Brew Water Quality
Sources: The Coffee Brewing Handbook by Rob Lingle (SCA, 2011); Espresso Extraction: Measurement and Mastery by Scott Rao (2013); The Physics of Filter Coffee by Jonathan Gagné (2020)
Water is never chemically pure in practice. It contains dissolved minerals, gases, and organic compounds in infinite combinations and concentrations. Because water constitutes 98–99% of the coffee beverage, even trace impurities can significantly affect cup quality — often at concentrations that are undetectable in the water itself but amplified in the finished brew. (source: SCA Coffee Brewing Handbook)
Ideal Water for Coffee Brewing
| Parameter | Ideal Range | Notes |
|---|---|---|
| Total Dissolved Solids (TDS) | 50–300 ppm | 100–200 ppm gives “crystal fresh” taste |
| Combined calcium + magnesium | Below 100 ppm | Above = hard water; scale build-up |
| Carbonate-bicarbonate alkalinity | Below 100 ppm | Above = slows water flow through coffee bed |
| Sodium + potassium | Below 50 ppm | Above = increases sourness/bitterness perception |
| Iron | Below 2 ppm | Above = causes greenish discolouration with cream |
| pH | 6.5–7.5 | Pure water = 7.0 |
| Chlorine | None | Must be removed |
| Odour | None | Hydrogen sulphide, chlorine, ammonia are all disqualifying |
Demineralised water (below 10 ppm TDS) is not recommended — some dissolved minerals are necessary for proper extraction and pleasant taste. (source: SCA Coffee Brewing Handbook)
The Chlorine Problem
Municipalities commonly add chlorine to water to kill bacteria. Residual chlorine is one of the most common and serious water problems in coffee brewing.
Chlorine by itself has a relatively high taste threshold (5 ppm in water). However, chlorine from the water combines with phenolic compounds from the coffee extract to form chlorophenol — a compound with an intensely objectionable medicinal taste detectable at concentrations as low as 0.001 ppm in the finished beverage.
Chlorine must be removed by an activated carbon filter before coffee brewing. Polyphosphate treatment (used to prevent scale) does not address chlorine. (source: SCA Coffee Brewing Handbook)
Total Alkalinity — The Most Critical Flavor Parameter
Gagné’s physics analysis identifies total alkalinity (bicarbonate HCO₃⁻ concentration) as the single most important water parameter for cup flavor — more impactful than hardness, pH, or TDS, apart from gross contaminants. The reason: alkalinity is a buffer that resists pH change. When coffee acids extract into alkaline water, they react chemically with HCO₃⁻ ions and are altered before they can reach the cup. Good-tasting coffee acids are irreversibly modified by high-alkalinity water — they cannot be tasted even if they were successfully extracted. (source: The Physics of Filter Coffee by Jonathan Gagné (2020))
Sensory research (Wellinger and Yeretzian 2015) found that tasters preferred water with 20 ppm as CaCO₃ alkalinity among those tested; the specialty community commonly uses 40–50 ppm as a practical target. Gagné finds 20 ppm too sour for light roasts in practice; the optimal point appears to depend on roast level.
For crafting custom water with controlled alkalinity, see Brew Water Crafting.
Carbonate-Bicarbonate Alkalinity and Flow
Beyond flavor, carbonates and bicarbonates also retard the flow of water through the coffee bed. At concentrations above 100 ppm, they significantly slow the brewing rate — extending contact time beyond the prescribed range and causing over-extraction (bitter, astringent).
The problem is compounded when water has been treated by a zeolite softening system (ion exchange replacing calcium and magnesium with sodium). The sodium bicarbonate that forms:
- Binds coffee particles together
- Blocks water flow passageways
- Extends brewing time severely
- Leads to heavy over-extraction
Water softening (sodium-for-mineral exchange) is explicitly not recommended for coffee brewing. It worsens brew quality and increases alkalinity. (source: SCA Coffee Brewing Handbook)
Calcium and Magnesium (Water Hardness)
Dissolved calcium and magnesium make water hard. At concentrations below ~150 ppm combined, they do not directly harm flavour — and at typical specialty-coffee concentrations they appear to improve extraction. The mechanism is not fully established, but researchers hypothesize that positively charged Ca²⁺ and Mg²⁺ ions increase the diffusion rate of specific chemical compounds. (source: The Physics of Filter Coffee by Jonathan Gagné (2020))
Practical distinction between the ions:
- Calcium (Ca²⁺): associated with body and sweetness perception
- Magnesium (Mg²⁺): associated with increased flavor complexity; gas chromatography data suggests Mg²⁺ favors extraction of some acids and disfavors quinic acid compared to Ca²⁺
Softer water (lower total hardness + lower alkalinity) mimics some characteristics of a lighter-roasted coffee — brighter acidity, less body. This makes water composition a subtle but meaningful variable in how a roast is perceived.
