Water Chemistry

Water acts as a solvent in coffee brewing. Its ability to dissolve coffee compounds depends on its mineral content. Minerals (calcium, magnesium) act as carriers that help dissolve coffee compounds; bicarbonates act as buffers that resist pH changes. The ideal brewing water has 50-150 ppm TDS, 40-75 ppm hardness (as CaCO3), and 40-75 ppm alkalinity (as CaCO3). Too few minerals and the water can't extract properly; too many and the water over-extracts or creates scale.

Key water chemistry parameters for coffee: (1) Total Hardness: the concentration of calcium (Ca2+) and magnesium (Mg2+) ions, measured as CaCO3. Calcium helps extract flavor compounds; magnesium extracts more fruity/acidic compounds. Ideal: 50-100 ppm. (2) Alkalinity (Buffer Capacity): the concentration of bicarbonate (HCO3-) ions, measured as CaCO3. Bicarbonate neutralizes acids, reducing perceived acidity. High alkalinity makes coffee taste flat; low alkalinity makes coffee taste sharp. Ideal: 40-75 ppm. (3) pH: should be 6.5-7.5. (4) TDS: should be 75-250 ppm (SCA recommends 150 ppm). (5) Chlorine: must be zero (removes with carbon filter).

Water's mineral content affects its ability to act as a solvent. Hard water (high calcium/magnesium) has higher ionic strength, which enhances the dissolution of polar coffee compounds but can cause over-extraction. Soft water (low minerals) has lower solvent power, leading to under-extraction. The physics of water treatment involves ion exchange (water softeners replace Ca2+/Mg2+ with Na+), reverse osmosis (forces water through a membrane, removing 95-99% of minerals), and remineralization (adding specific minerals back to RO water).

Water chemistry directly affects extraction yield and flavor. Calcium and magnesium ions form complexes with coffee acids and other compounds, enhancing extraction. Bicarbonate buffers the acids extracted from coffee, reducing perceived acidity. Research by SCA and others shows that water with 150 ppm TDS, balanced hardness and alkalinity, produces the best extraction. Water that is too soft (low TDS) under-extracts; water that is too hard over-extracts and produces scale. The 'ion balance' (ratio of hardness to alkalinity) is as important as absolute concentrations.

Water chemistry affects sensory perception in several ways: high alkalinity reduces perceived acidity (makes coffee taste flat and dull); low alkalinity increases perceived acidity (makes coffee taste sharp and bright); high hardness can increase body and extraction of bitter compounds; magnesium specifically enhances fruity and floral notes; calcium enhances body and sweetness. Professional cuppers use water with SCA-standard mineral content to ensure consistent evaluation.

The SCA water standard specifies: TDS of 75-250 ppm (target 150 ppm), calcium hardness of 50-175 ppm (target 68 ppm), total alkalinity of 40-75 ppm (target 40 ppm), pH of 6.5-7.5, and zero chlorine. The ideal water has a balanced ratio of general hardness (GH) to carbonate hardness (KH). Magnesium hardness is preferred over calcium hardness for flavor extraction because Mg2+ forms more favorable complexes with coffee acids. Water for espresso should be slightly softer (50-100 ppm TDS) to reduce scale formation in boilers.

For home brewers: use filtered water (carbon filter removes chlorine, which is essential). If your tap water is very hard (over 200 ppm TDS), use bottled water or a water filter. SCA-standard recipes: add 0.4g Epsom salt (MgSO4) and 0.4g calcium chloride (CaCl2) to 1 gallon of distilled water for approximate SCA-standard water. For espresso machines, use water with 50-100 ppm TDS to reduce scale. Never use distilled or RO water alone (no minerals = poor extraction). Test your water with a TDS meter ($15-30).

• •Distilled water makes the best coffee. False. Distilled water has no minerals, so it can't extract coffee properly. The result is flat, under-extracted coffee.
• •Bottled water is always better than tap. False. Some bottled water is too hard or too soft. Check the mineral content on the label.
• •Water temperature matters more than water chemistry. Both matter. Perfect temperature with bad water chemistry still produces poor coffee.
• •You need expensive water filtration. A simple carbon filter (like a Brita) removes chlorine, which is the most important step. For precise control, you can make SCA-standard water with Epsom salt and calcium chloride.

• •The SCA water standard (150 ppm TDS, 68 ppm hardness, 40 ppm alkalinity) was established through research by the Coffee Brewing Institute and sensory panels.
• •Research by Hendon et al. (2014) in the Journal of Agricultural and Food Chemistry showed that magnesium ions extract more flavor compounds than calcium ions.
• •Studies show that water with high alkalinity (>100 ppm as CaCO3) significantly reduces perceived acidity and can make coffee taste flat.
• •Research found that water hardness affects not just extraction but also the formation of crema in espresso.
• •SCA research shows that water with TDS below 50 ppm consistently under-extracts coffee.

• •Chapter 5: Extraction Science
• •Chapter 3: Core Brewing Variables

• •Hendon, C.H. et al. (2014). 'Computational prediction of the mineral composition of water.' Journal of Agricultural and Food Chemistry.
• •Fasano, V. et al. (2015). 'The Influence of Water Composition on Flavour and Crema of Espresso.' Chapter in 'The Craft and Science of Coffee.'
• •SCA Water Standard (2018). Specialty Coffee Association.

• •Specialty Coffee Association
• •Coffee Science Foundation
• •Rao, S. 'Water for Coffee'