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Can someone explain what exactly an alpha acid is and why should I care about the alpha acid "rating" on the hops I use.

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Alpha acids are compounds found in the flower of the hop plant and are the primary source of bitterness in beer. The alpha acids are isomerized in the boil to form iso-alpha acids. The degree of isomerization and the level of bitterness extracted is determined by the length of time the hops are boiled. The alpha acid rating is the amount of alpha acid by percentage of total weight of the hops. The higher the percentage alpha acid the more bitterness a hop will contribute during the boil. The alpha acids for a particular hop variety can vary due to growing conditions, drying methods, and age of the hop.

Alpha acids also contribute to foam stability and head retention in beer.

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Great answer to a deceptively complex question. –  Pulsehead Jan 10 '11 at 19:01
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Alpha acids are what cause the bitterness during your wort boil. The higher the %, the more bitter they will cause your beer to be during boiling.

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