Dry hopping

Hops are the flowers (also called seed cones or strobiles) of the hop plant Humulus lupulus. They are used primarily as a flavoring and stability agent in beer, to which they impart bitter, zesty, or citric flavours; though they are also used for various purposes in other beverages and herbal medicine. The hop plant is a vigorous, climbing, herbaceous perennial, usually trained to grow up strings in a field called a hopfield, hop garden (nomenclature in the South of England), or hop yard (in the West Country and U.S.) when grown commercially. Many different varieties of hops are grown by farmers around the world, with different types being used for particular styles of beer.

The first documented use of hops in beer is from the 9th century, though Hildegard of Bingen, 300 years later, is often cited as the earliest documented source. Before this period, brewers used gruit, composed of a wide variety of bitter herbs and flowers, including dandelion, burdock root, marigold, horehound (the old German name for horehound, Berghopfen, means "mountain hops"), ground ivy, and heather. Early documents include mention of a hop garden in the will of Charlemagne's father, Pepin III.

In addition to adding flavor to beer, hops are also used in brewing for their antibacterial effect over less desirable microorganisms and for many purported benefits, including balancing the sweetness of the malt with bitterness, contributing a variety of desirable flavors and aromas. Historically, traditional herb combinations for beers were believed to have been abandoned when beers made with hops were noticed to be less prone to spoilage.

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Wikipedia