Dry Hopping, the addition of hops to beer in the fermenting, conditioning, or serving vessel. For many styles of beer, hops are added throughout the brewing process. Hops added at the beginning of the wort boil are mostly contributing bitterness, those added at a mid-point of the boil add bitterness and some aromatics, and “late hopping,” the addition of hops at the end of the boil, is largely intended for hop aroma and flavor (although the reality is that heavy late hopping often adds a large proportion of a beer’s bitterness as well). During the boil, highly volatile hop oil components escape quickly with the steam, taking most of the aromatics out of the wort and up the kettle stack. See hop oils. Although the use of late hopping, whirlpool hopping, and hop backs can restore essential oils to the wort, some hop aromatics are later stripped away or chemically transformed during fermentation. The purpose of dry hopping is to infuse beer with additional fresh hop flavor and aroma. Dry hopping is a cold infusion technique that not only intensifies hop aromatics in beer but also adds aromatics that are substantially different from those achieved by late hopping. The alpha acids responsible for hop bitterness are not isomerized and therefore remain insoluble during dry hopping, but some tasting trials have shown clearly that the bitterness perception of beer can be increased by dry hopping, although international bitterness unit values may remain unchanged.

Although there can be little doubt that many brewing cultures have used dry hopping over the centuries, the practice is most closely associated with English pale ales and their variants. From the mid 1800s onward, the most common technique was the addition of a large handful of dried whole hops to the cask before filling. As the beer conditioned in the cask over days or weeks, the fresh hop aroma would suffuse the beer. Rather than simply being a tube, the cask tap is perforated with holes at the tapered end, which strains out the hop flowers so they do not end up in the consumer’s glass. Although it seems counterintuitive that one could add raw plant material to beer without causing spoilage, hops rarely harbor bacterial strains that can survive well in finished beer’s airless, low-pH, ethanol-rich environment.

At the dawn of the American craft brewing movement, many future brewers were inspired by the bright hop flavors of the cask-conditioned ales they had tasted in English pubs. Starved for hop flavor, American brewers took up dry hopping with particular fervor, and today it might be said that intense dry hop character is now widely considered a quintessentially American attribute when it comes to beer flavor.

One reason for this is the aromatic concentration that characterizes modern American hop varieties. Widely derided for decades by British and European brewers, American hops tend to contain considerably more essential oil than their overseas counterparts. A 2006 study showed popular aroma hop varieties such as Czech Saaz, East Kent Golding, and Styrian Golding with aromatic oil contents ranging from 0.4% to 1% of dry weight. When compared with the American hops Cascade (0.8%–1.5%), Chinook (1.5%–2.5%), and Simcoe (2%–2.5%), the difference becomes quite clear. In a very real way, the aromatics imparted by dry hopping have become a natural part of the American beer terroir.

Fresh dry hop aroma, sometimes evocatively described as “hop-sack aroma,” is most associated with the monoterpenoid fractions of hop oils. Monoterpenoids are aromatic compounds that are created by most plant species, including common flowering plants, wine grapes, and citrus fruits. In hops, the most important are myrcene (spicy, herbal, also a major aromatic in thyme leaves), gerianol (floral, rose-like, used in many perfumes), linalool (floral, citrusy, minty), limonene (oranges and lemons), and beta pinene (piney and woodsy, part of the aroma of rosemary and basil). Late hop aroma focuses more closely on oxidized forms of the less volatile sesquiterpenoid fractions of hop oils, including humulene (floral, cannabis, hay-like), caryophyllene (peppery, clove, camphor), and farnesene (green apples, lime).

Whereas dry hopping was originally carried out in a cask or barrel, it is now often performed in a fermentating or conditioning tank. The methods are various. The efficacy and quality of any infusion is based upon contact time, surface area, temperature, and agitation (or flow over the surface area). After fermentation, brewers usually cool the beer and allow most of its yeast to sediment out before dry hopping. Far less common is dry hopping during active fermentation, although some brewers do find it effective, feeling that the continuing yeast activity can remove the oxygen inevitably introduced by dry hopping.

