by Tamara Scully
It’s been eight years since the University of Vermont Extension Northwest Crops and Soils Team began hosting their annual Hops Conference. For the better part of a decade, the University’s researchers and extension educators have been working to assist farmers in growing the ingredients needed to propel the craft beer movement into a truly local phenomenon.
The UVM team offers a complete resource to growers interested in hopping on board the craft beverage train: The Hops Quality Testing Laboratory; variety and weed management trials; pest and disease management strategies; and fertility studies. From growing to glass, hops yard management is covered in-depth.
While growing is a big part of the equation, harvest timing and post-harvest handling can have tremendous impact on the quality of hops. Hop growers have as much to do after the crop reaches maturity as they do to get it there.
Harvest timing
The moisture content in hops relates to the alpha acids levels, which impart the desired quality of bitterness. Hops too high in moisture aren’t at peak alpha acid content. But those left too long in the field will be prone to more disease and pest issues, more susceptible to oxidation, and will degrade more quickly in storage.
To assist hops growers with harvesting at the best time, hops samples should be tested for moisture level. To do this, small representative sample cones are collected from the hopyard under dry conditions — no rain or dew. The samples are weighed, before being dried. Drying can be accomplished in a dehydrator, a microwave or a Koster Moisture Tester. Cones are dried down to a zero percent moisture level — when the weight stops changing — and weighed again. The final dried weight is divided by the starting weight and then multiplied by 100, providing the percent dry matter.
“We want to make sure that we’re harvesting at the right time for optimal moisture,” Chris Callahan, Agricultural Engineer with UVM Extension, said in a recent Hop Power Hour webinar. “We tend to want to harvest when there is between 20 and 23 percent dry matter in the hops.”
Post-harvest handling
While hops can be used wet, most growers aren’t harvesting their crops for immediate use by a maltster. In general, hops simply aren’t a pick and pack type of crop: they require post-harvest drying, which prevents mold and other concerns, and allows hops to be stored while remaining at peak quality.
Once hops are harvested at the proper moisture levels, drying those hops down to a moisture content of eight to 10 percent, without damaging the very qualities they add to beer, requires careful attention.
The basic idea is simply to remove water from the crop, without damaging the alpha acid content. The acid content is impacted by temperature, so too high of a drying temperature might enhance drying time, but not without consequences. Oxidation destroys the alpha and beta acids, the reason the hops are used in beer in the first place. And oxidation increases with heat exposure.
It’s a “race between evaporating that moisture… and oxidation,” Callahan said. “We want to get rid of as much moisture as we can, down to the storage moisture, as quickly as possible, within the limit of temperature, so we don’t negatively impact alpha acid content.”
And there is a lot of water to remove from the harvest. For every one pound of dry hops, more than three pounds of water needs to be removed, uniformly, from all of the crop.
Drying techniques
Drying can be done using ambient air, without any heat added.
“It’s challenging to get a good, even drying within a reasonable amount of time,” with only ambient air, Callahan said. “We need to make sure that air that’s hitting the hops is dry enough, in relative terms, to pick up more moisture as it flows by the hops.”
As the air temperature rises, the relative humidity is decreased, meaning that the air can pick up more moisture from the hops. So, warmer air has a greater capacity for removing moisture. With warmer air temperatures, the air flow through the hops can be decreased: you can dry more hops with the same air flow than if the air was cooler. Instead of heating the air, some growers utilize dehumidifiers, which work to decrease the air’s relative humidity without adding heat.
It’s also important to dry all of the hops uniformly. Moving air through all of the crop means spreading the hops out, typically on a screen or other perforated surface, to promote air flow. Keeping the moisture-laden air out of the drying area requires a fan to ventilate the air out. Using fans to promote air flow over the hops themselves can speed drying times.
Hops shouldn’t be exposed to air flow rates of more than one foot per second. Higher rates cause bouncing of the hops and shatter of the cones, damaging the quality, Callahan said.
A variety of systems to dry hops can work. Traditionally, oasts — buildings used for drying hops — were used. These oasts had a furnace. Hops were spread over a perforated floor above the furnace. Brimstone was added to the fire, to produce sulphur, which helps to open the cones for drying and aids in disease management.
Callahan and others at UVM have developed a cabinet-style oast, which is a low-cost option. The oast has fans, a system for adding fresh air and venting humid air, and a temperature control. Plans can be downloaded via Farm Hack at
Hops cones won’t all be at the same moisture levels, and the size of the cones will determine how quickly they dry. For better uniformity, hops can be conditioned, or allowed to sit for a time, after being removed dried to the appropriate moisture levels. This will help to eliminate variable levels of moisture within or between cones, resulting in a higher quality and more consistent product.
Dried hops are typically vacuum-sealed for storage. Alpha acids degrade quickly at room temperature, and storage below 40 degrees Fahrenheit for up to several weeks, or below freezing for long-term storage, is recommended.
Hops aren’t essential to beer, but they are the ingredient which installs the brew with the bitterness used to balance the sweetness of malt. And, they add aroma, as well as act as a preservative. While growing the crop is an important first step to brewing the beer, harvest timing and post-harvest handling of hops can make or break that pint’s potential.