Cannabis Drying & Curing: Science Explained
Explore our blog dedicated to cannabis community. Understand the science behind cannabis drying and curing processes to enhance your cultivation skills and achieve optimal results.
Wess
12/2/20243 min read
Drying and curing cannabis to perfection requires balancing complex environmental factors like water activity, vapor pressure deficit (VPD), temperature, relative humidity (RH), and absolute humidity. While these terms may sound technical, understanding their relationships is essential for achieving high-quality results. Let’s break them down and explore how they influence the drying and curing process.
Key Concepts
1. Water Activity (Aw)
Water activity measures how "free" water is available in a material for microbial growth and chemical reactions. It’s expressed on a scale from 0 to 1. For cannabis:
A safe water activity for curing is around 0.55–0.65, preventing mold but maintaining product quality.
Higher Aw (above 0.7) invites mold; lower Aw (below 0.4) leads to over-drying.
2. Vapor Pressure Deficit (VPD)
VPD is the difference between the air's saturation vapor pressure (the max it can hold) and the actual vapor pressure. It represents the drying "force" on cannabis:
A moderate VPD (e.g., 0.8–1.2 kPa) ensures consistent drying without over-drying or causing terpene loss.
A high VPD dries cannabis too quickly, while a low VPD slows drying and risks mold.
3. Temperature
Temperature controls how much moisture air can hold:
Higher temperatures increase the drying rate but risk terpene evaporation.
Optimal range: 60–70°F for drying cannabis.
4. Relative Humidity (RH%)
RH is the percentage of moisture air holds relative to its max capacity at a given temperature:
Target RH for drying: 45–55%.
During curing, RH should stabilize around 58–62%.
5. Absolute Humidity (AH)
AH is the actual amount of water vapor in the air, typically measured in grams per cubic meter (g/m³). While less intuitive, AH links closely with temperature and RH to control drying conditions.
How VPD and Environmental Control Work Together
VPD directly influences the drying rate of cannabis:
Low VPD (e.g., high RH or low temperature) slows evaporation, keeping buds too moist.
High VPD (e.g., low RH or high temperature) pulls moisture out too quickly, damaging cannabinoids and terpenes.
To balance VPD:
Use temperature and RH sensors to monitor the drying room.
Adjust dehumidifiers, humidifiers, and HVAC systems to maintain target conditions.
Controlling VPD in Cannabis Drying and Curing
Here’s a step-by-step mechanism to control VPD:
Monitor Environment: Use sensors to measure temperature, RH, and calculate VPD in real-time. Many modern systems integrate this data for automated control.
Adjust RH: Install dehumidifiers to lower RH or humidifiers to raise it. Control RH to influence the air's moisture-holding capacity.
Control Temperature: Use HVAC systems to keep the drying room cool (60–70°F). Cooling lowers saturation vapor pressure, reducing VPD.
Air Circulation: Fans maintain uniform conditions, ensuring even drying and avoiding hot or humid spots.
Data Logging: Use software to track VPD trends and fine-tune drying curves based on cannabis batch behavior.
Why These Factors Matter for Cannabis
During drying, a balanced VPD allows water to leave the buds slowly while retaining terpenes and cannabinoids. If water evaporates too fast, cannabis can lose its aromatic compounds and feel brittle. Too slow, and mold or mildew can develop.
In the curing phase, stabilized RH and VPD ensure the remaining moisture redistributes evenly, enhancing potency and flavor while preventing microbial growth.
Sample Drying and Curing Curve
Below is a simplified drying curve showing cannabis weight reduction over time:
Day 1-3: Initial fast moisture loss (VPD ~1.2 kPa).
Day 4-7: Slower drying as internal water diffuses outward (VPD ~0.8–1.0 kPa).
Day 8+: Transition to curing, maintaining stable RH and water activity.
Conclusion
The interplay between water activity, VPD, temperature, RH, and AH defines the drying and curing process of cannabis. By carefully monitoring and controlling these factors, cultivators can achieve a product that is flavorful, potent, and safe for long-term storage.
Investing in the right sensors and environmental controls ensures consistent quality, helping your cannabis stand out in a competitive market. Whether you're a hobbyist or a commercial grower, mastering these principles is key to success.