Introduction:Did you know? Over 98% of a cup of coffee is water, yet the choice of water is often overlooked. Different types of water—such as RO purified water, tap water, mineral water, soft water, and hard water—have a profound impact on coffee extraction efficiency and flavor expression. This article delves into the roles of calcium and magnesium ions and carbonate hardness in water, uncovering how they alter coffee extraction outcomes and guiding you to master the key to crafting ideal brewing water. Elevate your pour over coffee from the ground up in terms of depth and performance.
In the process of brewing coffee, water is the primary medium, making up over 98% of the final cup, yet it is often taken for granted. In reality, there are many types of water—such as RO reverse osmosis water, tap water, mineral water, soft water, and hard water—each having a distinctly different effect on coffee extraction and the resulting flavor.
In recent years, scientists have discovered that certain ions in water, particularly calcium, magnesium, and carbonate, directly influence the release efficiency of coffee flavor compounds. In 2014, Christopher Hendon from the University of Bath in the UK and barista Maxwell Colonna-Dashwood published a groundbreaking study indicating that three common cations—sodium, calcium, and magnesium—bind with acids, sugars, and other compounds in coffee to varying degrees, thereby altering the final extraction profile.
Their research found that, contrary to common belief, sodium ions contribute very little to flavor extraction and may even interfere with the release of some beneficial flavors, such as caffeine and eugenol (an aromatic compound with a woody character). Meanwhile, the divalent cations calcium and magnesium, due to their stronger binding ability, can extract various flavor components from coffee more efficiently—whether good or bad.
Interestingly, although both calcium and magnesium can enhance extraction yield, magnesium ions slightly outperform in overall extraction efficiency and are less prone than calcium ions to combine with bicarbonate ions to form scale, making them more device-friendly for both home and commercial use. Therefore, when designing brewing water, magnesium content often becomes one of the optimization priorities. But calcium is not without value—especially when brewing light roast coffee, where its extraction capability is quite similar to magnesium’s, helping to deliver a more layered mouthfeel.
Based on this research, Hendon and Colonna-Dashwood subsequently compiled and published the book Water for Coffee, which systematically explores the impact of water quality on coffee extraction. It also introduced the concepts of “total hardness (GH)” and “carbonate hardness (KH)” for the first time, emphasizing that the balance between these two factors is the key to influencing extraction performance.
Total hardness (GH) mainly consists of calcium and magnesium ions in the water, determining the water’s ability to “extract” coffee components. Carbonate hardness (KH), on the other hand, refers to the concentration of bicarbonate and carbonate ions in the water, which is responsible for stabilizing the pH during brewing and buffering the acidity inherent in the coffee, preventing overly sharp or dull flavors.
For example, when KH is too low, the acidity of the coffee may become overly pronounced or even harsh; conversely, if KH is too high, acidity can be neutralized, making the overall flavor flat or even powdery and sediment-like. When GH is too low, the coffee may taste watery and lack complexity; if GH is too high, over-extraction is likely, leading to a heavy or unpleasantly heavy mouthfeel with off-flavors.
In practical applications, many baristas adjust the hardness ratio of brewing water based on the roast level and origin of the coffee beans used. For instance, for high-acidity light roast beans, appropriately increasing GH can help boost extraction yield, bringing out more sweetness and complexity; whereas for dark roast beans, moderately controlling KH can prevent excessive suppression of acidity while maintaining a rich mouthfeel.
It’s worth noting that the book Water for Coffee also reminds readers not to overly pursue optimal values for a single ion, but rather to consider the overall balance of GH and KH to find the best combination suited to one’s own brewing style and bean variety. They further propose the innovative concept of “Roasting to water,” suggesting that since water quality varies greatly around the world, perhaps we should adjust roasting profiles and bean selection according to the characteristics of local water, rather than trying to change the water itself.
Although there are now many filters, additives, and water filtration devices on the market that can help adjust the hardness and acidity of household water, truly mastering the art of brewing water still requires a solid understanding of parameters such as TDS (total dissolved solids), pH value, GH, and KH. Only then can you adjust to the most suitable brewing conditions based on your personal taste preferences.
The above content is compiled by CoffeeHunters, a coffee news website.
