It is the alpha acid contained in the lupulin of the hop cone that gives beer its bitterness. Varieties classified as bitter hops, typically have alpha acid levels of at least 8–10% of dry‐weight. Some bitter hops are classified as high alpha hops, because they have alpha acid levels as high as 18%.
The varieties Columbus, Tomahawk® and Zeus, collectively referred to as CTZ, are three of the key bitter hops grown extensively in the USA. The variety Pride of Ringwood is Australia’s most recognised bittering variety.