Part (a) shows the chemical structure of humulone which is an example of an apha acid contained within hops. This acts as an antibacterial agent within the beer as it is being brewed. Part (b) shows the chemical structure of iso-humulone. When humulone is heated during the brewing process, its chemical structure changes and the atoms rearrange to form the compound iso-humulone. This new compound imparts a bitter flavour into the finished beer.