Bitters are on the short list of any bartender’s essential ingredients. Often dubbed “the spice rack” of the cocktail world, bitters are the mysterious elixirs found in tiny bottles — tiny because you don’t need a lot: Just a few drops pack a big punch.

History
Bitters go back centuries but are enjoying a resurgence thanks to the craft cocktail trend, as mixologists develop a renewed respect for classic techniques.

In their early days, bitters were used for medicinal purposes, to cure various ailments, before gravitating to the bartender shelf where they belong. (And some modern bitters are sold in medicine dropper bottles, as a kind of historical nod.)

Two kinds
Bitters consist of natural ingredients such as berries, leaves, seeds, and flowers, which are infused in alcohol, then aged until the alcohol picks up their flavors.

There are two kinds: “digestive” bitters, also known as aperitifs, such as Campari, which can be combined with soda for a tart but friendly drink; and cocktail bitters, which boast flavors so concentrated that you wouldn’t want to drink them solo.

But used in a cocktail, they open things right up, adding not only a jolt of flavor but also complexity, aroma, and depth.

Big names
The two best-known brands are Angostura, noted for its spicy clove taste and used in whiskey cocktails like the Manhattan  and the old-fashioned ; and Peychaud’s, the cherry-red bitter with a subtle anise flavor, used in trendy cocktails such as the Sazerac  and the Vieux Carre.

But as bitters have become more popular, more flavor innovations have emerged, from citrus-flavored bitters such as grapefruit and blood orange, to exotic flavors such as mint and chocolate.

Bitters and bourbon
Having a collection of bitters on hand is not only the sign of a well-stocked bar, but also provides opportunities to experiment with flavors and put bourbon in a new light.

If you’re looking to acquire a few bottles, be sure to consider how they’ll work with bourbon’s trademark flavors such as vanilla, toffee, oak, caramel, and smoke. They pair nicely with citrus and chocolate bitters, but not as well with herbal-flavors. (Bourbon and cucumber? Not and ideal match.)

Orange bitters such as Regan’s Orange Bitters No. 6 are a solid pick and can be used in place of, or in concert with, Angostura, for a clove-and-orange one-two punch.

Chocolate bitters made by small artisanal companies such Portland Bitters Project — and sometimes made by bartenders themselves — have become a huge flavor, so popular that Angostura recently introduced its own chocolate bitter made with cacao nibs. A drop or two of chocolate bitters in your bourbon is an invention unto itself.