Filtered water enhances the extraction of specific organic molecules and aromatic compounds from coffee beans through the precise management of mineral content and chemical purity. Dissolved minerals like magnesium Mg2+ and calcium Ca2+ coordinate with the chemical motifs in coffee to facilitate the extraction of essential sugars, starches, and acids. Magnesium is particularly effective at pulling flavorful compounds from the bean, but an excess of minerals—common in hard tap water—can lead to over-extraction and a chalky, bitter taste. Furthermore, chlorine and chloramines in tap water react with coffee phenols to create chlorophenols, which impart a medicinal or plastic-like aftertaste. Filtered water also balances alkalinity, ensuring that the bright, pleasant acidity of the coffee is not neutralized, which would otherwise leave the drink tasting flat and dull.
References:
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Hendon, C. H., et al. (2014). The Role of Dissolved Cations in Coffee Extraction. Journal of Agricultural and Food Chemistry, 62(21), 4947-4950.
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Specialty Coffee Association (SCA). (2018). Water Quality Handbook.
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Navarini, L., & Rivetti, D. (2010). Water quality for Espresso coffee. Food Chemistry, 122(2), 424-432.