Why is magnesium key in coffee water?

Magnesium (Mg²⁺) is the most important mineral ion for coffee aromatic extraction. It specifically promotes the solubilisation of fruity and floral compounds, and has a higher affinity than calcium for the organic acids and aromatic molecules in coffee. The SCA recommends a minimum of 10 mg/L; optimised water recipes often contain 50-90 mg/L.

The importance of magnesium in coffee water is supported by scientific research conducted since the 2000s, notably by food chemistry teams comparing the extraction of different metal ions on aromatic compounds in roasted coffee.

The mechanism involves coordination chemistry: Mg²⁺ ions form complexes with carboxylic acids, chlorogenic acids and other polyphenolic compounds in coffee. This chelation process increases the solubility of these molecules in water, resulting in more complete extraction of fruity, floral and bright aromatic notes. By comparison, calcium (Ca²⁺) extracts the same compounds but with less selectivity, tending to contribute body and richness rather than aromatic finesse.

A frequently cited study within the specialty community showed that with high-magnesium, low-calcium water, an Ethiopian natural coffee could express strawberry, jasmine and hibiscus notes with clarity and intensity far superior to the same water depleted of magnesium. Conversely, excessive magnesium (> 150-200 mg/L) can create a sense of excessive lightness and reduce perceived body.

In practice, advanced water recipes — such as Barista Hustle's or Third Wave Water Classic range — favour a high Mg/Ca ratio, often above 1:1 and sometimes 3:1 or 4:1 in favour of magnesium. This is the inverse of most European tap water, where calcium dominates strongly (Ca/Mg ratio often 3:1 to 5:1).

The most commonly used practical magnesium sources in DIY recipes are magnesium chloride (MgCl₂) or magnesium sulfate (MgSO₄, also known as Epsom salt), available from pharmacies. It is important to note that both salts also introduce an anion (chloride or sulfate) that itself influences taste — sulfate tends to accentuate bitterness and dryness, chloride to round and soften. The choice of magnesium source is therefore an integral part of water recipe design.

Magnesium vs calcium: compared extraction effects