Why is my coffee sour?

Why does it taste sour? Sourness is a sensory response to acids. Harsh, thin sourness may indicate insufficient or uneven extraction, while pleasing citrus-like brightness can belong to the coffee itself. The old explanation that acids extract first, sugars second, and bitters last in three rigid stages is an oversimplification. To test extraction, brew the same dose and water quantity with a slightly finer grind, then taste. If needed, adjust contact time or water temperature on the following brew, one variable at a time. For espresso, grinding too fine can cause uneven flow, so monitor results rather than assuming finer is always better. Brewing water's alkalinity also affects how acidity is perceived. Cold brew demonstrates that low water temperature alone is not a defect. Beans are not automatically sour because they are more than four weeks past roasting. Compare several coffees and water sources before assigning a single cause.