Cannabis plants are complex to begin with, but when you consider the intricacies associated with their use (fiber, medicinal, and recreational), issues such as stability and use-by-dates only increase the complexity. Perhaps the best way to address stability is to consider the intended use.
Cannabis for smoking and vaping
The quality and safety of cannabis for smoking are dependent on many parameters, including temperature, dryness (humidity), oxygen, strain, harvest time, part of the plant, packaging, and processing history. Stability testing should include samples stored under conditions normally used by consumers. In contrast, reference materials may be stored crushed and bone dry. Some analytical profiles reference “dry weight” to remove the variability due to water content or “water activity.”
Thus, for representative storage conditions for consumers, I consulted Leafly for guidance.
Temperature: Molds thrive in cannabis when moist and stored between 77 and 86 °F. However, excessive heat will drive off the cannabinoids and terpenes. The latter are responsible for the aroma and flavor. The dry residue increases the concentration of less volatile essential oils, which results in a hot, harsh smoke.
- Humidity: Controlling the humidity of cannabis is also essential to protect against growth of mildew and molds. Cannabis is best stored when the relative humidity in the container is in the range of 59–63% RH. Above 65% RH, the risk of molds increases. Below 55% RH, the trichomes dry out, which squeezes out the essential oils. The humidity can be controlled with systems from companies such as Boveda and Desiccare.
- Sunlight: Protection against sunlight is also important for prolonging the stability of cannabis products. Essential oils degrade faster in light.
- Oxygen: Oxygen is another important factor to control. Vacuum pump apparatus is helpful in lowering oxidation of cannabis.
- Storage containers: Clean glass jars such as Ball or Kerr Mason jars provide airtight seals. Cannador and The Bureau provide products for cannabis storage. FoodSaver systems facilitate vacuum sealing of Mason jars.
The following should be avoided:
- Storage in a refrigerator, since the humidity can be high, which increases risk of mold or mildew.
- Plastic bags often have static electricity, which can attract precious trichomes.
- Storage near heat sources (heater ducts, above ovens and stoves), refrigerators, etc. Store cannabis near the floor.
- Humidors are not suitable for long-term storage. Some use propylene glycol for humidity control, which is not optimized for cannabis.
- Avoid storing cannabis near smoking pipes, grinders, etc. The ash and resin can easily contaminate the product.
A related article by Patrick Bennett claims the useful shelf-life can exceed 10 years. However, he cautions that its appearance and smell need to be examined carefully. Visible mold or a musty smell can indicate danger.
Other notes for chemists: Many analytical protocols use accelerated testing to predict behavior, without waiting for the product to age naturally. These accelerated tests should be calibrated periodically to check the accuracy of the predictions. The tests should be part of the sampling plan. Remember to include these retention samples in the sample requirements and storage facility.
Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected]