Which ingredient primarily contributes to the bitterness of beer?

Hops are the ingredient primarily responsible for the bitterness in beer. They contain alpha acids that, when boiled during the brewing process, isomerize and contribute a distinct bitterness to the beer. This bitterness balances the sweetness from the malt, adding complexity and depth to the flavor profile. Hops also impart additional characteristics such as aroma and flavor variations depending on the variety used, making them a crucial and multifunctional ingredient in brewing.

While yeast is essential for fermentation and contributes to the beer's flavor and aroma profile, it does not contribute bitterness. Malt provides sugars for fermentation and adds sweetness as well as flavors like caramel and toastiness but also does not contribute bitterness. Water is a vital ingredient for brewing but does not impact the bitterness of beer directly. Thus, hops are the key ingredient responsible for the bitter taste that many beer styles showcase.