Humulones, Lupulones and Other Constituents of Hops

Abstract

The hop (Humulus lupulus) is cultivated to provide raw material for the Brewing Industry and its chicf value to the brewer lies in the organoleptic properties and the resistance to infection by bacteria which constituents of the plant impart to beer. Hop cones, which consist of a series of sets of four bracteoles and two bracts arranged about a central stalk or “strig”, develop from the inflorescence of the female plant and in fertilised plants, seeds form at the base of the bracteoles. In the British system of cultivation fertilisation is encouraged by including a number of male hop plants in each garden but in Continental practice male plants are rigorously excluded from the gardens as the production of seed is held to impair the quality of the flavour and the aroma principles of the hop-cone. As the cones ripen minute cup shaped glands appear at the base of the bracts and bracteoles in the vicinity of the seed or the abortive seed site. These glands contain an oily resinous material which is yellow in colour and which is at first transparent but which becomes opaque as ripening progresses. The yellow material is known as lupulin and is comprised of a mixture of resins, which are chiefly responsible for producing the bitter flavour of beer, and of essential oils which contribute significantly both to aroma and flavour of beer. In this latter connection it is noteworthy that traditional brewing processes involve the boiling of hops with wort so as to extract the resins and transform them into the bitter principles of beer and that a considerable portion, indeed the majority, of the hop-oil is lost at this stage. Hence the practice of dry-hopping i.e. introducing a small quantity of hops into the finished beer, has been introduced in many breweries so as to obtain a satisfactory contribution to flavour and aroma from the oil.