What causes the astringency in coffee?

Astringency is often confused with bitterness, but they are distinct sensory experiences. While bitterness is a flavor detected by the tongue, astringency is a physical sensation caused by the interaction of compounds like tannins with the proteins in your saliva. This interaction creates a drying, rough, or 'sandy' feeling in the mouth, similar to the sensation of drinking a very strong, over-steeped black tea or a tannic red wine.

In the context of coffee, astringency is almost always a sign of over-extraction. During the brewing process, water extracts compounds in a specific order: first the acids and fats, then the sugars, and finally the plant fibers. If the extraction process continues too long or is too intense, the water begins to break down the structural components of the coffee bean. These fibers and complex phenolic compounds are what contribute to that unpleasant, drying mouthfeel.

Several factors can lead to this over-extraction. Grinding your coffee too fine increases the surface area significantly, allowing water to extract those structural compounds much faster than intended. Similarly, using water that is too hot can accelerate the extraction of these compounds, even if the contact time is relatively short. Even the quality of the coffee can play a role; some processing methods or lower-quality beans may naturally contain higher levels of compounds that contribute to astringency.

To fix astringency in your daily brew, the goal is to dial back the extraction. If you are using a pour-over, try a slightly coarser grind or a lower water temperature. If you are pulling espresso, you might need to decrease your yield or shorten your brew time. By focusing on extracting the sweet, aromatic compounds while avoiding the structural fibers, you can eliminate that drying sensation and achieve a much smoother, more pleasant cup.