The Condition of the Grinds
If you wish to go further and start factoring the TDS of the interstitial liquid of a brew into your extraction calculations, then you have arrived at “the boundaries of coffee!” The world of advanced barista training hasn’t worked this out yet. This is a great example of what makes it so exciting to be a barista and to be a part of the first generation of specialty coffee.
A shift in thinking on this issue has recently occurred, explored in an influential blog post by Scott Rao. Scott identified that no one has yet instituted a reliable system for comparing the condition of coffee grinds at the precise moment a barista considers a brew to be finished brewing. You see, the interstitial liquid has some coffee extract in it, and even though it didn’t get into the cup, it still came out of the grinds.
There are a few approaches to best ascertaining how much TDS is in the interstitial liquid. The challenges here are three-fold:
If you dehydrate the brew liquid, it assumes the interstitial liquid in the spent grinds is just water. This is where Earl E. Lockhart signed off.
Spent grinds will reabsorb the extracted mass in the interstitial liquid if you dehydrate them.
Dehydration is a very slow and expensive way of measuring extraction, in spite of its reliability. A better way to apply this technology, in the opinion of BH, is to use dehydration to create a multiplier to compare drip coffees relative to immersion coffees. (As we conduct ongoing research in this area, we will notify you if we reach any interesting conclusions).