How do I stop my espresso from tasting sour?

When your espresso tastes sour, it is almost always a symptom of under-extraction. In the world of coffee science, extraction happens in stages: first, the acids are pulled out, followed by the sugars and sweetness, and finally the bitter compounds. If your shot is sour, you have stopped the extraction process while it was still in the 'acidic' phase, failing to reach the sweet, balanced middle ground that makes for a great cup.

To troubleshoot this, look first at your grind size. If your grind is too coarse, water flows through the puck with too little resistance, resulting in a fast, watery, and sour shot. Adjust your grinder to a finer setting to increase the contact time between the water and the coffee. A good target for a standard double shot is to aim for a 1:2 brew ratio—meaning if you put 18 grams of dry coffee in, you should aim for 36 grams of liquid espresso out in about 25 to 30 seconds.

If your grind is already fine and you are still getting sourness, consider your water temperature. If your machine is running too cool, it won't have the thermal energy required to extract the deeper, sweeter compounds. Ensure your machine is fully preheated and, if possible, increase the temperature by a few degrees. Additionally, check your distribution; if the coffee bed is uneven, water will 'channel' through the path of least resistance, leaving parts of the puck under-extracted even if the overall timing seems correct.

Finally, remember that the coffee beans themselves play a role. Very light roasts are naturally more acidic and can be difficult to extract. If you are using a light roast, you may need to push your extraction further than you would with a medium or dark roast. Don't be afraid to let the shot run a few seconds longer or use a slightly higher water temperature to coax out the sweetness you are looking for.