RO Water and Coffee: Why Mineral Content Changes Everything

Water is roughly 98 percent of a cup of coffee.
It is about dialing in a brew variable that most home baristas overlook entirely.
What Flat Water Does to Extraction
Reverse osmosis removes calcium and magnesium down to near zero.
The pressure and heat of espresso extraction amplify whatever the water chemistry is doing.
The difference is most noticeable in light roasts, where the flavor compounds are more delicate and extraction precision matters more.
Targeting the Right Profile for Coffee Brewing
This is higher than the general drinking water target of 50 to 150 ppm, since coffee benefits from a slightly more mineral-rich water source.
Magnesium is the more important mineral for coffee extraction.
For espresso,  reverse osmosis water filter and remineralization  adds body to the shot without reducing clarity.
Getting Mineral Content Right for Coffee
For serious home baristas, this level of control is worth the extra cost.
The limitation is that cartridges do not allow ratio adjustment, so you get whatever calcium to magnesium balance the media delivers.
Check the label of the mineral water for magnesium content in mg/L and blend to hit a target in the 80 to 150 ppm range.
Testing Your Coffee Water
A reading outside that range will show up in the cup before you taste it, usually as flatness below range or bitterness above it.
Remineralizing  remineralize water after reverse osmosis  for coffee is one of the highest-return adjustments a home brewer can make.