The Aroma Unit (Au) was proposed by Nickerson and Van Engel to quantify the “hoppiness” of a given hop sample just as bitterness in beer is quantified using the international bitterness unit. They define the AU as the sum of the 22 hop aroma component profile (HAPC) constituents, measured in parts per million (µL/kg of hops or µL/L of wort or beer).

Because hop aromatic character is attributable to hop essential oil, it would not be unreasonable to think that the amount of oil in a hop sample would provide a means of estimating its hop aroma content. Gas chromatography has helped researchers to identify more than 250 essential oil components. Of these, 22 have been reported to affect hop aroma, and they have been divided into groups: humulene and caryophyllene oxidation products, floral-estery compounds, and citrus-piney compounds. It is these 22 that constitute the HACP (see table).

A hop’s total oil content can indicate the overall quality of a hop sample, especially if there is a good idea of what the normal oil content for that specific hop should be. This method may be valuable to brewers when they are buying hops from afar and sight unseen. But because many factors affect essential oil production and preservation in hops, every growing season holds the potential for significant variation in both the total amount of oil and the composition of the oil, even for a single variety. Hops do essentially have “vintages,” and some are notably better, or at least different, than others.

The total oil content figure, therefore, does not provide useful information about the actual composition of the hop oil and is not necessarily a good indicator for evaluating a hop sample for potential hoppiness and/or aroma quality. Additionally, no significant relationships were found between AUs and the key hop oil constituents myrcene, humulene, or alpha acid levels. However, some commercial breweries report that taste panels have found that hop aroma, hop taste, and dry hop aroma have correlated very well with AUs. The aroma unit therefore remains a controversial concept and individual brewers must determine whether it is a measurement that they find useful and are willing to trust.