Why is my espresso sour and how do I fix it?

Sourness in espresso is often misunderstood. While some coffees are naturally bright and acidic, a 'sour' shot that lacks sweetness and leaves a sharp, unpleasant sensation on the tongue is almost always a sign of under-extraction. This happens when the water passes through the coffee puck too quickly or at too low a temperature, failing to dissolve the sugars and heavier compounds that balance out the acidity.

To fix this, start by looking at your grind size. If your shot is pulling in under 20 seconds, your grind is likely too coarse. By grinding finer, you increase the resistance in the puck, forcing the water to spend more time in contact with the coffee. This extra contact time allows for more thorough extraction, which will bring out the sweetness and body you are missing.

If your grind is already fine and the shot is still sour, consider your yield. If you are aiming for a 1:2 ratio (e.g., 18g in, 36g out) and it tastes sour, try pulling a slightly longer shot—perhaps 18g in, 40g out. That extra water will pull more solubles from the puck. Alternatively, check your water temperature. If your machine is set too low, the water won't have enough energy to extract the coffee properly. Increasing the temperature by a few degrees can often resolve sourness in light-roast coffees.

In practice, always check your puck preparation. If you have 'channeling'—where water finds a path of least resistance through the puck—the rest of the coffee remains under-extracted, leading to a sour, uneven cup. Use a distribution tool or a leveler to ensure your coffee bed is flat and even before tamping. By addressing these variables one by one, you can transform a sour, unpleasant shot into a balanced, sweet, and complex espresso.