Unlocking the Secrets of Water Chemistry in Coffee Extraction

How the Minerals in Your Water Influence the Flavour of Your Brew

When we think about brewing the perfect cup of coffee, we often focus on the beans, the roast, or the brewing method. However, one critical element that frequently goes unnoticed is the water we use. Believe it or not, the minerals dissolved in your water can dramatically alter the flavour profile of your coffee—from smooth and balanced to astringent and bitter—even if everything else remains constant.

As a coffee roaster, it is essential to understand this phenomenon to help our customers enjoy coffee as we intend, without requiring them to jump through hoops to achieve that perfect cup.

Water: More Than Just a Solvent

Water is not just a neutral medium; it is a solvent that extracts flavours from coffee grounds. Dissolved within it are various minerals and additives introduced during processing. While the impact of water's mineral content on coffee flavour has been studied for years, it may have been the chemist Christopher Hendon who brought this topic into mainstream awareness.

In 2014, Hendon and his colleagues published an article titled "The Role of Dissolved Cations in Coffee Extraction" in the Journal of Agricultural and Food Chemistry [1]. This paper delves into how different minerals, specifically dissolved cations like sodium (Na+), magnesium (Mg2+), and calcium (Ca2+), interact with coffee compounds during extraction. Although our understanding of this topic is still developing, I think it is interesting to mention this work from 2014. Remember, though, we should be open minded to the fact that this post summarises one article, and there may have been updates to the knowledge since.

The Science Behind Mineral Interactions

In the introduction, the authors cite the previous work by Lockhart et al. [2] and later by Pangborn et al.[3] Both studies also describe the role that water impurities play in the flavour of coffee.

The researchers used computational chemistry to explore how these cations bind with various coffee constituents. They focused on seven compounds commonly found in roasted coffee:

1. Lactic Acid – Contributes sour notes.

2. Malic Acid – Also adds sourness.

3. Citric Acid – Provides a sweet flavour.

4. Quinic Acid – Associated with pungent, unpalatable tastes.

5. Chlorogenic Acid – Another source of less desirable flavours.

6. Caffeine – The well-known bitter alkaloid.

7. Eugenol – Offers delightful woody notes.

Their goal was to quantify the "binding energy" between these cations and the coffee compounds. In simpler terms, they wanted to see how strongly these minerals attach to coffee molecules, affecting their extraction into your cup. The thermodynamic relative binding energies between two compounds was defined by

ET = Eab − (Ea + Eb)

where the relative binding energy, ET, is equal to the difference between the bound product, Eab, and the individual components, Ea and Eb.

The authors considered the interaction of dissolved cations with the nucleophilic motifs of the dissolved coffee compounds. This interaction may be understood with:

Ur = (qi x qs )/r^2

where the interaction energy Ur is proportional to the charge of both the ion and solute ( qi and qs ) divided by the interatomic separation, r^2. Therefore, it would be expected that the more localised charged species should interact more strongly with molecular multipoles.

A table was presented summarising the interaction lengths of each metal ion to the nearest/most nucleophilic site on the seven compounds and water. Mg2+ had the shortest distances (in angstrom, Å) for all seven compounds and water, with Ca2+ second and then Na+.

Key Findings

• Magnesium (Mg2+) showed the highest binding energy with all coffee compounds. This means it has the strongest interaction, enhancing the extraction of flavours.

• Calcium (Ca2+) had a slightly lower binding energy than magnesium but was still effective in extracting coffee constituents.

• Sodium (Na+), on the other hand, was less favorable for extracting compounds like caffeine and eugenol because it binds less effectively than water does.

For the displacement of the metal cation coordinated with water, the compounds 1 to 7 require a higher ET than the metal-water coordinated molecules. Considering this, the authors found that Na+ would not favour the extraction of caffeine and eugenol, because the binding of Na+ to these compounds are less favourable than to water.

The relative binding energy of (1−7)−Mg2+ was highest in all cases, with the ET proportional to the charge density of the compounds 1−7.

What This Means for Your Coffee

• High Magnesium Content: If you're aiming for maximum extraction—say, for making instant coffee—water rich in magnesium is ideal.

• Balanced Flavours: For lighter roasts where a balance of flavours is crucial, both calcium and magnesium-rich water work well. Magnesium has the added benefit of reducing scale formation in your equipment.

• Bicarbonate Levels Matter: The presence of bicarbonate and other bases in water can neutralise certain acids in coffee, altering the flavour. For instance, bicarbonate interacts strongly with chlorogenic acid, reducing some of the less desirable acidic tastes. (The effect of carbonates was not directly studied but included as a comment in the discussion.)

No One-Size-Fits-All Solution

It's important to note that there's no universally "perfect" water composition for all types of coffee. The ideal mineral content depends on various factors, including the coffee bean's origin, roast level, and personal taste preferences. As the researchers concluded:

"It should be noted that there is not one particular composition of water that produces consistently flavorsome extractions from all roasted coffee. Rather, there is water that has the most extracting ability (i.e., cation-rich), and the resultant flavor depends on the balance between both the cations in solution and the quantity of bicarbonate present (acting as a buffer). Furthermore, each bean is roasted to taste optimal when brewed with the water it was roasted to."

Final Thoughts

Understanding the role of water chemistry in coffee extraction opens up new avenues for both roasters and coffee enthusiasts to explore flavour profiles more deeply. While more research continues to emerge in this field, being mindful of the minerals in your brewing water is a significant step toward enjoying coffee as it's meant to be savoured.

Although beyond the scope of this post, I would like to mention that there are more recent studies quantifying the levels of coffee constituents when brewed with water containing metal cations versus adding the cations after extraction. Their findings elude to the cations not having a discernible effect on the levels of the acid constituents and it might be that any influence on the flavours are taking place after the brewing process.[4]

References:

[1] Hendon, C., Colonna-Dashwood, L., & Colonna-Dashwood, M. (2014). The Role of Dissolved Cations in Coffee Extraction. Journal of Agricultural and Food Chemistry, 62(21), 4947–4950. https://doi.org/10.1021/jf501687c

[2] Lockhart, E. E.; Tucker, C. L.; Merritt MC. (1955). The effect of water impurities on the flavor of brewed coffee. J Food Sci, 20. https://doi.org/https://doi.org/10.1111/j.1365-2621.1955.tb16874.x

[3] Pangborn, RM, Trabue IM., Little, AC. (1971). Analysis of coffee, tea and artificially flavored drinks prepared from mineralized waters. J Food Sci, 36. https://doi.org/https://doi.org/10.1111/j.1365-2621.1971.tb04061.x

[4] Bratthäll, T., Figueira, J., & Nording, M. L. (2024). Influence of divalent cations on the extraction of organic acids in coffee determined by GC-MS and NMR. Heliyon, 10(5), e26625. https://doi.org/10.1016/J.HELIYON.2024.E26625