Which ingredient primarily contributes to the beer's aroma?

Hops are the primary ingredient that contributes to the aroma of beer. They contain essential oils that impart a wide range of aromatic qualities, from floral and citrus to earthy and spicy, depending on the hop variety used. During the brewing process, especially during the boiling stage and added in the dry-hopping phase, these aromatic compounds are released and infused into the beer.

While malted barley does influence the aroma through the sugars and Maillard reaction products produced during the malting and brewing process, its contributions are generally more focused on sweetness and nuttiness rather than the distinctive hoppy aromas. Yeast plays a role in producing esters and phenols, which can also contribute to the beer's overall aroma, particularly in certain styles, but the most significant and recognizable aromas in many beer styles come from hops. Water, while essential for the brewing process, does not contribute any aroma on its own.