How to tell if coffee is under-extracted?

Extraction is the process of pulling flavor, oils, and solids out of the coffee grounds and into your water. When you under-extract, you are essentially stopping the process too early. Because the different compounds in coffee dissolve at different rates—acids first, then sugars, and finally bitter compounds—an under-extracted cup is dominated by the initial, sharp, acidic phase, without the balancing sweetness of the sugars that come later.

The result is a cup that tastes aggressively sour, sometimes even salty or grassy. It lacks the 'body' or mouthfeel that makes coffee satisfying, feeling instead like thin, flavored water. The finish is often short and leaves a lingering, unpleasant sensation on the tongue. If you find yourself adding sugar or milk to mask a sharp, biting flavor, you are likely dealing with under-extraction.

There are several common reasons for under-extraction. The most frequent is a grind size that is too coarse, which allows the water to pass through the grounds too quickly without picking up enough flavor. Another cause is water that is not hot enough, as lower temperatures lack the energy to dissolve the necessary compounds. Finally, if your brew time is too short, you simply haven't given the water enough contact time with the coffee to do its job.

In practice, fixing under-extraction is straightforward. If your coffee tastes sour, try grinding your beans finer to increase the surface area and slow down the water flow. Alternatively, you can increase your brew time or slightly raise your water temperature. Make one change at a time so you can taste the difference. By adjusting these variables, you will eventually find the 'sweet spot' where the acidity is balanced by sweetness, resulting in a full, complex, and delicious cup.