The Oxford Companion to Beer definition of
Lupulone is one of four beta acid analogues in hop resin, the others being adlupulone, colupulone, and prelupulone. See adlupulone and colupulone. Lupulone levels vary across hop varieties from roughly 30% to 55% of total beta acids. Colupulone varies, from roughly 20% to 55%, but adlupulone stays within a narrower range of 10% to 15%. Structurally, these analogues are very similar, but not identical, to their alpha acid counterparts. The slight structural difference, however, prevents these beta acids from isomerizing in the kettle. See hop isomerization. When beta acids oxidize into hulupones, however, which happens as hops age, they become wort soluble and confer bitterness. See hulupones. As hops oxidize, the bitterness that comes from iso-alpha acids diminishes, but this is somewhat offset by the contribution of bitterness from the hulupones. The ratio of alpha to beta acids ultimately determines the extent to which the bitterness will diminish as hops oxidize; higher levels of alpha compared with beta acids in raw hops mean a slower decline of bittering power as hops degrade over time.
Thomas Shellhammer and Val Peacock