The water is the unsung hero of a cup of coffee. In the South Bay, we've been dealing with hard, heavy water for a while - some trick filtration has allowed us to dial our water into the SCAA spec. of 150 parts per million total dissolved solids and our coffee has done excellently at that standard.
The concept for Radio began after our return from the roaster's guild retreat about a year ago and conversations we had there about the Scandinavian roast style and water available in the region - typically glacial melt water of low gravity/TDS. We began discussing regional roasting traditions, those water sources, and what possibilities existed if you could dismiss as a given, what water you were working with.
In short, what we've found is that the shorter the roast, the lighter* the water we want to use, and the longer the roast, the heavier the water. In light roasts, the lighter water seems to be able to better pull sweetness from the bean - for instance, the Radio coffees are dropped between thirty seconds and a minute after the start of first crack which continues, rolling, in the tray for a bit. We like to brew these coffees at about 60 PPM TDS, with water immediately off the boil, and for espresso, we're pulling shots at 20 PPM, with water coming out of our group at about 95 Celsius (203 F).
The inverse seems to hold true as well - when dialing in the espresso roast of last year's harvest of La Maria, we took one batch into the neighborhood of Second and were brewing really excellent, lively cups of it at around 450 PPM and about 85 C (185 F). The coffee had all of the positive qualities you would expect to find in a darker roast: thick, chewy body, high perceived sugars-browning sweetness and heavy spice qualities, but also lacked the pungent, smoky, fuzzy funk that you typically would expect when brewing coffee at that degree of roast and presented a very surprising liveliness.
What's interesting about using water gravity as a parameter is that it seems to allow us, in some cases, to brew the same coffee the same way, changing only the mineral content of the water and allowing us to see different sides of the coffee; until now, we've typically thought of dialing in an extraction to be an optimization problem - that a given coffee and a given brew method had an "optimum" set of parameters (extraction temperature, grind size, total water contact time, agitation, etc.) and that all you could do was taste the coffee and try to hit those optimum values. What we've found using water gravity as a parameter, is that the coffees are sometimes lends itself to variations with this parameter, and present balanced extractions at different values; we like to brew the Radio coffees at about 60 PPM TDS, because that tends to produce a sweet cup, with lots of round fruit acidity and intense character, whereas when we brew closer to 20 PPM the cup is typically more soft, tea-like with an almost demure acidity and lighter body, and closer to the SCAA spec of 150 these coffees are really wild - screaming acidity (although when brewed hot and long enough we're still able to brew balanced cups at higher gravities) and very densely packed flavors.
We’re certainly not claiming to be experts, or know the exact channels of effect the different water gravities or specific water chemistry have on extraction, but we’re excited about being able to explore the possibilities that are presented with a little more attention to the water we’re using.
*There’s lot’s of good science on water quality as it pertains to the extraction of coffee. We’ve done a brief survey of the literature available on the subject, most of which is beyond the scope of this project - so far we’ve exclusively dealt with the amount of dissolved solids, holding the actual makeup of minerals coming in as a given. Calling higher or lower TDS water “Hard” or “Soft” is a bit of a misnomer, as they are terms that refer more to the balance of double (Calcium/Magnesium) and single (Sodium, Potassium) charge cations; for the purpose of referring to water with more or less total dissolved solids, we've taken to using the concept of water gravity (we’re all home brewers over here) and while we’re not measuring our water with a hydrometer, the concept has served us.