Why does my coffee taste sour when I grind too coarse?

The relationship between grind size and flavor is governed by the science of extraction. When you grind coffee, you are essentially increasing the surface area of the beans, allowing water to access the soluble compounds inside. If your grind is too coarse, the water flows through the coffee bed too rapidly, and the individual particles are too large for the water to fully penetrate. This leads to under-extraction, where the coffee only releases its most easily dissolved compounds—primarily acids—while the sugars and deeper, more complex flavors remain trapped inside the grounds.

In the cup, this manifests as a sharp, sour, or salty flavor profile. You might also notice that the coffee feels thin or watery, lacking the body and sweetness that characterize a well-balanced brew. Because the water hasn't had enough contact time with the coffee to extract the balancing bitter compounds, the acidity dominates the palate. This is a common issue for beginners who may be using a grind setting intended for a French press when they are actually brewing with a pour-over or drip machine.

To fix this, you need to increase the extraction by making your grind finer. A finer grind increases the surface area and slows down the flow of water, giving it more time to dissolve the sugars and heavier compounds that provide sweetness and balance. If you are using a burr grinder, adjust your setting toward the 'fine' end of the spectrum in small increments. If you are using a blade grinder, you will find it difficult to achieve the consistency needed to avoid this issue, as blade grinders produce a mix of boulders and dust that leads to uneven extraction.

It is important to note that sourness can also be caused by water temperature that is too low or a brew time that is too short. However, if your technique and temperature are consistent, the grind size is almost always the culprit. By dialing in your grind to be slightly finer, you allow the water to extract the full spectrum of flavors, transforming a sharp, sour cup into one that is sweet, balanced, and satisfying.

In practice, think of your grind size as a dial for flavor. If your coffee is sour, move the dial toward finer. If it is bitter or astringent, move it toward coarser. Through small, incremental adjustments, you can find the 'sweet spot' where the acidity is bright and pleasant, rather than overwhelming and sour.