My coffee is stealing my coffee! A few notes on absorption

Lately I’ve been brewing with the Eva Solo during the day and the Aeropress at night for an evening cup once the kids are down. As I’ve mentioned before, my usual practice with the “Aeropress + DISK Fine” combo is to stop pressing when I hear the hiss, i.e. I don’t squeeze out that last bit of liquid. The idea is to not force too much sediment through the metal filter, though I’m not sure it matters much.

Either way, what’s important for this post is that two nights ago I got curious about how much coffee I was forgoing by stopping the press prematurely. Out came the scale, and up went the eyebrows. The answer was 35.3 grams, or 16.8% of my total brew weight. In other words, 210 g. of water entered the Aeropress, but only 174.7 g. of coffee exited. Now I should quickly hedge by admitting that it’s unclear how much of this loss was due to my not plunging all the way vs., say, absorption by the coffee grounds. 

I realized I’d never paid much attention to absorption, so I spent a few minutes searching online and found that the issue is well-known but not often seriously discussed. One exception—and there are surely others I missed in my half-assed “research”—is this 2-year-old post by Nick Cho, at the end of which he cites the following rule of thumb in a footnote:

“Coffee grounds generally retain water in the amount of double the mass of the grounds (i.e. 20 g. of coffee will retain 40 g. of water).”

How well does this rule describe the absorption I observed? Very well, it seems. My Aeropress dose was 17 g. for the cup described above. Double that and you’ve got 34 g., which isn’t far from the 35.3-g. gap I experienced. Cool.

I decided to do the same sort of before and after measurements with my next two Eva Solo batches:

Eva Solo batch #1: 23.5 g. coffee; 364.5 g. water in, 289 g. coffee out; 75.5 g. lost, i.e. 20.7%
Eva Solo batch #2: 23.5 g. coffee; 360.5 g. water in, 279.6 g. coffee out; 80.9 g. lost, i.e. 22.4%

Twice the dose would be 47 g., and yet to my surprise absorption was much higher in both cases—more than triple the mass of the grounds, in fact (at 75.5 and 80.9 g., respectively).

Why should this be? I did pour from both sides of my Solo in order to coax as much liquid from the carafe as possible, so technique isn’t the problem. It’s probably not the coffee either since I chose different beans for each batch. In comparison, the “drip” from the Aeropress is assisted by light pressure on top of gravity, which could certainly account for some of the difference. I’m curious now to check what level of absorption I get from a Clever or pourover cone since I’ve been in the habit of weighing the water I use to brew but *not* weighing coffee output with these methods as well. Taste isn't a problem, as I have no complaints with what I’m drinking; rather, I’d just like to understand what’s going on. Any ideas?

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