The clear wort is boiled in a kettle with hops, the flowers of a climbing plant. Boiling extracts bitterness from the hops, kills unwanted microbes, and removes undesired flavors. Prolonged boiling can darken the wort and introduce malty or
Kettle at Philadelphia Brewing Company
roasted flavors. In addition, the boiling causes proteins in the wort to coagulate so they can be removed. The removal of proteins yields beer that is less likely to develop haze (cloudiness).
In many breweries, the boiling hot wort is sent to a tank called a whirlpool where it is made to flow in a circle, causing solids from the protein and the hops to collect in the center where they are easily removed.
This topic is covered in The Chemistry of Beer: The Science in the Suds, Chapter 8.