Same difference:
http://en.wikipedia.org/wiki/Alpha-amino-acid_esterase
Also:
The degree of bitterness imparted by hops depends on the degree to which otherwise insoluble
alpha acids (AAs) are isomerized during the boil, and the impact of a given amount of hops is specified in International Bitterness Units (IBUs). Unboiled hops are only mildly bitter. On the other hand, the (non-bitter) flavor and aroma of hops come from the
essential oils, which evaporate during the boil.
The four major
essential oils in hops are
Myrcene, Humulene, Caryophyllene, and Farnesene which comprise about 60–80% of the essential oils for most hop varieties
Myrcene, or β-myrcene, is an olefinic natural organic compound. It is classified as a hydrocarbon, but more precisely as a monoterpene. Terpenes are dimers of isoprene and myrcene is one of the most important. It is a component of the
essential oil of the several plants including bay, ylang-ylang,
wild thyme, and hops
Were does Thymol come from ?