The growing and harvesting of hops or hemp is only half the challenge in getting to a marketable crop. Drying and processing the crop to ensure quality is equally important to be able to sell a crop which the market wants.
Plants have a high level of moisture (75 to 85%), so this moisture needs to be removed in order to safely store, package, and maintain product quality and shelf life. Typically, bringing moisture levels down to about 10% will prevent spoilage due to mold, but too much drying can undermine product quality. The longer the crop is exposed to air, especially heated air, the more oxidation will occur. Oxidation reduces quality by chemically changing the properties of the desired chemical compounds in these dried products.
UVM Extension Agricultural Engineer, Chris Callahan explains, “Drawing out the water which is 70% of a crop’s weight needs to be done in a controlled way. There are about 168 gallons of water per ton of crop and that water needs somewhere to go. If the air is too warm and dry, you may drive off desired volatile compounds and dry only the outer layer. Too cool or humid and the drying process is prolonged leading to greater oxidation and potentially mold. View UVM Extension webinar and slides here.
There are a number of drying methods ranging rom passive to more active. Passive methods rely on ambient conditions and require drying over longer time periods. These include, for example, hanging product in a barn or hoop house.
have a high level of moisture (75 to 85%), so this moisture needs to be removed in order to safely store, package, and maintain product quality and shelf life. Typically, bringing moisture levels down to about 10% will prevent spoilage Agricultural crops mold, but too much drying can undermine product quality. The advantages of passive methods are their simplicity and availability – no moving parts, low tech, and most farms have ample barn or high tunnel space. The disadvantages are that the crops hanging up from beams or on racks need sufficient space for air circulation and depend on the outside or ambient conditions. such as oxidation and
Barns are subject to condensation during nights as cooler air will hold less water vapor: air at 41 F holds three times less water vapor than air at 68 F. Moreover, humidity will vary with the weather. While Vermont is most humid in middle of summer, winter is typically dry. However, the months which are dry have wide variability from day to day, and between night and day. If farm buildings are leaky, moisture can be reintroduced from outside more humid air. Sealing the building envelope can help reduce the infiltration of outside air and using fans for air circulation within the space can hep prevent condensation.
Farm buildings can be designed or renovated to be effective drying spaces with heating and ventilation as long as the building envelope or shell is adequately sealed to prevent moisture and heat moving between inside and outside areas. Contacting Efficiency Vermont to conduct an energy audit and blower door test to measure the amount of air leakage will help prioritize where and what improvements can be made to a barn space used for crop drying and storage.
More active methods for drying include adding heat and using forced air. Drying equipment can be rotary or plenum dryers, some of which can be repurposed from tobacco farms. With dedicated equipment, temperatures can be set and regulated. For safety reasons and potential fire risk, it’s important to check combustion clearance on this equipment. Such systems can be complex, initially costly, and require effective controls as they run faster and hotter. See UVM Extension presentation here.
Dry air is passed over crop to enable moisture to be driven from the crop to the air. And then as the air becomes humid, that moist air needs to be removed. Over time, the moisture content and crop weight is reduced. While determining how much drying is enough is often done by feel or observation, moisture meters can be used, but plant stems, leaves, and flowers all have different moisture levels so it’s more difficult to get a representative sample. A simple way to test moisture of a crop at harvest and at anypoint during drying is demonstrated on the UVM Extension NW Crops and Soils page.
Dehumidification systems work with a refrigeration cycle or heat pump. These include a compressor, condenser, expansion valve, and evaporator which are purpose built. For effective functioning, these systems need to be adequately sized to meet farmers’ needs with controls and monitors. Given the high moisture load to be removed from the crop, it’s important that these are suited to the end product markets whether retaining terpenes for CBD product quality, fiber hemp or seed saving. For example, for CBD products, curing or drying at lower temperatures helps maintain visual and aroma qualities.
Efficiency Vermont offers technical assistance and customized services to meet farm business needs including design review, energy modelling, and connecting farmers to contractors and engineers. Incentives and/or rebates are available for:
- Efficient fans,
- Energy star dehumidifiers,
- Improving the Building envelope
- Mechanical drying
For more information on UVM Extension’s hemp research and Extension programs, visit the UVM Extension Industrial Hemp web pages.
For technical advice on crop drying and storage, contact Chris Callahan, UVM Extension, Chris.Callahan@UVM.EDU
For further information on fire and safety, contact Landon Wheeler, Vermont Department of Public Safety, Landon.Wheeler@vermont.gov
For further information on energy efficiency, contact Lauren Morlino at Efficiency Vermont, email@example.com
“Drying hops on a small scale” 2013