Why water chemistry matters for brewing

Coffee is roughly 98–99% water, so water chemistry shapes the cup as much as the coffee itself does. Four properties matter most. TDS (total dissolved solids) measures overall mineral content; water that's too low in TDS under-extracts and tastes flat and empty no matter how well the brew is dialed in, since there's little mineral content available to bind with and carry flavor compounds, while water too high in TDS can taste harsh or mineral.

Hardness specifically measures calcium and magnesium content — some hardness is necessary since magnesium in particular helps extract flavor, but too much accelerates scale buildup inside boilers and pipes. Alkalinity (often measured as buffering capacity or KH) resists pH changes; overly alkaline water can mute coffee's natural acidity and make a bright coffee taste flat, while very low alkalinity offers little buffering and can make brewing less consistent. pH itself measures acidity/alkalinity directly, with most brewing water sitting close to neutral. In practice, most specialty coffee guidance (including the SCA's) targets water around 75–250 ppm TDS with roughly 150 ppm as an ideal middle point, and general hardness in a moderate range as CaCO₃ — treating water as a variable to measure and adjust rather than assuming tap water is automatically fine.

Filtration approaches range from simple carbon filters (remove chlorine taste and some sediment, but don't meaningfully change mineral content) to reverse osmosis (strips nearly all dissolved minerals, requiring minerals to be re-added afterward for good extraction) to blended systems (mixing filtered and remineralized water to hit a specific target profile) — the right choice depends on how far local tap water is from target and how much control a venue needs. Espresso and filter brewing don't necessarily want identical water: because espresso is a much shorter, more concentrated extraction, some roasters and machines are more sensitive to water minerality and scale risk on the espresso side, which is part of why serious commercial setups often run separate filtration or monitoring for espresso machines versus general brewing water.