Sour espresso is under-extracted. The water left the coffee before it had dissolved enough of it, so the acids came through and the sugars that balance them did not.
Most people hear that and reach for the grinder. Grinding finer is the right instinct, but it only works if the water is actually passing through the whole puck — and very often it isn't.
The usual culprit is channeling, not the grind
Water takes the easiest path. If one part of the coffee bed is less dense than the rest — a clump, a crack, a low corner from a tilted tamp — the water races through that spot and barely touches the coffee around it. The shot comes out fast and tastes sour, and you can grind finer all morning without fixing it, because the problem is that most of your coffee is being bypassed.
This is why the same dose and the same grind setting can give a sour shot one morning and a decent one the next. Nothing changed except how evenly the puck was built.
How to tell which one you have
- Shot runs noticeably fast and tastes sour and thin — likely channeling.
- Shot runs at a normal pace and still tastes sour — more likely genuinely too coarse, or the dose is too low.
- Puck is wet, cratered, or falls apart when you knock it out — channeling.
- Sour at the start, better by the end of the cup — uneven extraction across the bed.
What actually fixes it
In this order, because each step removes a different cause:
- Stir the grounds before tamping. Grinders deliver coffee in clumps. Breaking them up with fine needles — the technique is called WDT — removes the densest spots before they can become channels.
- Tamp level, at a repeatable pressure. A two-degree tilt is enough to channel and most people cannot feel it. A calibrated tamper fixes the pressure; a palm tamper fixes the angle.
- Then adjust the grind. Once the bed is even, a grind change does what it is supposed to do, and you can read the result.
What will not fix it
Tamping harder. Past roughly 15 kg the puck is as compressed as it is going to get, and leaning into it mostly makes the tamp less level. Also not a new machine: sourness from channeling happens on a €200 Dedica and a €2,000 E61 alike, because it starts in the basket.
Honest note — if your beans are more than a month past roast, or you are pulling a very light roast on a machine that cannot hold temperature, puck preparation will only take you so far. Fix the bed first, because it is free to diagnose and it is the most common cause. Then look at the beans.