Perceived as a mouth feel rather than a flavor, astringency stimulates free nerve ending throughout oral cavity. It is a mouth-puckering membrane-contracting sensation that is comparable to chewing on grape skins or grape seeds found mostly on the roof of the mouth, back of the mouth, underside of the tongue and insides of the cheeks.
Wine makers naturally extract tannins from grape skins, seeds and stems. It gives the wine astringency or character. Tannin is increased in wines when using oak. In some cases wine tannin needs to be added to wines which are naturally low in tannin like peach and apricot. Tannin helps clear and prolong the life of a good wine. Balanced tannins are required for a good quality wine. Too little will result in a wine that is insipid and lacks depth of character while too much will cause a rough flavor and mouth-puckering dryness.
In Brewing astringency is often produced by the extraction of tannins from grain husks due to over crushing, over sparging, or sparging with alkaline water, high sulfate water or water above 170° or boiling adjunct grains. Over hopping or hops in contact with wort too long can result in astringency. Think about your oral cavity sensations after drinking a really hoppy IPA. That's astringency. That ugly brown scum thrown up during vigorous fermentation will contribute astringency as it falls back through the beer once fermentation had completed. It's best to separate the brown scum by using an oversized carboy so it will cling to the glass or a blow-by hose so it goes down the hose into the receiving water vessel. Over-attenuation and low dextrin levels can increase the perception of astringency. In General astringency may be produced by polyphenols that result from spoilage by acetobacter or wild yeast, letting beer or must sit too long on the trub or lees, or oxidation, in which case the responsible compounds are polyphenols and aldehydes. Polyphenols can be reduced by fining with Polyclar. Spices such as coriander, orange peel and cinnamon also contribute astringent flavors, but these tend to mellow with age.