What is the primary compound in hops that contributes to bitterness?

The primary compound in hops that contributes to bitterness in beer is alpha acids. When hops are added to the brewing process, particularly during the boil, alpha acids undergo a chemical transformation through isomerization due to the heat. This process converts the alpha acids into iso-alpha acids, which impart the characteristic bitter flavor to the beer.

Alpha acids are crucial in defining the bitterness level, which is measured in International Bitterness Units (IBUs). The contribution of alpha acids to the final bitterness profile is significant, and brewers carefully select hop varieties based on their alpha acid content to achieve desired levels of bitterness in their beers.

In contrast, beta acids, though present in hops, do not contribute to bitterness as effectively as alpha acids. Terpenes are aromatic compounds that influence hop flavor and aroma rather than bitterness. Phenols are primarily known for their role in flavor and aroma as well, particularly in specific beer styles, but they are not responsible for bitterness. Therefore, the correct answer highlights the essential role of alpha acids in providing the bitterness that is a fundamental aspect of many beer styles.