I moved a mile above the sea and my coffee went wrong in a way I could not name. It took two weeks and a thermometer to find out that the water had changed, not the recipe.
I have moved to a town a mile above the sea, and my coffee has gone wrong in a way I cannot name.
The recipe is the same and the beans come from the same roaster, shipped from the same place, and the grinder came in the moving truck wrapped in a towel. Boulder sits at the foot of the Rocky Mountains, and in November the sky here is a hard, bright blue I did not know sky came in, the blue of a gas flame, and the cold has a clean, dry smell like a freshly ironed shirt, and every cup I have made in the last two weeks has been thin and a little sour, like tea that has been dunked once and taken out.
Rob, who sits across from me at the laboratory where I now work, keeps a dented steel thermos on his desk and says the altitude will sort itself out, and it does not sort itself out, and it took me two weeks and a kitchen thermometer to learn that the kettle had been honest with me all along and the water had been lying.
I left Alabama in the summer, and left Sync Energy there with it, and took a job as a scientist at a national laboratory here, modelling the electricity grid on machines that fill a room and hum like a server farm in a film. What I build now, on the side, at night, is a company called Plexflo, which lives almost entirely on a screen, in video calls with a small team in Bengaluru whose mornings begin as my evenings end, so that my day has two shifts and a sunset between them. I have told myself this job is a bridge, and I am not sure it is, but I am sure that the water here boils at a temperature it did not boil at before, and that this is the kind of problem I can actually solve, so I am going to solve it.
The machines at the laboratory hum, and the first time I heard them through a closed door I was small again, on the terrace of my grandfather's house in Tamluk, in Purba Medinipur, looking down into the backyard. The ground floor and the yard had been rented to the West Bengal State Electricity Board, and the yard was full of distribution transformers, grey steel boxes in rows, each with two white porcelain bushings standing up from its lid like the horns of Mahishasura's buffalo in a Durga Puja pandal, and all of them humming one low, steady note under the crows and the pressure cookers and the long wet afternoons, a note I now know is steel flexing a hundred times a second to the fifty cycles of the Indian grid, and it is the earliest thing I remember. I grew up listening to the grid before I knew there was a word for it, and I seem to have spent my working life walking back toward that terrace, and here, a mile above the sea, in a building full of computers pretending to be power lines, the note has found me again.
Water as an ingredient
The short version comes first, because it is most of the benefit: coffee is nearly all water, and if your water tastes bad cold it will taste bad hot with coffee in it, so that chlorine comes straight through with its swimming-pool sharpness and a stale, flat taste comes straight through too. Filter it, or use bottled water with some minerals in it, and never brew with water you would not happily drink on its own from a glass, and if you stop reading here you will have most of what matters.
Water as a solvent, which nobody explains
Water is also the thing that dissolves the coffee, the way warm water dissolves the sugar at the bottom of the mug and cold water leaves it sitting there, and how well it does that depends on what is already dissolved in it. Very hard water is a Howrah bus at rush hour, already carrying a heavy load of minerals and full to the footboard, with little room left to pick up the good things waiting in the grounds, and so it makes a poorer solvent; water with nothing in it at all goes wrong in its own way, because completely demineralised water makes flat, hollow coffee, coffee with the colour and none of the weight, and a few passengers of the right kind help the water do its job.
Which minerals, then? A few years ago a young chemist whose day job is simulating materials with quantum mechanics sat down with two coffee professionals and computed how strongly the common ions in water bind to the acids and aromatic compounds in coffee, and their paper found that magnesium is the better extractor and calcium is mostly what furs your kettle white and bicarbonate buffers the acidity away. They wrote a small book about it afterwards, and the chemist's stated aim, which I have pinned above my desk between a photograph of my son and a printout of a feeder model, was that science is here to make coffee simpler.
The brewing-water range published by the Specialty Coffee Association, with three readings from my own kitchens: New York's published average, and the tap and filtered water in Whitefield, Bengaluru, from a cheap meter.
The trade body publishes targets for brewing water, with ranges for the total dissolved minerals and for hardness and for alkalinity, and no chlorine, and a pH near neutral. I have put the main one on the chart with three readings from my own kitchens, and two things on it surprised me. New York's tap water, which everyone praises, is actually very soft, below the target, and so is Boulder's, which means soft water can be as wrong as hard, only in the other direction. And the two Indian readings come from a later kitchen, in a suburb of Bengaluru called Whitefield, where the tap water reads at the far right of the chart, hard as a hotel towel, and the filter under the sink brings it down to the far left; I did not have that meter yet this winter, and I wish I had.
The meter that reads total dissolved solids costs about as much as a takeaway dinner and is the cheapest diagnostic in coffee, and it is a conductivity meter in disguise, turning how easily a small current flows between two metal pins into parts per million, which would please my father, a professor of electrical engineering who has spent his life teaching what a current does when it meets a material. It reads one number, the total, and cannot tell hardness from alkalinity, so treat it as a compass that tells you roughly where you stand and leaves the map to you.
Making your own water
If your tap is very hard, or very soft, or simply unknown, you can make brewing water at home, which is fiddly, like sewing on a button, and no harder.
Start with distilled or reverse-osmosis water, never tap.
