Water Chemistry for Dual Boilers: Magnesium, Calcium and the Extraction of Dark Roasts
Water chemistry for dual boilers: magnesium, calcium and the extraction of dark roasts
Water makes up more than 98 per cent of what's in your cup. With a dual boiler, it's also the factor that decides how long the machine will last. We've laid the available research, the SCA recommendations and the actual water values from Zurich and Bern side by side.
Magnesium extracts a little more efficiently than calcium – that much is documented. That it tastes better is not. The SCA Water Quality Handbook says in so many words that the sensory difference is still unresolved.
The more important factor is alkalinity (bicarbonate). It buffers away the coffee acids, and in Zurich and Bern it's considerably higher than the SCA recommends. The problem here isn't hardness, it's bicarbonate.
With a dual boiler, the steam boiler sets the upper limit: that's where water evaporates, while the minerals stay behind and build up. That's why hardness has a double budget – once for flavour, once for the machine's service life.
What magnesium and calcium actually do
Distilled water extracts poorly. That's undisputed, and the reason lies in the dissolved cations. In 2014, Hendon, Colonna-Dashwood and Colonna-Dashwood calculated in the Journal of Agricultural and Food Chemistry how strongly sodium, magnesium and calcium bind to typical coffee molecules – five coffee acids, caffeine, and eugenol as a stand-in for an aroma compound. The cations coordinate to nucleophilic sites in the molecule and form soluble complexes. The ranking: magnesium ahead of calcium, sodium trailing far behind.
1 · Brew water: Magnesium and calcium are dissolved in the water. 2 · Coffee bed: The ions move in and bind aroma molecules there. 3 · Cup: Whatever has been bound ends up in the drink.
A heavily simplified model. The ranking of magnesium ahead of calcium ahead of sodium comes from a quantum-chemical calculation (Hendon et al. 2014) — not from a measurement of the drink and not from a tasting.
Important for context: Hendon et al. (2014) didn't brew or taste a single coffee – the paper is a quantum-chemical calculation. Bratthäll, Figueira and Nording (2024) put it to the test experimentally: at around 100 ppm, in other words typical of drinking water, they found no significant difference between magnesium and calcium; it only became measurable at 1,000 ppm. We show the model here because it explains the mechanics well – not because it has been proven.
Peer reviewThis re-examination came in 2024 from Bratthäll, Figueira and Nording in Heliyon. They added magnesium and calcium chloride before and after brewing and measured citric, malic, lactic and quinic acid using GC-MS and NMR. Their finding: at concentrations typical of drinking water, the acid content changed only to a limited extent; only at ten times the salt concentration did the change become clear. The authors state explicitly that a scientific basis for the coffee industry's water recommendations is long overdue.
The SCA Water Quality Handbook, written by Wellinger, Smrke and Yeretzian at the Coffee Excellence Center of ZHAW Wädenswil, puts it cautiously: preliminary trials pointed to a slightly higher extraction efficiency for magnesium, but the difference in sensory properties remains unclear and is a subject for future research. We rather like that restraint – and it matches our own experience that, with water, the big mistakes matter far more than the fine balance of ions.
Alkalinity – the underrated lever
Alkalinity, which is practically synonymous with bicarbonate content, isn't a flavour compound. It's a buffer. Bicarbonate neutralises the acids dissolved out of the coffee: HCO3− plus H+ becomes carbonic acid, which breaks down into water and CO2. The SCA handbook sums it up in one short formula: the higher the alkalinity, the lower the perceived acidity.
At very high alkalinity, above roughly 100 ppm CaCO3, so much CO2 is produced that, according to the handbook, the extraction time can lengthen and the extraction can tip towards over-extraction. Elevated sodium amplifies this – one reason why classic sodium ion exchangers are the wrong technology for espresso machines: they lower the hardness but leave the bicarbonate in place.
Conversely, too little alkalinity is no solution either. Below 40 ppm CaCO3, the handbook explicitly warns of corrosion, because even small amounts of dissolved CO2 can push the water below pH 6. Water that's too soft is more dangerous for an espresso machine than water that's slightly too hard.
