Several readers recently requested a post explaining the differences in measuring extractions of immersion vs. percolation (drip) brews.  I have always wanted to write about this, but didn’t think anyone would want to read such a post :).   I’m glad I’ve got a few geeky readers out there.  

Writing this post got me to thinking, measuring, and questioning.  What I found was surprising, so I discussed it with my friend Dan Eils, my frequent partner in coffee debates.  I think Dan and I are onto something, and it brings into question the way we have all been calculating extractions.  This post wouldn't have been possible without Dan's help and our enjoyable weeks-long debate over these issues.  

Comments are welcome, and sharing this post with other coffee enthusiasts is also welcome, as I'd like to get feedback and ideas from many sources to gauge whether it may be worth pursuing some new extraction equations.

First, a little primer...

Concentration Gradient

When water mixes with coffee grounds, extraction occurs by two mechanisms:  first the water washes exposed solids off the ground surfaces, and then coffee solids dissolve in the water and move from the grounds to the surrounding slurry by diffusion (surface washing dominates espresso extraction, while drip and immersion methods favor inner-particle diffusion.) The rate of diffusion is determined primarily by the concentration gradient, the difference in solids concentration between the grounds and the slurry.   The higher the gradient, the more quickly extraction will occur. 

Immersion Brewing 

During immersion brewing, upon water contacting the grounds, the concentration of coffee solids in the slurry increases rapidly, and then continues to increase at a progressively slower rate, asymptotically, for the rest of the brew.  An example of this is how the color of coffee in a French press hardly looks much darker from a few seconds into the brew until a few minutes later.   Another example is how in cupping, brew strength doesn’t change much in the time window of 9:00–-20:00 in which you likely slurp and spit (the lower temperatures of a long immersion also contributes to the slowing extraction rate.) 

Brew and serve a French press, and you’ll notice that the strength of the coffee left behind in the grounds, the “interstitial liquid," is nearly identical to the strength of the coffee in the cup.  If you were to measure the drips from a French press after decanting the liquid that readily poured out, you'll find the strength of those drips would be approximately the strength of the liquid in the coffee served, or perhaps slightly stronger. 

Percolation brewing

Percolation refers to brews in which liquid extracts coffee solids by passing through the bed of grounds.  Just to be clear, I’m not referring to those nasty old “percolators” you may remember your mom breaking out for large gatherings when you were a kid.  And trust me, there are few more memorable aromas than baked-in, rancid robusta oils from a dirty percolator in 1977.   

In a percolation method such as batch brew, during most of the brewing cycle, fresh, clean water is added to the slurry while solids-packed liquid leaves the slurry.  That causes the slurry’s solids concentration to decrease throughout the brew.  So, while the strength of the slurry of a French press is always increasing, the strength of a slurry during percolation is always decreasing.  This is important, and is the primary reason percolation is a more efficient extraction method than immersion. 

Why your software has separate modes for drip and immersion

Standard percolation extraction measurement

This is a short version of what I was taught years ago: at the completion of a percolation brew, the slurrys TDS is extremely low, almost zero. Therefore, when considering how much coffee solids extracted during a percolation brew, you can more or less ignore the dissolved solids remaining in the slurry liquid.  That is why the VST Coffee Tools app (née Extract Mojo) uses the brewed-coffee weight as the multiplier for the TDS to determine total extraction-- almost all of the extracted solids are assumed to be in the cup. 

Standard immersion extraction measurement:

When measuring the extraction of an immersion brew, the liquid left in the slurry is full-strength coffee (or possibly a tad stronger).  That is why the software uses the brew-water weight to calculate the extraction of an immersion brew: the slurry liquid is just like brewed coffee, left behind: 

*Please note: this equation is slightly incorrect-- to be precise, you should increase the total water weight by the weight of the solids dissolved in the water.  For example, if the total water poured is 100g and final TDS is 1.5%, then the total-water weight should really be 101.5g, not 100g.  The difference in calculated extraction will be about 0.3% (eg. 20.3% vs. 20.0%).

Theory vs. Reality

Here's the surprising part: I decided to sample a few ml's of liquid from the spent grounds of several v60s and batch brews, and they measured anywhere from 0.8%--1.3% TDS.  For reference, the brews were all in the 1.3%--1.4% range.  This shocked me-- you know how the last bit of liquid flowing from the basket in a batch brew looks like very weak coffee?  That had always fooled me into not questioning what I had been taught about the slurry liquid TDS being extremely low.

Two liquids in the slurry

If you use Coffee Tools software, you've seen the term LRR:  liquid retained ratio.  LRR varies with brewing method: the coarser grind of a batch brew holds a touch less liquid than the finer grounds of a v60, and a siphon has a very low LRR due to suction literally vacuuming the interstitial liquid from the coffee bed.  For the sake of this discussion, let's keep it simple and assume a percolation LRR of 2g/g of dry grounds. 

It's speculative, but Dan and I agree that the liquid trapped within the grounds at the end of a brew is probably pretty strong, or at least stronger than the interstitial liquid.  The dissolved solids in the liquid trapped within the grounds are, by definition, not extracted, so they should not be a part of any extraction calculation.  Therefore, we may be overstating the extractions of immersion brews.  The relevance of the solids within the interstitial liquid is more debatable.  In my view, the solids in the interstitial liquid have been extracted, they just never made it into the cup. Given the revelation that the TDS of the interstitial liquid is rather high, if you believe the solids dissolved in the interstitial to be "extracted", as I do, then its arguable that we've been understating the extraction levels of percolation brews. .

A Proposal

For the five  of you who made it this far, thanks for hanging in there.  And I'm sorry you don't have better things to do than read my blog.  :)

I propose that we consider changing the way we calculate extraction: perhaps we should count the solids in the interstitial liquid but not the solids still trapped within the grounds.  The current equation to calculate immersion extraction counts the solids-packed liquid still trapped within the grounds, but it probably shouldn't.  Conversely, the percolation calculation doesn't count the extracted solids in the interstitial liquid, but it probably should. 

The original draft of this post proposed new extraction equations, adjusted to reflect the solids remaining in the interstitial liquid but not the solids dissolved in the liquid still trapped within the grounds.  I’m hesitant to share them publicly without first gathering some feedback about this post.  I imagine many coffee professionals won’t want to change the way they calculate extraction, and others may be excited by equations that would potentially harmonize the results of immersion and percolation brews.  Currently, a "20%" percolation and a "20%" immersion brew don't taste as similar as most of us would like.  If the new equations were to make the relationship of extraction numbers and coffee flavor more similar for the two types of brews, that would be a huge win.  If the numbers don't harmonize the flavor/number relationship, then it's probably not worth going to the trouble of adjusting the system we use to calculate extractions. 

I’d love to hear from you, even though there are no contest prizes on offer this time  :).