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In page Tannin:

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In addition to the alpha acids extracted from hops to provide bitterness in beer, condensed tannins are also present. These originate both from malt and hops. Trained brewmasters, particularly those in Germany, consider the presence of tannins to be a flaw[citation needed]. However, in some styles, the presence of this astringency is acceptable or even desired, as, for example, in a Flanders red ale.[1]