Why can coffee dry your mouth? Does it have anything to do with over extraction?
Dryness – or astringency?
The dry feeling in the mouth that you can sometimes get after drinking coffee is properly called ‘astringency’. This is a dry, sandy feeling on the tongue — the same feeling that you get from drinking strong black tea, or from eating an unripe banana.
While astringency is the sensation we’re interested in here, it’s worth noting that there are some other sensations that can be described as ‘dry’. For example, a dry white wine doesn’t typically contain much tannin, but is called ‘dry’ because it has low residual sweetness — so the acidity can give a similar puckering sensation in the mouth. However, this can be distinguished from astringency as it doesn’t linger on the tongue in the same way.
What causes astringency?
Astringency is caused by a group of molecules called tannins and pseudo-tannins. These are a large family of polyphenols, which have one thing in common: they bind to proteins in solution and make them precipitate. When you drink something containing tannins or pseudo-tannins, they bind to proteins in your saliva, making them precipitate onto your tongue. This literally drops small particles of aggregated protein onto your tongue, and also removes the lubricating effect of the proteins, leaving your tongue feeling dry and sandy.
Because tannins and pseudo-tannins have a similar effect, and both show up in many tests for tannins, both classes of molecules are often just called ‘tannins’ in the coffee industry, and this is generally how we refer to them. The most common examples of these molecules in coffee are not actually tannins in the strictest sense, however.
This sensation is generally unpleasant, but a bit of sweetness both reduces our perception of tannins and brings them into balance. In small amounts, tannins bring ‘structure’ and complexity to red wines, and perhaps to coffee too. So balance is the key – and achieving balance is all about controlling extraction.
Tannins and extraction
Many tannins are not very soluble in water, which means they only make it into the coffee at high extraction levels. So in general, over-extracted coffee will have a higher proportion of tannins, and taste more dry, than under-extracted. However, the largest tannins are so insoluble that they’ll only make it into the brew at very high extractions, approaching complete extraction (around 30%).
In a typical brew or espresso, larger tannins will only be extracted near channels. In and around a channel, the flow of water can be so great that localised extraction will approach 30%, even if the overall brew remains under-extracted. In a brew or espresso that channels, you’ll get increased tannins from the over-extracted areas around the channels, as well as reduced sweetness from the rest of the coffee bed, which can make the tannins overwhelming. This explains why coffee can be dry even if your extraction numbers are spot on.
Improving extraction will help limit tannins in the brew and keep them in balance. However, there are a few other factors that can affect our perception of tannins as well. We already mentioned that sweetness reduces the perception of tannins but on the other hand, acidity and saltiness increase the perception of tannins.
Perception of tannins also increases at lower temperatures, which is part of the reason that many people prefer some sugar in their iced coffee to bring it into balance. Incidentally, this effect is also part of the reason that white wines are served chilled, as they typically contain very little tannin.
Finally, tannins are also partially blocked from binding proteins in the presence of fats or oils. This is particularly relevant in espresso, where the high proportion of oils can help reduce the drying effect of tannins.