Some styles, like the brown ale or hefeweizen, minimize the apparent bitterness and use just enough delicately-flavored hops to bring things into balance. Others, like IPAs, intentionally use assertive hops and showcase the bitter qualities (along with other hop flavors.)Read the full story at Serious Eats.
When you buy hops, you'll probably find an %AA marked somewhere on the package. This number is the percentage of alpha acids by weight in the package. Alpha acids produce the bitterness in hops, so this percentage suggests how bitter one hop variety is compared to another. For example, a popular American hop called Cascade is generally rated 6%AA, while another type called Magnum is around 12%. If you make two versions of the same beer recipe, one using Cascade hops and the other using the same amount of Magnum hops, you will find the second one much more bitter (and differently flavored in general.)
The percentage of alpha acids is usually pretty consistent for each variety, but it can vary from crop to crop and year to year. If you're following a recipe that calls for 7%AA Amarillo hops, and you purchase a package listed as 13.5%, you should cut the amount you use in half or you'll end up with a very different beer than you intended. If a recipe calls for 3.5% AA Saaz hops and you buy 4%, I would consider that close enough and not really adjust the recipe.
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