds and water in the basket feel due to the pressure gradient), with no slurry of stuff mixing around, and certainly no opportunity to stir or alter the flow patterns.
In Chemex and other drip brewing (I'm just going to call all automatic and manual gravity-powered percolation methods drip for this post) methods, on the other hand, the water may enter the top and exit the bottom, but what happens in between is quite different. Unless you were to use extemely coarse, very old grounds (i.e. no gas to create turbulence) the water will dwell in the slurry for some time. The slurry is also turbulent because the water from the sprayhead or pouring vessel agitates the slurry. Because of this turbulent-slurry effect, the water is not just going straight from top to bottom and the grounds are shifting their locations in the coffee bed, so there is not a definitive set of 'top' or 'bottom' grounds.
But in the case of espresso extraction, the top always extracts more than the bottom (in a cylndrical basket, anyway) because the top always gets the fresher water, which is a more effective solvent.
In the case of drip brewing, the amount a given section of grounds will extract, relative to other sections, is based on several factors (I'm going to stick to just the biggies here):
-temperature (let's call that almost-irrelevant for this discussion)
-the concentration gradient in that area (affected by how clean the water is and how much solids are in the grounds)
-how much contact time the grounds in that area have with the water.
In espresso extraction, the concentration gradient is dominant in determining the relative extraction of different areas of the coffee bed. in drip brewing, the contact time is the dominant factor.
In drip brewing the relative concentration gradients of different areas are hard to figure out due to the turbulence in the slurry. However, proper stirring at the right times can help rebalance the concentration gradient to limit extraction favoritism of one area over another (also mentioned in that book, I think. I no longer have it memorized!)
But the most important factor with Chemex or manual-drip cones, at least according to the combined efforts of my taste buds and my ExtractMOJO refractometer, is that the dwell time of the high-and-dry grounds may be as little as HALF the dwell time near the bottom of the filter. For instance, in James' video (James, I am not picking on you; I just have no other video to reference right now) the total extraction time is 3 minutes and 45 seconds but a mass of grounds starts sticking high-and-dry 2 minutes and 20 seconds before the bottom is done extracting! An area of grounds that spends half as much time extracting will extract less than an area extracting for twice as much time.
This is why it is important to not let any grounds stick up on the filter walls. Even in my Abid brewer, with a 1:00 dwell time and a mere 1:00 drawdown time, if I let a small mass of grounds stick up around the rim of the filter, I can predict the brew strength will drop by 0.1%. (roughly the same as extracting 1.4% less of the total dry grounds mass.) A biggish mass, and brew strength will drop by 0.2% or more. God forbid the entire filter wall is coated, as it is in most peoples' Chemexes, and total extraction levels drop precipitously. My Abid coffee is full, round, and balanced ONLY when no grounds stick to the sides of the filter, and all of the grounds are in the bottom dome-shaped mass.
As for many people who have enjoyed Chemexes, I will posit a new possibility: If you overdose (in other words, use much more grounds than the standard 17:1) enough and use a coarse enough grind, pour in many stages, and stir appropriately you can avoid the astringency of overextraction, and you can get a clean-tasting cup. BUT you will definitely have sacrificed sweetness and roundness, and will have wasted grounds in the process. Much like a Clover, the Chemex design is flawed, and the only way to create a non-astringent, reasonably decent coffee with standard brew strength is to overdose, underextract, sacrifice some sweetness, and waste a lot of expensive coffee.
I will now hunker down and prepare for you to shred all of my pseudo-scientific assertions.
-S. Rao, charter member of the A. Schecter Fan Club