Resins in hops are chemical constituents produced by the hop plant, Humulus lupulus, as secondary metabolites. The term “secondary metabolite” refers to substances produced by a plant that do not participate in the primary metabolic process but are nonetheless necessary for the development and life of the plant. Resins are found in the lupulin glands of mature hop cones along with hop oils and some polyphenols. The resins are characterized by their extractability in different solvents, whereby soft resins are soluble in cold methanol and hard resins are soluble in diethyl ether. From a brewing perspective, only the soft resins are considered important because they contain the precursors to bitter flavor in beer. The hop acids, which are part of the soft resins, are composed of two chemically similar groups of compounds, alpha acids, or humulones, and beta acids, or lupulones. Neither of these compounds is bitter per se, but isomerized alpha acids as well as oxidized beta acids are. Isomerized alpha acids are the principal source of bitterness in beer. Therefore, hops are characterized to a large extent by their alpha acid content. Hop resins are not very soluble in water and even less so in beer because of beer’s acidic nature. Virtually no amounts of beta acids and only small amounts of alpha acids (less than 14 parts per million) can be found in beer. Hop resins are extracted during wort boiling, during which alpha acids are isomerized to iso- alpha acids. Hop resins can also be extracted from hops using supercritical CO2 or ethanol. These can then be used during brewing in their pre-extracted form.
