Water in coffee extraction: The active solvent
In various brewing methods, water accounts for an absolute majority. For espresso, water makes up approximately 90% of the final product's weight, while in filter coffee methods, this ratio ranges from 98.5% to 99%. Given its dominant volume, water quality and chemical composition directly impact the ability to dissolve and release flavors from coffee grounds.
Water is not merely a passive solvent. Dissolved minerals within water act as active chemical agents, altering solubility and surface tension, thereby regulating the efficiency of extracting groups of compounds such as organic acids, aromatic compounds, and polyphenols.
The binding mechanism of Calcium (Ca2+) and Magnesium (Mg2+) ions
Divalent ions, primarily Calcium (Ca2+) and Magnesium (Mg2+), play a crucial role in enhancing extraction speed. Within coffee bean cell structures, flavor compounds contain numerous nucleophilic sites with weak negative charges or free electron pairs.
When water comes into contact with coffee grounds, Ca2+ and Mg2+ ions form coordination complexes with these nucleophilic sites. The dissolved metal-ion complexation process helps break down the structural barrier of the coffee bean, allowing these molecules to dissolve faster than using distilled water devoid of minerals.
The difference in binding affinity between Magnesium and Calcium for flavor compounds
According to a study published in the Journal of Agricultural and Food Chemistry by Hendon, Colonna-Dashwood, and Colonna-Dashwood, Magnesium (Mg2+) and Calcium (Ca2+) ions do not have identical effects on various flavor groups. The study's results indicate that Magnesium has a stronger binding affinity than Calcium for many important flavor compounds in coffee.
Magnesium has a smaller ion radius and higher charge density than Calcium, allowing it to easily interact and extract complex compounds, fruity flavors, and organic acids from coffee grounds. Conversely, Calcium still supports extraction but tends to favor heavier compounds and may lead to lime scaling in equipment when heated.
SCA water standards for brewing
To ensure optimal extraction efficiency without damaging equipment, the Specialty Coffee Association (SCA) has established standardized chemical water parameters. Maintaining total hardness and alkalinity within recommended ranges prevents both under-extraction and equipment scaling due to excessive minerals. | Chemical parameter | SCA recommended range | Ideal target | Impact on extraction & equipment | | --- | --- | --- | --- | | Total Dissolved Solids (TDS) | 75 – 250 ppm | 150 ppm | Determines the total amount of minerals supporting solvent dissolution | | Total Hardness | 50 – 175 ppm CaCO3 | 50 – 68 ppm CaCO3 | Provides Ca2+ and Mg2+ ions to form complexes with flavor compounds | | Total Alkalinity | Approximately 40 ppm CaCO3 | 40 ppm CaCO3
| pH buffering, maintaining natural acidity without making it too bright |
If total hardness is too low, water lacks divalent cations to bind with flavor molecules, resulting in weak and sour coffee. Conversely, if hardness and alkalinity exceed recommended levels, water will neutralize natural acidity and cause scaling in the steam wand of an espresso machine.
References
- Hendon, C. H., Colonna-Dashwood, L., & Colonna-Dashwood, M. (2014). The Role of Dissolved Cations in Coffee Extraction. Journal of Agricultural and Food Chemistry.
- Specialty Coffee Association (SCA). SCA Water Standard.