Kilning Me Softly

It’s a refrain hop growers, researchers and brewers utter again and again. “The kiln is the easiest place to ruin a perfectly good hop.” Cook out the delicate aromas coveted by brewers large and small and the final product is worthless.

Drying hops is like curing a fine cut of meat. The softer you kiln, at lower temperatures for longer times, the more flavorful and aromatic the final batch will be. At Sierra Nevada we once sun dried a portion of our estate hops in 100°F temperatures, spreading several hundred pounds on a large tarp in an open field for 24 hours. The quality of the sundried hops was truly remarkable.

Kilns are key to the hop harvest, pumping hot, dry air through millions of pounds of hops each year. Dating back centuries, British kilns (or oast houses) were singular structures capable of sparking the imagination. They also occasionally went up in flames, since dried cones loaded with essential oils are incredibly combustible. These older kilns that dried tons and tons of hops in their lifetimes still inspire awe. Modern hop kilns are not so eye-catching—some of them look more like inner city blight than a functional system for drying hops.

But if modern hop kilns look more like small penitentiaries, they still possess a memorable ambience during harvest. Seas of green hop cones flood inside via conveyors and are piled on the kiln floors. A typical bed ranges from 24 to 36 inches high, depending on variety, grower kilning philosophy and throughput necessities. As the burners get going and the hops begin to heat up, the kiln transforms into a hop sauna. The moisture, the grassy wet hop character and some of the precious essential oils begin to evaporate out of the hop bed, filling the kiln space with an incredible array of aromas. Onlookers will soon find themselves sweating buckets.

Newer kilns produce lots of data. Today’s kilns take the art of drying hops into the realm of science. Using airflow, pressure differential, and temperature controls coupled with extremely sensitive moisture content probes, they produce vital information, which helps create beautifully consistent hops. The days of touch and feel drying are numbered. Even as technology makes the art of drying hops more reproducible, I hope hop growers never abandon the intimate relationship they have with the their “kills,” an affectionate term many professionals use to refer to a batch in the kiln.

After “dropping the kill,” the hops then sit for a period of 12–24 hours to allow the moisture within each cone to equilibrate and the moisture content of the pile to homogenize. The moisture content of hops from the field is usually about 75 percent. The goal of the “Kiln Master” is to effectively dry the entire batch down to about 9 or 10 percent. Hop kilns in the US are a single level and hops on the bottom of the kiln bed dry much faster. There is also a pronounced stratification of moisture content from the bottom to the top of the kiln bed. The very dry hops at the bottom can have a moisture level as low as 4 or 5 percent, making for an extremely brittle hop that will shatter in your hands. The hops dried at the bottom also lose more aromatic oils and can take on objectionable onion and garlic tones. Hops at the top still have up to 20 percent moisture. These whole cone hops will retain the structure and much of the original aroma of wet field hops.

Recent studies conducted by the nonprofit Hop Quality Group show marked differences in quality for hops dried at 130°F instead of 150°F. Larger, more efficient kiln fans also reduce drying time and add to the feasibility of drying hops at lower temperatures. These more powerful fans push more dry air through the bed of wet hops, significantly lowering the amount of time needed to complete the process. Shallower kiln beds help produce more consistent hops from top to bottom too, yielding a more valuable product to the brewer and lending a cleaner, more intense hop aroma to beer.

Thanks to IPAs and the craft boom, a renewed focus on quality and the art of hop drying has taken shape. Today, reinvestment on the hop farm is at an all-time high. ■