Hulupones are oxidation products of hop beta acids. Beta acids, lupulones, make up part of the soft resins in hops. They have very low solubility in wort (∼1 part per million); thus, only trace amounts survive the brewing process and end up in finished beer. Beta acids are fairly reactive with oxygen and can oxidize to a set of compounds called hulupones, each of which is derived from its beta acid analogue; for instance, cohulupulone comes from colupulone. Because they are not bitter and are only marginally soluble, beta acids do not contribute to beer flavor. However, hulupones are bitter and can contribute substantially to the final flavor of beer. Anecdotal claims suggest that hulupones have an unpleasant bitterness quality. Hulupones are relatively stable once formed and can survive all stages of the brewing process. They can be formed via the oxidative degradation of hops during storage. As hops oxidize, the bitterness that comes from iso-alpha acids diminishes because their precursors, alpha acids, are lost as a result of oxidation, but this is somewhat offset by the presence of bitterness from the hulupones. The ratio of alpha acids to beta acids ultimately dictates the degree to which the bitterness potential will diminish as hops oxidize. Higher levels of beta acids in the raw hops will result in a slower decline of bittering power as hops degrade oxidatively because of the resultant higher levels of hulupones. Hulupones can also be formed during wort boiling because the high temperatures accelerate the reactivity of beta acids. Alternatively, hulupones can form on trub following spent hops separation from boiled wort where the environment is hot and there is plenty of available oxygen. Bitterness contributions can be substantial in the latter case if the trub pile is added to a subsequent batch of beer, as is the case in some breweries.
