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Water makes up around 98 per cent of a cup of espresso and over 99 per cent of a pour-over. The minerals dissolved in it are not neutral carriers — they actively participate in extraction, affecting which compounds are pulled from the coffee and how they taste. Tap water that is too hard scales your machine and mutes flavour. Water that is too soft over-extracts and tastes sharp. Getting water right is one of the highest-leverage changes a home barista can make.
TDS measures the total concentration of dissolved minerals in water, usually expressed in parts per million (ppm) or milligrams per litre (mg/L). The SCA guideline for brewing water is 75-250 ppm, with 150 ppm as the target. Extremely low TDS water (below 50 ppm) — including distilled and reverse osmosis water — extracts too aggressively, pulling harsh and astringent compounds. Very high TDS water (above 300 ppm) is already so mineral-laden that it extracts less from the coffee, producing flat, muted cups. You can measure TDS cheaply with a pocket TDS meter.
Calcium and magnesium are the primary minerals that extract coffee compounds. Calcium contributes body and a slightly heavier mouthfeel. Magnesium is particularly effective at binding with aromatic compounds — research from the University of Bath suggests magnesium-rich water extracts more of the flavour-active molecules responsible for fruity and floral notes. Most home brewing water targets 50-70 ppm of calcium hardness and 10-30 ppm of magnesium. Products like Third Wave Water or the Barista Hustle water recipes provide precise mineral ratios you can reproduce with distilled or reverse osmosis water.
Bicarbonate (measured as alkalinity or temporary hardness) acts as a buffer against acidity. High bicarbonate levels neutralise the organic acids in coffee, making it taste flat and reducing perceived brightness. SCA guidelines recommend bicarbonate below 40 ppm. The problem is that bicarbonate is also the dominant mineral in many hard tap water supplies — areas with chalk or limestone geology often see 200-300 ppm bicarbonate, which is far too high for brewing and also causes the scale buildup that damages heating elements. A filter or water recipe that controls bicarbonate separately from general hardness is the most effective solution.
Distilled and reverse osmosis water contain virtually no minerals — they are essentially pure H2O. For espresso machines, this causes two problems. First, pure water extracts too hard: without the buffering and extraction-moderating effects of dissolved minerals, flavour compounds are pulled too aggressively and the cup tastes harsh and astringent. Second, some machines use water hardness sensors or flow-control mechanisms calibrated around a minimum mineral content — pure water can confuse these systems. The practical fix is to use RO water as a base and add a mineral concentrate to reach the right composition.
For most home baristas, the simplest approach is to check local tap water TDS with an inexpensive meter. Below 150 ppm and low bicarbonate means tap water is probably fine. Above 200 ppm or with high hardness, consider a Brita filter (which reduces temporary hardness and chlorine but not all minerals), or use a 50/50 blend of filtered tap and bottled still water. For precise control, Third Wave Water sachets mixed into distilled water give a known mineral profile. The Barista Hustle and Lotus Water recipes are freely available online and can be made with food-grade mineral salts if you want full control without ongoing cost.
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