The Science of Extraction
Water makes up around 98% of your coffee , yet it remains the most overlooked brewing variable. The chemical composition of water directly influences how flavor compounds are extracted from coffee grounds through a complex interplay of dissolved minerals and acids. Coffee extraction works similarly to diffusion principles, where compounds travel from areas of high concentration to those of low concentration. The amount of total dissolved solids (TDS) in brewing water directly correlates with its extraction efficiency.
TDS matters because it affects the level of extraction you can get out of your coffee. Coffee requires minerals in the water you brew with so you can pull the desired flavor out of the grounds. Empty water (water with a TDS of 0) will leave your coffee tasting dull and flat because it doesn’t have any of the required solids to extract the flavor from your coffee. This explains why distilled water produces disappointing results despite its purity.
Total Dissolved Solids and the SCA Standard
The recommended TDS is a target of 150 with an acceptable range of 75-250 TDS , according to the Specialty Coffee Association. The SCAA specifies a TDS level of 75-250 mg/L (ppm) for brewing water, with an ideal target of 150 mg/L. This standard emerged from extensive research to identify the optimal mineral concentration for balanced extraction without overwhelming the coffee’s natural flavors.
Water with very low mineral content tends to overextract coffee, while water with high TDS values tends to underextract.
Too low water TDS (very soft, distilled, or straight RO): coffee often tastes thin, sharp, and hollow because the water lacks minerals to pull flavor out. Too high water TDS (very hard): extraction can turn muddy and bitter, and scale builds up in kettles and espresso machines.
Professional brewing requires understanding that TDS alone doesn’t tell the complete story. Two waters can have identical TDS but taste completely different depending on their mineral composition (Which minerals are used).
Mineral Composition: Calcium, Magnesium, and Their Roles
Hardness is determined mainly by the concentration of calcium (Ca2+) and magnesium (Mg2+) ions dissolved in water. These are essential minerals, they interact with coffee compounds to enhance flavour and extraction efficiency. However, recent research challenges long-held assumptions about mineral extraction mechanisms.
From a sensory standpoint, Magnesium enhances fruitiness and complexity while the calcium tends to emphasise roundness and body in coffee.
Calcium and magnesium are the key players. Calcium contributes to body and mouthfeel, helping create a fuller, rounder cup. Magnesium is responsible for extracting the bright, fruity, and complex flavors that make specialty coffee so exciting.
A 2024 study published in the journal Heliyon provides compelling evidence that questions traditional extraction theory. At concentrations typically found in drinking water, the salts resulted in limited variation of the acid content, while ten-fold higher salt concentrations produced more pronounced variations. Comparisons between pre- and post-brew additions showed similar acid content in most cases, suggesting that extraction of acids proceeds independent of the water composition. This research suggests that perceived flavor of coffee is most likely due to interactions taking place post-brew rather than during extraction itself.
Alkalinity and pH Buffering
Alkalinity is — by far — the single most important factor in how water chemistry will affect coffee flavor. Simply put, higher alkalinity makes coffee less acidic. Lower alkalinity makes coffee more acidic. This relationship holds regardless of pH values.
Alkalinity measures water’s buffering capacity—its ability to neutralize acids. Coffee naturally contains various acids that contribute brightness and complexity to the cup. Maintaining alkalinity between 75 and 250 mg/L helps balance mineral content, though the specific SCA recommendation sits at 40-70 ppm.
Specialty coffee is defined by its acidity: citrus, berry, floral, and wine‑like notes. High alkalinity water neutralizes those acids. Research summarized by the Specialty Coffee Association, a BRITA‑sponsored study, and practical guides from water specialists such as Scott Rao all point toward the same pattern: when alkalinity is high, coffee tastes dull, bitter, and flat.
My personal preference for lightly roasted, well-developed beans is an alkalinity level of 30—40ppm , notes coffee expert Scott Rao.
The relationship between alkalinity and extraction method matters significantly. For espresso preparations only 10% of the alkalinity is available to buffer coffee acids when compared to the amount of alkalinity available to buffer the same amount of coffee acids present in filter coffee preparations. It follows that water’s alkalinity can be much higher for espresso brewing before it makes a significant impact on the cup’s acidity.
Practical Application and Water Treatment
Understanding water chemistry enables targeted improvements through various treatment approaches. Activated carbon filtration is widely used to eliminate chlorine, unpleasant odors, and organic compounds, making it an effective first step toward SCA-compliant water. Carbon filters remove chlorine completely while reducing some hardness minerals, though they typically don’t provide comprehensive mineral balancing.
Municipal water suppliers often disinfect with chlorine or chloramine. Even at low levels, these compounds are noticeable in coffee. The Specialty Coffee Association and sensory specialists from BRITA point out that chlorine can react with organic residues in water, producing off‑odors and unpleasant tastes.
For comprehensive control, reverse osmosis followed by remineralization offers precision. Reverse osmosis technology removes most undesirable ions, heavy metals, and dissolved contaminants, creating ultra-pure water as a starting point. After RO filtration, minerals such as calcium, magnesium, or bicarbonates can be reintroduced to achieve the desired hardness and pH balance.
Advanced home brewers can create custom water profiles using mineral salts. The most suitable RO water should have a TDS level between 5-8 ppm. The minerals we need are Magnesium from Epsom salts, Bicarbonate from baking soda, and Calcium Chloride. These minerals can be found in food-grade off-the-shelf products.
Equipment Protection and Scaling
Water chemistry affects not only flavor but equipment longevity. Hard water is one of the most common issues for coffee brewers. High mineral content (especially calcium and magnesium) can lead to over-extraction, resulting in bitter, harsh flavors. Hard water also causes mineral buildup (scale) in kettles, coffee makers, and espresso machines, which can damage equipment over time and affect brewing temperature consistency.
Most studies agree that 50-80 ppm CaCO3 hardness is ideal for both flavor and equipment performance. This range provides sufficient minerals for extraction while minimizing scaling risks. Professional operations require regular descaling regardless, but optimal water chemistry reduces maintenance frequency and protects expensive equipment investments.
The balance between performance and protection explains why many commercial operations invest in sophisticated water treatment systems. I always ask my clients to get a water test from a laboratory before choosing a water system. I have my preferred labs, and I would personally pay the $100-$200 for a test rather than get a free test from a water-treatment company , emphasizes Scott Rao, highlighting the importance of understanding your specific water composition before implementing treatment solutions.