At concentrations above 200 ppm combined, Ca+Mg cause equipment problems:
- Scale and lime deposits on boilers, thermostat relays, and feed pipes
- Blocked inlet pipes and valve failure
- Equipment malfunction and inefficient heating
Water above 150–200 ppm combined Ca+Mg should be treated — but by filtration or reverse osmosis, not by zeolite softening.
pH and Coffee Acidity
Pure water has a pH of 7.0 (neutral). Coffee naturally brews to approximately pH 5.0:
- Acidy Arabica coffees: pH 4.7–4.8
- Mild Arabica coffees: pH 4.9–5.1
- Robusta coffees: pH 5.2–5.5
This lower pH of Arabica relative to Robusta reflects the higher organic acid content of Arabica — a quality marker for the specialty segment. Using alkaline brewing water (pH above 7.0) neutralises the coffee’s acidity and flattens the flavour, particularly damaging for coffees whose distinguishing characteristic is bright acidity. (source: SCA Coffee Brewing Handbook)
Other Problem Compounds
- Iron above 2 ppm: Combines with coffee phenols to produce a greenish discolouration in the finished beverage — immediately detectable when cream is added.
- Sodium/potassium above 50 ppm: At low concentrations, these salts add sweetness. As concentration rises, potassium increases bitterness perception; sodium increases sourness perception.
- Hydrogen sulphide (rotten egg odour): Detectable at 0.05 ppm in water, 0.12 ppm in coffee. Treated by adding chlorine before brewing.
- Cleaning compounds: Alkyl sulfonates in dishwashing detergent, if not thoroughly rinsed from cups and brewing equipment, dissolve into the beverage at detectable concentrations above 100 ppm.
Water Chemistry for Espresso (Rao)
Rao’s target ranges differ slightly from Lingle’s broader guidelines and are calibrated specifically for espresso extraction:
| Parameter | Rao’s target | Note |
|---|---|---|
| TDS | 75–250 ppm | Narrower than Lingle’s 50–300 ppm |
| Hardness (Ca + Mg) | 50–175 ppm | |
| Alkalinity | 40–70 ppm | Key constraint — see below |
| pH | ~7.0 | Close to neutral |
Missing these targets by up to 15% is acceptable except for pH, which should be very close to neutral. (source: Espresso Extraction: Measurement and Mastery by Scott Rao (2013))
Alkalinity and softening — a critical espresso risk: Water with high bicarbonate content, when put through a sodium-based softener (ion exchange), produces sodium bicarbonate. In espresso, this dramatically increases extraction time, forcing the barista to use a significantly coarser grind. A coarser grind for espresso directly reduces extraction — with some source waters, reaching 19% extraction becomes impossible after softening. Rao recommends a careful cost–benefit analysis before softening, and notes that machine manufacturers recommend softening primarily to prevent scale damage, not to improve cup quality. (source: Espresso Extraction: Measurement and Mastery by Scott Rao (2013))
Scale prediction: Use the Langelier Saturation Index (LSI) formula to predict whether a given water chemistry will deposit scale in the boiler. Input TDS, hardness, alkalinity, and pH at various boiler temperatures. Municipality water chemistry can change seasonally, so test periodically.
Recommended Treatment
| Problem | Recommended Treatment |
|---|---|
| Chlorine | Activated carbon filter |
| Sediment and insoluble materials | Mechanical sediment filter |
| Excess dissolved solids | Demineralisation or reverse osmosis |
| Odours (chlorine, H₂S) | Activated carbon treatment |
| Scale (Ca, Mg) | Polyphosphate treatment (does not affect flavour) |
| Sodium bicarbonate hardness | Water bypass device; not zeolite softening |
Relevance to Kaiserblick
For Kaiserblick’s local distribution to coffee shops in El Salvador and for its export customers in Europe, water quality is a practical concern:
- El Salvador: Municipal water quality varies; chlorine treatment is standard. Coffee shops should use activated carbon filtration.
- European markets: Water hardness varies significantly by city and region. German cities in particular have variable hardness; some Swiss cantons have very hard water. Kaiserblick’s export customers should be aware that water quality will affect the expression of the coffee they buy.
- Acidity: Kaiserblick’s Arabica coffees from the Apaneca-Ilamatepec region have characteristic acidity (pH ~4.7–4.9). Alkaline water will suppress this quality — a specific risk in hard-water regions.
Related pages
- Brew Water Crafting
- Extraction
- Advection and Diffusion
- Pourover Technique
- Espresso Extraction
- Coffee Brewing Control Chart
- Brewing Standards
- Brewing Methods
- SCA Coffee Brewing Handbook
- Apaneca-Ilamatepec
- Scott Rao — Espresso Extraction (Source Summary)
- “The Physics of Filter Coffee — Jonathan Gagné (2020)”