Get Epsom salt (magnesium sulphate, from any chemist) and baking soda (sodium bicarbonate, already in your cupboard), both food grade.
Expect a fraction of a gram of each per litre: a pinch smaller than a pinch, below what a kitchen scale can read.
Either mix a large batch, four litres at a time, so the weights become readable.
Or make a concentrate: dissolve the minerals in a small bottle of distilled water, then add a few millilitres of it to each litre you brew with. One small bottle lasts the way a bottle of vanilla essence lasts a family a year.
There are commercial sachets that do the same thing, sold in the United States, and when I later looked for them in India I could not find them; India is do-it-yourself territory for water, and I think it is better that way, because a meter and a bottle of concentrate teach you what a sachet hides.
Your kettle changes with altitude
Temperature, and the part this note is really for
Most brewing advice says to use water a little below the boil, around ninety-three degrees, give or take a few, and you will see a narrower range quoted everywhere, but the wider one is what the trade body actually publishes, and the width matters, because the width is where altitude lives. Darker roasts like the lower end, and I like darker roasts, so I live at the lower end, while lighter roasts like the upper, and you should leave this dial alone until grind and ratio and time and water quality are settled, as the extraction note explains.
What I did not know when I unpacked the kettle from its towel is that at sea level water boils above the brewing range, so the standard advice is to boil and then wait half a minute for it to cool, the way you let a bath run before you get in; in Boulder, water off a rolling boil is already in the middle of the range, and half a minute in an open kettle puts you near the floor of it, and every degree lost to a cold ceramic cone or a cold mug is a degree you cannot get back.
Open-kettle boiling points for towns in these posts. The shaded band is the 90 to 96 °C range most brewing advice assumes. Values are approximate and move with the weather.
So the advice inverts, and up here you pour straight off the boil and preheat everything, the brewer and the filter and the cup, until they are warm to the palm like a plate out of the dishwasher, and I had been doing the opposite, faithfully, for two weeks. A roastery in this town built an entire brewing method around exactly this, pulling their batch brews at a temperature that would ruin coffee at sea level and cooling them fast over ice, and one of their people calls the result indoor cowboy coffee, which I find funny and also correct, since I have a soft spot for cowboy coffee, black and boiled and oily, which is closer to what I drink than most of the trade would admit.
There is a second consequence, because cooler water extracts more slowly, so at altitude you grind a little finer, or extend the contact a little, or both, to get back to where you were. A brewer who works at a similar height told an interviewer that he favours a pressure brewer up here because you can make up for the low air pressure with a little pressure of your own, and the batch machines sold nationally are calibrated at sea level, and someone who repairs them says they give a thin cup in the mountains, which was my cup exactly, thin as the air it was brewed in.
Espresso is mostly exempt from all this, because a pressurised boiler sets its own boiling point whatever the room, and the exceptions are small: steam is a little cooler at altitude for the same gauge reading, and the gas in fresh coffee comes out of solution faster in thinner air, which one local roaster describes as crema with the texture of popping candy on the tongue. Never having owned an espresso machine here, I am repeating what owners tell me, and the espresso note will one day say why I probably never will.
What I could not find
I looked for a published figure for Bengaluru's water, since half of my company drinks it every morning, and I could not find one that I trusted. People assert that the piped water from the Kaveri is soft and the borewell water is hard, and they are probably right, but nobody cites anything, and I have spent enough of my working life chasing uncited numbers about the Indian grid to know what that is worth. It is the same Kaveri that was turning turbines at Shivanasamudra in 1902, at one of the first hydroelectric stations in Asia, sending power a hundred and forty kilometres to the gold mines at Kolar, so that in that corner of India water and electricity have shared a sentence for more than a century, and the river has been measured in megawatts far more carefully than it has ever been measured in a glass. When I did get there, years later, I bought the meter and took my own readings, which is what I would tell you to do wherever you live, since your kitchen is the only kitchen that matters.
A green field below the Flatirons in Boulder, Colorado. Photo by Malachi Brooks on Unsplash.
The cheapest upgrades in coffee
In the order I now understand them, each one a small act of attention that happens to cost a little money:
A roast date on the bag.
A scale.
A water meter.
Water you mixed yourself.
If you do one thing this morning:
Boil the kettle.
Put a thermometer in it.
Find out what your own water does.
Decide whether you wait thirty seconds for a reason or out of habit.
The next dial is the biggest one, the grinder, and Rob still says the altitude sorts itself out, and he drinks his coffee from the thermos, made at home before dawn, black and probably too hot, and he is happy, and I have decided not to tell him anything.
Questions this note answers
Does water really change how coffee tastes?
Yes, twice. You taste the water itself, so chlorine or a stale flavour comes straight through. And the minerals in the water decide how well it dissolves the coffee: very hard water extracts poorly, and fully demineralised water extracts poorly too.
What temperature should water be for coffee?
Around 93 degrees Celsius, give or take three. At sea level that means boiling and waiting half a minute. Above about 1,500 metres the kettle already boils inside that range, so pour straight off the boil and preheat everything.
Can I make coffee water at home?
Yes. Start with distilled or reverse-osmosis water and add a little Epsom salt and baking soda, weighed on a scale. Mixing a concentrate in a small bottle and adding a few millilitres per litre is the easiest way to get the tiny amounts right.
