The TDS tests may be of the greatest interest to Home-barista readers. The test methodology was blind tasting by six tasters at the SCAA Lab in Long Beach. From page 31:
In a tasting conducted by the Technical Standards Committee of the SCAA, coffee was brewed with different levels of TDS to determine if significant flavor differences existed and how much difference actually existed. ... The same coffee, grind, and brewer were used and the same standard combination of minerals was used. The only difference was the concentration of the minerals in the brewing water. The first tasting was conducted using three water samples: one contained TDS at a level of 45 mg/L, one at 150 mg/L, and one at 450 mg/L. The coffee that was brewed with 150-mg/L water was chosen as far superior by all who judged the coffee.
A second tasting was conducted using 125 mg/L, 150 mg/L, and 175 mg/L samples to determine if minor variations in water quality would have an effect on flavor and extraction. The minor changes in the TDS of water were unanimously discernable by the panel. Acid and body balances were perceived to be off at both 125mg/L and 175mg/L TDS, and the 150 mg/L TDS brew was rated superior.
Different treatment methods are discussed, with only one being flatly condemned: the cation salt exchange method commonly sold with home espresso machines. From page 37:
The flow-rate problem through the bed of coffee is further compounded if the water has been treated by a zeolite [cation] water softening system, the most common type of water softening system, and which is required to be regularly recharged with salt. Through ion exchange, this process replaces the minerals in the water—principally calcium and magnesium—with sodium. When combined with the bicarbonates already in the water, the sodium bicarbonate forms a shiny, slimy material that binds the coffee particles together and blocks the passageways through which the water would normally flow. As shown in Table 7, this extends the brewing time, thereby causing over-extraction of the flavoring material and leading to excessive astringency and bitterness. As a result, brewing coffee or espresso with softened water is not recommended. [Emphasis added]
Table 7, shows the increase in extraction time that cation exchange causes in seven different cities, with the hard-water regions of Dallas and Los Angeles showing the greatest impact (over 40%).
Finally, I'm only reporting here. I have never used a cation system and have no personal basis for judging them.
The Handbook is available from the SCAA Store. http://www.scaa.org