Water chemistry and coffee extraction

Water as an ingredient

When you brew a cup of coffee, around 98–99% of what you drink is water. This makes water more than a solvent: it is the dominant ingredient by volume, and its composition directly affects how coffee solubles are extracted and how we perceive them in the cup.

Using water without the right minerals can make even an excellent coffee taste flat, dull, or strange.

GH and KH: the two metrics that matter most

Water composition can be characterized by many parameters, but for coffee the most relevant are:

GH (general or total hardness) Measures the concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. Expressed in German degrees (°dH) or parts per million (ppm) of CaCO₃.

Calcium and magnesium are the main extraction agents. They act as "bridges" that help soluble compounds from the coffee pass into the water. Very low GH (very soft water) has little extraction capacity: the cup can taste dull and bodyless. Very high GH can over-extract undesirable compounds or create metallic flavors.

The range recommended by the Specialty Coffee Association (SCA) is 50–175 ppm of CaCO₃ for total hardness, with an ideal point around 75–150 ppm.

KH (carbonate hardness or alkalinity) Measures the concentration of bicarbonates (HCO₃⁻). KH acts as a pH buffer: it neutralizes coffee's acidity during extraction.

High KH can mute coffee's acids and make the cup taste flat or dull. Very low KH can leave the coffee with too much acidity and sour flavors. The recommended range is 40–75 ppm of CaCO₃.

pH: the indirect effect

The pH of water (the acidity/alkalinity scale) is related to KH. Water with a pH of 7 is neutral. Above 7 it is alkaline; below 7, acidic.

For coffee, the ideal is a pH close to neutral, between 6.5 and 7.5. Very low pH can amplify acidity too much. High pH (very alkaline water) usually mutes nuances and gives a flat profile.

The relationship between KH and pH is not linear and depends on other ions present, but in practice KH is the most useful indicator because it directly measures buffer capacity.

Tap water vs. bottled water vs. remineralized water

In many cities, tap water has a variable composition that can be problematic for coffee: it may be too hard, contain chlorine or chloramines, or have very high alkalinity.

The most common practical options are:

Bottled water Some brands of mineral water have a composition close to the ideal range for coffee. Evian, for example, has a high GH that some baristas use blended with soft water. It is a simple solution but varies by brand and region.

Filtered water Activated carbon filters remove chlorine and improve taste, but do not modify minerals. Reverse osmosis filters remove almost all minerals: the resulting water is very soft and needs remineralization.

Remineralized water The most precise solution: you start from very soft water (osmosis or distilled) and add controlled mineral salts. You can prepare it at home with salt blends available from specialty stores. This lets you precisely adjust GH and KH to the desired values.

How water interacts with the brewing method

The impact of water is not equal across all methods. In general, methods with a higher water ratio (filter, cold brew) are more sensitive to water composition because water has a greater proportional presence in the final drink.

In espresso, water also matters, but contact time is so short and pressure so high that the effects of GH and KH are perceived differently.

Immersion methods like French Press or AeroPress have more contact time, which can cause high-KH water to buffer more of the coffee's acidity.

The simulator and water

In coffee-sim, advanced mode allows you to enter GH and KH values. The simulation engine uses them as secondary modulators on the extraction index and sensory axes.

High GH values tend to increase estimated body and shift extraction. High KH values reduce perceived acidity in the result. It is not a complete physical model, but it reflects the most important qualitative trends.

Experimenting with those parameters in the simulator can be a good starting point before changing the water you use at home.