Lupulin is the fine yellow powder or resin contained in the lupulin glands at the base of hop cone bracts, holding high concentrations of the bitter acids and essential oils of hops. It carries the alpha-acids (humulones) that isomerise during wort boiling into the iso-alpha-acids responsible for beer’s bitterness, the beta-acids (lupulones) with antimicrobial activity, and the volatile oils (myrcene, humulene, caryophyllene) behind hop aroma. Brewers prize it as the source of bitterness, flavour and foam stability; concentrated lupulin products — CO2 extracts and enriched lupulin powders — now allow precise dosing and reduced vegetal matter. It must be stored cold, dark and oxygen-free to protect its volatile oils and acids.