Which beer characteristic is primarily measured in IBUs?

The characteristic that is primarily measured in International Bitterness Units (IBUs) is bitterness. IBUs provide a quantitative measure of the bitterness contributed primarily by hops during the brewing process. Hops contain alpha acids that, when boiled, contribute to the beer's bitterness, which is an important flavor component in many beer styles, particularly in IPAs and other hop-forward beers.

Measuring bitterness in IBUs helps brewers and consumers alike understand the intensity of this flavor aspect in beer. While other aspects like color, aroma, and alcohol content are critical in evaluating a beer, they are not quantified through IBUs. Color is measured in specific gravity or color units, aroma is assessed through sensory evaluation, and alcohol content is typically reported as a percentage by volume. The primary function of IBUs is to specifically address the bitterness level in a beer, making it a key metric for assessing this characteristic.