The target values and where Zurich and Bern sit
Further to the right, the water gets harder; further up, it's more strongly buffered. Both axes are linear and use the same unit. For the Swiss scale: 100 ppm CaCO₃ equals 10 °fH.
Rectangles = measured ranges of variation over the year, not single values. Data: Trinkwasserqualität Stadt Zürich 2024 · Energie Wasser Bern · SCA Water Quality Handbook.
The earlier version of this chart had a compressed x-axis that made Bern look closer to the core zone than it really is – we've corrected that. Zurich's total hardness is still within range; Bern's no longer is. On alkalinity, both cities sit above the recommendation – Bern at around two and a half times the upper limit of 75 ppm. That's exactly why softening alone isn't enough here: a sodium ion exchanger removes the hardness and leaves the alkalinity where it is.
| Parameter | SCA recommendation | Zurich | Bern |
|---|---|---|---|
| Total hardness | 50–175 ppm CaCO₃, for espresso machines 50–150 | 139–160 ppm | approx. 210 ppm |
| Alkalinity | 40–75 ppm CaCO₃ | 127–148 ppm | 176–203 ppm |
| pH | 6–8 | 7.6–7.9 | approx. 7.6 |
| Chloride | below 5 mg/l (common rule of thumb, not an SCA standard) | 5.2–8.6 mg/l | 4.4–6.4 mg/l |
| Sodium | keep low | 6.6–9.1 mg/l | 3.4–4.3 mg/l |
A note on hardness for the Swiss scale: Zurich's cantonal laboratory classifies water below 15 °fH as soft and 15 to 25 °fH as moderately hard. Zurich sits at around 14 to 16 °fH, Bern at about 20 °fH – both converted from the ppm values above, not our own measurements. We've described step by step how to put together your water deliberately in Composing your own coffee water, and in the water chemistry simulator you can play hardness and alkalinity off against each other directly.
Why the steam boiler sets the limit
This is where the real difference between a single boiler and a dual boiler lies. Water flows through the brew boiler: water in, water out. The steam boiler is the only place in the machine where water leaves the liquid phase. The steam leaves mineral-free, the ion load stays inside – and is topped up again with every automatic refill. The steam boiler concentrates.
The steam leaves mineral-free; calcium and magnesium stay behind. Limescale precipitates more heavily the hotter the water gets — calcium carbonate becomes less soluble with heat, not more. That's why the steam boiler scales up first, and more heavily than the brew boiler.
Bar lengths drawn to scale, values from the SCA Water Quality Handbook. The two boiler temperatures of 130 °C and 95 °C are typical operating values — assumed, not measured on a specific machine.
For context: the 30 and 12 mg/l aren't formation rates but the amounts the SCA handbook sets as maximum precipitable – that's where the right-hand boundary of the core zone comes from. You can project what ends up in the boiler: at 100 drinks a day of 0.1 litres each, 30 mg/l adds up to around 110 grams of scale per year, 100 mg/l to 365 grams, and 200 mg/l to around 730 grams. This is a model calculation: the grams come from the formula, not from a boiler, and 100 drinks a day means hospitality use – at home the amount is considerably lower. That's why, in Bern and Zurich, every dual boiler consultation starts with water and only then turns to the machine.
This is exactly why the right-hand boundary of the SCA core zone is set not by taste but by the steam boiler. And it's why dual boilers are also more vulnerable than single boilers to the consequences of the wrong water treatment.
ManufacturerLa Marzocco states in its own water specification: TDS 90–150 ppm, total hardness 70–100 ppm, alkalinity 40–80 ppm, pH 6.5–8, chloride 0–30 ppm, free chlorine 0.05 ppm maximum. The fact that the chloride limit there is 30 ppm, while the rule of thumb common in the coffee scene is 5 mg/l, isn't a mistake on either side – incidentally, the SCA standard doesn't give a chloride value at all – they're simply different risk thresholds. A warranty limit on one side, best practice on the other. It's good evidence that there is no binding standard here: the SCA currently doesn't maintain any published water standard, and the much-quoted table comes from the legacy SCAA document from 2009.
