Astringency is often listed as a “taste” but is as much a physical sensation of beer as it is a flavor. Although not particularly hard to define, it is often confused with bitterness and is easily misdiagnosed in tasting profiles. While some balanced astringency can be pleasant in certain beer styles, notable astringency is generally considered to be a flavor fault in beer.
A simple analogy of astringency without the bitterness is the dryness produced by a strongly brewed cup of tea. Tea releases similar phenolic compounds to beer and causes similar physical responses of a dry mouth, a slightly grainy feel and, after time, a thirstiness—in part to clean the palate.
Astringency results from phenolics, particularly polyphenols in beer. Phenols arise from the husks of malt and the stems of hops and polymerise to polyphenols during brewing and in beer maturation. These polyphenols include drying, mouth-puckering tannins. Polyphenols are attracted to protein molecules causing them to co-precipitate both in the boil and later as beer matures. However, some remains to provide astringency when beer is tasted.
In the mouth polyphenols are also attracted to proteins—particularly those of the mouth epithelium. A consequence of this is that they contract the mouth surface, providing an impression of dryness.
As with most flavors an optimal amount is acceptable, excess is not. Well-crafted beers will contain only enough astringency to balance bitterness and sweetness. Poorly crafted beers cause uncomfortable dryness and a desire to move to a different drink. Excess levels may arise from specific ingredients but most often from poorly managed sparging, after the mash, particularly with high pH liquor.