Hops

Sugar from malt has already been converted to alcohol by yeast.  So why do we need hops?  The hops play two key roles.  Firstly, the residual sweetness of any unfermented sugar is pleasantly balanced by the bitterness provided by hops.  Secondly, the hops have anti-microbial properties that help prevent the beer from spoiling.  Several other plants have been used to the same effect, for example different types of roots in root beer, but for various historical and geographical reasons, hops won out.  Modern-day Germany is the epicenter for hop production as the climate there is suited to their cultivation.  It is not co-incidence that the Germans are then famous for their beer, while their Southern neighbors made wine from the grape vines thriving in the mediterranean regions (and those further north made vodka from whatever they could get to ferment).

The bitter compounds in hops are a class of chemicals called the alpha acids.  These acids are present in the hop cones, which are the female flower clusters of the hop plant.  Interestingly the alpha acids have to go through an isomerization process to become bitter.  Isomers are two different arrangements of the sample atoms.  The isomerization occurs during boiling, so a key process in brewing is the boiling of the hops in water also containing the sugars extracted from the malt.  Isomerization is mostly complete after 1 hour of boiling, so hops boiled for one hour contribute maximum bitterness.  Later hop additions provide other aromatic compounds that contribute to flavor and aroma.