There are four basic ingredients to beer, but only three are essential: malted barley, yeast and water. However, beer made with only these three will be sickly-sweet and dull. Throughout brewing history, brewers have added something extra—usually a plant part of some sort—to give their beer balance and depth.

They’ve added heather flowers, spruce tips, borage or bog myrtle. In the Middle Ages, brewers flavored their beer with a mixture called “gruit” that combined herbs and spices in recipes that varied from place to place.

But by the 15th century, one vigorous weed crowded out all the others as the fourth ingredient in beer: hops. Hop plants are climbing vines (more accurately, bines: vines without tendrils). The plant part used in brewing beer is the hop flower, a delicate, pale green, papery cone full of perishable resins. They give a beer bitterness when used early in the brewing process, and aroma when added at the end. As a bonus, hops are a preservative, and extend the life of beer.

In the hands of American microbrewers, hops have moved from their position as the supporting actor in the beer ensemble to the starring role.

West Coast microbrewers led the way in creating beers where the character of hops—bitter, piney, grassy, floral, or grapefruity—took center stage. Beer lovers took pride in seeking out the brews with higher and higher IBUs—international bittering units, the measure of the concentration of hop compounds in beer.

High-hopped beers are not for every taste. But for the hop lovers out there, there is a stunning array of hop varieties—with new ones being developed all the time—that brewers employ singly or in combination.

Now, American brewers have boosted the hopping levels of their IPAs to such an extent that a new beer style has emerged: so-called double or “imperial” India pale ale. These big beers feature even more hop power and alcohol to match.

Now, learn more about yeast.