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Sour espresso is one of the most common complaints from home baristas, and it has a clear cause: under-extraction. When not enough soluble compounds are dissolved from the coffee grounds during the shot, the acids in the coffee dominate the cup. The result tastes sharp, lemony, or vinegary rather than balanced and sweet. The good news is that under-extraction is diagnosable and fixable once you know what to look for.
Espresso extraction is the process of water dissolving solids out of ground coffee. The first compounds extracted are acids and some fruity esters — pleasant in balance, but overwhelming when they are the majority of what ends up in your cup. Sweetness, body and bitterness develop later in the extraction. Under-extract and you get mostly the sharp early acids without the balancing sweetness and body.
Grind too coarse is the most common culprit. If the particles are too large, water flows through the puck too quickly, touching less surface area and dissolving less. A shot that runs in under 20-22 seconds (for a standard 1:2 ratio) is almost certainly extracting too fast. Other causes include: using too little coffee (low dose leaves too much headspace for water to channel through), too low a brew temperature (most modern machines run at 93-96°C; older or cheaper machines may run cooler), short pre-infusion or no pre-infusion (the puck does not have time to fully saturate and swell before full pressure hits), and channeling, which causes some parts of the puck to under-extract while other parts over-extract.
Start with the grind. Make it finer in small increments — on most grinders, one or two clicks at a time — and pull another shot. Watch the time: a balanced espresso at 1:2 ratio should generally run 25-35 seconds from first drip to end of shot, depending on the coffee and machine. If time is too short, go finer. If that does not fully resolve the sourness, check your temperature: increase brew temperature by 1°C if your machine allows it. Also check your dose: a standard double basket should hold 17-20g depending on basket size. Do not underdose.
Not all acidity is bad. A well-extracted Ethiopian Yirgacheffe is supposed to taste bright and fruit-forward. The difference between pleasant acidity and unpleasant sourness is balance: if the cup has sweetness and body alongside the sharpness, it is acidity. If it tastes one-dimensional, vinegary or mouth-puckering, it is sourness from under-extraction. Light-roast coffees with high natural acidity will always read as sharper than dark roasts; this is normal. The question is whether sweetness and structure are present too.
Before you change the grind: check dose weight is correct, check your puck is level and distributed evenly, check there is no obvious channeling visible after the shot. If everything else looks right and the shot still pulls fast and tastes sour, grind finer. One change at a time makes it easier to understand what is actually moving the flavour.
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