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A cup of filter coffee is around 98 per cent water. We put a great deal of care into the other two per cent: where the beans are grown, how they are processed, how we roast them, how finely you grind them. The water, for most of us, comes straight from the tap without a second thought.
Yet if you have ever made your usual coffee at a friend's house or a holiday cottage and found it tasted noticeably different, the water is the most likely reason. A study published this autumn by chemists at the University of Copenhagen has sharpened our understanding of exactly what in that water matters. The answer is not the one most people would guess, and the clue is probably sitting on your kitchen worktop.
Hard water, soft water and the UK's chalk line
When people talk about water and coffee, they usually talk about hardness. Hard water carries more dissolved calcium and magnesium, picked up as rain filters through the rock beneath our feet.
In the UK, the dividing line is largely geological. Much of the south and east of England sits on chalk and limestone, which dissolve readily, so London, the South East and East Anglia have some of the hardest tap water in the country. Scotland, Wales, north-west England and much of the South West draw their water from upland catchments and harder rocks such as granite, which give up far less, so their water is much softer.
The long-standing assumption in coffee has been that hardness itself shapes the flavour. The new research suggests that is only part of the story.
What the Copenhagen study found
The team, publishing in the Journal of Food Science in September, started with ultrapure water and added carefully measured amounts of three things found in ordinary tap water: calcium, magnesium and bicarbonate. A trained tasting panel then compared coffees brewed with each version against a reference.
The results were clear. Adding calcium and magnesium on their own made no marked difference to the flavour, even when calcium went beyond the levels normally found in drinking water. Bicarbonate was a different matter. Across the whole range tested, more bicarbonate meant less acidity, less fruit and less complexity, along with more roasted and bitter notes. Even small increases were enough to change the cup. The researchers concluded that alkalinity, rather than hardness, was the main driver of the change in flavour.
Why does bicarbonate matter so much? It is a buffer, which means it neutralises acid. In coffee, acidity is not sourness. It is the brightness and juiciness that carries the fruit, from acids such as citric and malic acid, the same ones found in lemons and apples. Bicarbonate in your water neutralises some of those acids during brewing, leaving the cup flatter and heavier.
It is worth being clear about the limits. The study used one Colombian coffee and laboratory-made water, so it is not the final word. A well-known 2014 study from the University of Bath, by chemist Christopher Hendon and UK Barista Champion Maxwell Colonna-Dashwood, used computer modelling to suggest that magnesium helps draw flavour from the grounds. The Copenhagen panel could not taste that effect. On the central point, though, the two studies agree: high bicarbonate is bad news for flavour.
The clue in your kettle
If you want to know whether your water is high in bicarbonate, look inside your kettle.
In chalk and limestone areas, much of the calcium in the water arrives as calcium bicarbonate. Heat it, and some of that breaks down into calcium carbonate, which settles out as the white crust of limescale. A furred-up kettle is a visible sign of the same bicarbonate that flattens coffee. A single boil only removes part of it, so plenty still ends up in your cup.
Water companies publish quality reports by postcode, and many list alkalinity alongside hardness. In much of London and the South East, it typically runs at several times the level recommended by the Specialty Coffee Association's water standard. If you live in a soft water area, the good news is that your tap water is naturally low in bicarbonate. According to this research, that is the part that counts.
Which coffees have the most to lose
If bicarbonate dulls acidity and fruit, the coffees most affected are the ones that depend on them. That includes some of the world's most celebrated origins, among them the bright, washed coffees of East Africa we have been writing about in recent weeks.
Our Kenya Nyeri AA is a good example. Its blackcurrant character and lively acidity are exactly what high-bicarbonate water takes away. The same is true of the elegant, aromatic cup from our Ethiopia Irgacheffe. If you have ever found a celebrated Kenyan coffee a little flat at home, the coffee may not be the problem.
Coffees built around chocolate, nuts and body have less fruit to lose, so they tend to be more forgiving. They are not immune, though: the same study found that bicarbonate pushed coffee towards heavier, more bitter flavours.
What you can do about it
Use a filter jug, and keep it fresh. Most standard filter jugs contain an ion-exchange resin designed to reduce "carbonate hardness", the bicarbonate-linked kind behind limescale, along with carbon to take out chlorine. They will not remove everything, but they bring bicarbonate down noticeably. Change the cartridge on schedule, because in hard water areas it works harder and runs out sooner. As a bonus, less limescale means a longer life for your kettle and coffee machine.
Don't count on a water softener. Salt-based softeners swap calcium and magnesium for sodium, but leave the bicarbonate where it is. They protect your boiler and pipes, not your coffee. The Bath study also found that sodium-rich softened water did nothing to improve flavour.
Read the label on bottled water. Mineral waters list their bicarbonate content on the label, sometimes written as HCO3. For coffee, lower is better. The Specialty Coffee Association's water standard works out at roughly 50mg per litre, and plenty of familiar brands list considerably more.
If your water is soft, keep it simple. Fresh, cold water from the tap, run through a jug if chlorine bothers you, is likely to serve you well.
Taste the difference for yourself
The easiest way to understand all of this is to try it. Brew two cups of the same coffee, with the same grind, dose and method. Make one with water straight from the tap and the other with freshly filtered water or a low-bicarbonate bottled water. Let both cool for a few minutes, then taste them side by side.
A bright coffee makes the difference easier to spot. Our Kenya Nyeri AA is ideal, and you will find it alongside our other East African coffees in our African coffee collection. Notice where the fruit sits in each cup, how lively the finish is, and whether one tastes heavier or more bitter than the other.
Every coffee we roast has been through a lot of careful hands: the farmers who grow it, the washing stations that process it, and finally us. The last step happens in your kitchen, and the water you pour is as much a part of it as the beans. A few small changes can make sure the flavour that set out from Nyeri or Yirgacheffe actually reaches your cup.
