Hops are next: boiled into the wort. These green flowers carry essential oils and alpha and beta acids that contribute to flavor and aroma. Alpha acids are dual purpose purifiers and bitterness compounds. Alpha acids fight off undesirable bacteria and produce a good habitat for yeast. In boiling wort, alpha acids undergo a process called isomerization, which means that these acids change shape at a molecular level. Isomerized alpha acids add bitterness to the wort.
Hops also carry essential oils. These oils give hops their aroma and add flavors and aromas to beer. The trick with essential oils is that they evaporate right out of the wort during the boil. So by adding hops several times, brewers can get the bitterness they want from the alpha acid isomerization and the flavor they want from the oils.