Old School beer kits consisted of a can of malt extract syrup with a pack of dry yeast under the plastic lid. The directions were simple: stir the contents of the can in with 4.5 gallons (17 liters) of water, add a couple of pounds of sugar, then add the yeast, and wait. Having tasted some of those brews, I’m amazed that people stayed with the hobby. Aside from being too dependent on simple sugars for fermentation, these beers skipped out on a vital step in the brewing process: a vigorous boil.
Later, Charlie Papazian’s The Complete Joy of Homebrewing would address both of these issues for a new generation of homebrewers. In particular, he recommended boiling the wort for at least 45 minutes. Today, a 60-minute boil has become the accepted practice, but you’ll come across recipes that range from 30 minutes to 2 hours (e.g., this recipe for a Russian imperial stout). There are even some outliers that run for several hours.
Understanding what goes on during the boil can make all of this a little bit clearer.