Hops

All commercial hops belong to Humulus lupulus, the common hop; the genus is related to Cannabis. Hop plants are perennial and dioecious, with male and female flowers on separate plants. Each spring a crown and rhizome send up annual climbing bines that can grow up to about 7.5 m (25 ft) in a season. Only the female flowers, papery cones, are used in brewing, and growers remove male plants to prevent pollination. Inside the cones, lupulin glands hold alpha acids, beta acids and essential oils.

The alpha acids (humulone, cohumulone and adhumulone) are barely soluble until a vigorous boil turns them into iso-alpha acids, which are more soluble and more bitter. The essential oils give aroma but are volatile, so aroma hops go in late. Hops also give beer a measure of protection against bacteria. Medieval writers knew this: around 1158 Hildegard of Bingen noted that the bitterness of hops inhibits spoilage in drinks.

Because hopped beer kept and travelled, it won the trade. From the thirteenth century hopped beer from Bremen and Hamburg was exported widely, and from the fourteenth century beer brewed with the herb mixture called gruit was in decline throughout the Low Countries.