Dry hopping with whole flowers is traditional but also relatively difficult. Dried hops float rather vigorously, so keeping them submerged in beer is not always easy. Brewers commonly tie mesh bags full of hops to the bottom of a tank interior, purge the tank of air, and then send beer in, looking to suspend the floating hop bag somewhere in the center of the tank. Alternatively, they will sometimes weigh down a hop bag, hoping to sink it with a heavy piece of sterilized stainless steel equipment. They are often disappointed (and somewhat impressed) to see the bag float back to the surface. Others depend on some form of steel mesh cage inside the tank, usually held in place with a rod. Whatever the method, infusion of flavor from whole hops is relatively slow, because the lupulin glands of the hop flowers are still intact. Oxidation of the hop oils during storage is also a particular concern with baled whole hops. That said, some brewers say they prefer the flavor of beer dry hopped with whole flowers.

In the United States it is more common to dry hop with hop pellets. In hop pellet production, the whole hop flower is milled into a powder, extruded into the pellet form, flushed with inert gas, and then vacuum packed. See hop pellets. In the milling process the hops’ lupulin glands are ruptured, exposing the contents to the beer during dry hopping. The pellets quickly break up into small particles upon contact with liquid, and evolution of hop oil character into the beer is relatively rapid. Brewers commonly climb to the tops of tanks and simply pour the pellets in, often through a small pipe fitting that holds a pressure-relief valve. Others have special dry hopping ports built into the tops of tanks. But methods range from the mundane to the ingenious. Many breweries load hop pellets into a small dosing tank and then circulate their beer through it. The most amusing method must be the “hop cannon,” a setup that uses a blast of pressurized CO₂ to blow a charge of hop pellets from a chamber through a snaking steel tube into the waiting tank. One brewer reports that dry hopping by this method is highly effective, but takes up to six “shots” of the hop cannon to complete. Apparently the tube of the cannon actually heats up after a few shots, and the brewer must wait for it to cool down before finishing the task.

During the dry hopping phase, it is not uncommon for brewers to either recirculate the beer to resuspend the hop fragments for greater contact or rouse the hops by sending CO₂ bubbles through the bottom of the tank. In cylindroconical tanks, the hops can later be removed in the form of a green sludge. Although this material is usually discarded or composted, it retains its bittering potential and it is technically possible to reuse it for wort bittering.

Regardless of the method, the attainment of dry hop character is also temperature dependent. Hop aroma transfer is faster at warmer temperatures than at colder temperatures. Dry hopping performed at 4.4°C (40°F) may easily take twice as long to achieve similar intensity as the same procedure performed at 15.5°C (60°F). The aroma profile will also be different, perhaps reflecting the differing volatility of the hop oil fractions. Unpleasant grassy, vegetal, chlorophyll flavors can result from extended contact time with dry hops, so most brewers who dry hop heavily try to keep contact time to 1 week or less.

Although lightly applied dry hopping in beer can result in beautifully balanced and aromatic beers, the most expressive use of dry hopping is surely seen in the American India pale ale (IPA) style and the even more intense “double IPA” or “imperial IPA” styles that are becoming increasingly popular. Unfortunately, this bright, hoppy character has a relatively short life. Even when oxygen pickup is minimal, dry hop character tends to oxidize or otherwise degrade notably within a matter of 2 or 3 months, becoming first reminiscent of tropical fruit and finally arriving at a tea-like quality that rarely pleases. Hop polyphenols leached out during dry hopping also can give rise to hazes, although many brewers and beer drinkers do not seem to mind. Beer enthusiasts are enamored with the flavors and aromas that dry hopping brings, but the technique makes beer inherently less stable. Perhaps it is true that the candle that burns twice as bright also burns half as long.