Why does my pourover taste sour?

You brewed it the way you always do, and it tastes sharp and thin — like biting a lemon, with the flavour gone almost as soon as you swallow. That cup is telling you something specific, and once you know what, you can fix it in one change.

Sour is almost always under-extraction

Coffee does not give up its flavours all at once. Brewing is a dissolving process, and different compounds dissolve at different speeds.

The acids tend to come out first. They are small, highly soluble molecules, and within the first stretch of contact with hot water most of what you will get of them is already in your cup. The sugars and the compounds that read as sweetness and body follow, more slowly. The bitter and drying compounds come last. The real chemistry overlaps rather than arriving in tidy stages, but as a working model this is close enough to brew by.

So a cup that tastes sour is a cup you stopped too early — not in the sense of pouring for fewer seconds, necessarily, but in the sense that not enough of the coffee actually dissolved. You got the front of the sequence and none of the middle.

Sour coffee often tastes thin alongside it, and it is worth being precise about why. If you are brewing at the ratio you always use, pulling less out of the grounds means less of everything ends up in the same amount of water — so sourness and a watery body show up together. But they are two different problems, not one: how much you extract and how strong you brew are separate dials. A heavy dose of under-extracted coffee can taste sour and not remotely thin.

Sour is not the same as acidic. A good Ethiopian washed coffee is bright — it tastes like citrus or stone fruit, and that brightness sits on top of sweetness and lingers. Sourness is sharp without sweetness underneath, and it disappears fast. If you are unsure which one you have, ask what happens after you swallow. Brightness leaves something behind; sourness leaves nothing.

Change your grind first

Of everything you could adjust, grind size moves extraction the most, so it is the first thing to change and usually the only thing you need to.

Grinding finer does two things at once. It increases the surface area the water can work on, so more dissolves per second. And it slows the water down as it moves through the bed, so there is more time for that to happen. Both push in the same direction: more extraction, more sweetness, less sourness.

Go a couple of steps finer than you did last time and brew the same coffee the same way. One change, one cup, then taste. If it is better but still a little sharp, go finer again. You are looking for the point where the sourness turns into sweetness — and if you overshoot, the cup goes bitter and dry, which tells you the sweet spot is just behind you.

A rough sanity check while you are there: a single-cup pourover that finishes draining in well under two minutes is almost certainly too coarse. Around 2:30 to 3:00 from first pour to last drip is the usual neighbourhood.

Then check the water temperature

If the grind change helped but did not finish the job, temperature is the next lever. Cooler water dissolves everything more slowly, which lands you in the same under-extracted place by a different road.

Off-boil is the working range for most home brewing — roughly 92–96 °C, or about thirty seconds after the kettle stops if you are not measuring. Two things quietly sabotage this: pouring into a cold brewer and cold mug, which can drop your real brewing temperature several degrees below what the kettle said, and light roasts, which are denser and less broken down by the roast — there is simply less soluble material for the water to find, and the bean resists being wetted. If you are drinking a very light roast, use water straight off the boil and grind finer than you would for anything else.

Then look at how evenly you poured

If the cup tastes sour and bitter at the same time, the problem is not the average — it is the spread. Water found a channel through the coffee bed, over-extracting a narrow path while barely touching the rest. You are tasting both failures in one sip.

Two habits fix most of it. Bloom properly: pour about twice the coffee's weight in water, wait thirty to forty-five seconds, and let the coffee release its trapped carbon dioxide before the real brew begins — a fresh coffee that has not degassed will push water away from itself. And wet everything: pour in slow circles that reach the outer edge of the bed, and give the slurry a gentle swirl at the end so the grounds settle flat instead of stranded up the sides of the filter.

The order matters more than the list

Grind, then temperature, then pour technique. That order is not arbitrary — it runs from the largest effect to the smallest, and it means you will usually be done after the first step.

The real discipline is changing one thing at a time. Change grind and temperature together and you learn nothing about either: if the cup improves, you cannot say which one did it, and next week you are guessing again. One variable, one cup, one note about what happened. That is how a recipe stops being something you copied and starts being something you know.

Keep track of what you changed

The reason this problem keeps coming back for most people is not that the fix is hard. It is that six weeks later, with a different bag, nobody remembers what the grind was when it finally tasted right.

That is exactly what Buna is for: log the brew, note the change, and the next time a cup goes sour you are adjusting from a record instead of from memory.