The Oxford Companion to Beer definition of
Lupulin, small glands that contain hop acids and essential oils. They are observable as a fine yellow powder found deep within the hop cone—a fruiting body, which is technically called a strobile. See hops. Pine cones (which have a similar shape) are also strobiles. The cone is made up of bracts and bracteoles that are attached to a central rachis (or strig). The lupulin glands, which are technically called glandular trichomes (specialized epithelial cells in plants that contain essential oils and other secondary metabolites), are found attached to the base of the bracteoles and to a lesser degree to the bracts. They are small, slightly pear-shaped glands roughly 200 μm (0.2 mm) in diameter. Their concentration in hops differs from one variety to another. Given that most of the brewing value of the hop cone is found within the lupulin glands, there have been a number of technical approaches to concentrating or preserving this component. A process for washing hop cones of their lupulin with water followed by sieving, drying, and storage in an inert environment has been developed and patented. Type 45 hops are a pelletized product whereby the hops are milled and then sieved in order to concentrate the lupulin. In this process, 100 kg (220 lb) of whole hops yields roughly 45 kg (99 lb) of concentrated hop pellets, hence the name. Supercritical CO2, liquid CO2, and ethanol extraction of hops are techniques for dissolving lupulin, removing it from the hop cones, and recovering it as a solvent-free (in the case of CO2) or ethanol-based extract.