A recent study from the University of Copenhagen challenges common assumptions about water chemistry for coffee brewing. Published in the Journal of Food Science, the research indicates that bicarbonate levels, which determine water's alkalinity, have a far more significant impact on a coffee's flavor profile than the mineral hardness contributed by calcium and magnesium.
In the study, researchers brewed a single-origin Colombian coffee using ultrapure water modified with various concentrations of calcium, magnesium, and bicarbonate. According to the findings from a trained sensory panel, high levels of bicarbonate (300 mg/L) drastically reduced perceived acidity and fruitiness while increasing bitter and roasted notes. In contrast, even high concentrations of calcium and magnesium created no statistically significant flavor difference compared to the pure water control brew.
These results suggest that the industry's focus on achieving specific hardness levels may be less critical for flavor than previously believed. For roasters and quality control professionals, the primary takeaway is the importance of measuring and managing water alkalinity. By controlling bicarbonate, brewers can better preserve the delicate acidity and complexity that define specialty coffees, though the study's authors note that further research is needed across different roast levels and origins.