The experience of drinking coffee often includes a distinct, slightly rough sensation on the tongue and the inside of the cheeks. This feeling is frequently described as the coffee “sticking” or leaving a dry, textured coating in the mouth. This physical mouthfeel is technically known as astringency, which can make the oral tissues feel constricted or rough. This common physiological reaction is the result of a direct interaction between compounds extracted from the coffee bean and the lubricating fluid in the mouth.
Coffee’s Chemical Culprit: Polyphenols
The chemical agents responsible for initiating the drying sensation are naturally occurring compounds called polyphenols. These molecules are present in many plants and contribute to the color, bitterness, and overall sensory profile of coffee. Within coffee, the most abundant and active polyphenols are chlorogenic acids, often referred to as tannins due to their similar mouthfeel effect in beverages like wine and tea. These polyphenols are highly soluble in water, which is why they are effectively extracted into the cup during the brewing process. Their mechanism remains consistent: they are designed to bind with proteins. The presence of these protein-binding compounds in the coffee sets the stage for the physical sensation experienced on the tongue.
The Science of the “Sticky” Sensation
The physical feeling of stickiness or dryness is a consequence of these polyphenols disrupting the mouth’s natural lubrication system. Saliva contains specialized proteins, primarily mucins, whose function is to coat the oral tissues and provide a smooth, protective film. This film allows the tongue and cheeks to glide with low friction.
When coffee is consumed, the polyphenols rapidly encounter these salivary proteins and begin to bind to them. This chemical binding causes the proteins to clump together, or precipitate, effectively removing the lubricant from the saliva. The loss of this protective film increases the friction between the tongue and the roof of the mouth and the cheeks.
It is this sudden increase in friction that the brain interprets as the rough, dry, or “sticky” sensation of astringency. The feeling is not actual dryness due to a lack of water, but rather a temporary loss of the smooth, lubricating coating. The mouth’s glands quickly work to restore the protein film, which is why the sensation usually fades shortly after the coffee is swallowed.
Why Some Coffees Are Worse Than Others
The intensity of the drying sensation is related to the concentration of polyphenols in the final cup, which is influenced by several factors.
Bean Variety
The type of coffee bean plays a role, as the Robusta variety generally contains a higher amount of chlorogenic acids compared to the Arabica bean.
Degree of Roasting
The degree of roasting is a major determinant, with lighter roasts retaining significantly more polyphenols than darker roasts. The high heat and extended time of dark roasting break down many astringent compounds, resulting in a less drying mouthfeel. A light roast, by preserving these compounds, naturally produces a more astringent beverage.
Brewing Technique
The brewing technique can also amplify the sensation. Methods that lead to over-extraction, such as using a grind that is too fine or allowing excessively long contact time with the grounds, pull out more polyphenol molecules. This increased extraction concentrates the astringent compounds, making the resulting cup feel rougher on the palate.