The concentration of humulene varies among different varieties of the plant but can be up to 40% of the essential oil.[3] Humulene and its reaction products in the brewing process of beer gives many beers their “hoppy” aroma. Noble hop varieties have been found to have higher levels of humulene, while other bitter hop varieties contain low levels.[4] Multiple epoxides of humulene are produced in the brewing process. In a scientific study involving gas chromatography–mass spectrometry analysis of samples and a trained sensory panel, it was found that the hydrolysis products of humulene epoxide II specifically produces a “hoppy” aroma in beer.[5][6]