Dark roasts: what's proven and what isn't
Now to the part where we had to correct our own expectations. The industry formula goes: dark roast equals more soluble equals higher yield. In 2024, Lindsey and colleagues showed the opposite in Scientific Reports – although their work primarily examined caffeine content, with yield as a secondary finding. For filter coffee, extraction yields fell once roast loss exceeded around 12 to 14 per cent. A natural-processed Ethiopian reached around 21 per cent yield as a light roast and around 15 per cent as a dark roast under identical conditions.
What dark roasts actually do: with given parameters, they reach their optimum yield faster, but that optimum is lower. They're also more brittle, produce more fines at the same grinder setting and so change the way water flows through the coffee. That's a process effect, not a solubility effect.
The widespread recommendation to choose higher alkalinity for dark roasts so that the bitterness becomes rounder is plausible, but not proven. We haven't found a single publication that systematically tests roast level and water composition against each other. Anyone who tells you otherwise should be able to name the study. Our take: try it, take notes, decide for yourself.
What is proven for espresso, on the other hand, and often overlooked: the SCA handbook states that for espresso, alkalinity can be left at up to 150 ppm CaCO3 before a clear buffering effect on the acidity becomes noticeable. The reason is the brew ratio: at 1:2, around seven times less buffering capacity reaches each gram of coffee than with filter at 1:15 (a figure calculated from the two brew ratios, not a measurement). So filter coffee needs less alkalinity than espresso, not more.
Frequently asked questions
Is magnesium really better than calcium for coffee?
What is documented is a slightly higher extraction efficiency for magnesium. At the same time, the SCA Water Quality Handbook states that the sensory difference between calcium and magnesium remains unclear. An experimental re-examination (Bratthäll et al. 2024) found only limited differences at concentrations typical of drinking water. So “magnesium tastes better” is experience-based knowledge, not a measured result.
What water values are ideal for an espresso machine?
As a guide: total hardness 50–150 ppm CaCO₃, alkalinity 40–75 ppm CaCO₃, pH 6–8, chloride as low as possible – the SCA gives no value for this; in practice, below 5 mg/l is the rule of thumb – and no free chlorine. For espresso, according to the SCA handbook, alkalinity can go up to 150 ppm before the buffering effect becomes clear. Some manufacturers deviate from this – La Marzocco, for example, allows up to 30 ppm chloride.
Is a standard water softener enough for my dual boiler?
Usually not. Classic sodium ion exchangers swap calcium and magnesium for sodium, but they don't lower the alkalinity. And in Zurich and Bern, alkalinity is precisely the critical value. Cartridges that reduce alkalinity, or reverse osmosis followed by remineralisation, make more sense. La Marzocco explicitly advises against sodium-based softeners in its own specification.
Why is water that's too soft a problem?
Below 40 ppm CaCO₃ alkalinity, the SCA handbook warns of corrosion: even small amounts of dissolved CO₂ can push the water below pH 6. On top of that, very low-mineral water extracts less well. Demineralised water doesn't belong in an espresso machine.
Do dark roasts need different water from light roasts?
Here's the honest answer: there's no study that tests roast level against water chemistry experimentally. It's plausible that higher alkalinity dampens the acidity, making dark roasts taste rounder. But it isn't proven. We recommend trying it with your own coffee rather than following a rule of thumb.
Roastery tip
If you want to test a change of water, use a coffee you know by heart. Cozy Chocolate and Nutty Delight respond noticeably to alkalinity – too much bicarbonate takes away their definition. Wild Peach shows you most clearly when the acidity is being buffered away, because that's the one with the most acidity to lose. This is our observation from the roastery – judged by taste, not measured in a lab, and one of many.
Water treatment and descaling affect your machine directly. The figures on this page are guide values drawn from studies, manufacturers' specifications and our own experience. They replace neither a water analysis nor your manufacturer's requirements.
Your manufacturer's operating manual and water specification always take precedence. If you deviate from them, you risk your warranty and guarantee claims. If in doubt, have your water tested and stick to the values your manufacturer specifies.
We accept no liability for damage to equipment, for limescale damage or for the loss of warranty or guarantee claims arising from the application of these recommendations. All information is provided without guarantee.
