Drying and curing are some of the final steps prior to the packaging of dry flower. However, little published research exists on the effects of drying and curing on potency, terpene profile and microbial load and industry-standard practices can vary.
The Purpose of Drying Cannabis
Much like in the food and tobacco industry, drying is used as a means of preserving harvested cannabis which can hold upwards of 75% moisture content when freshly cut down. The removal of this moisture prevents the growth of yeast, mould and bacteria thereby extending the shelf-life of properly dried cannabis flowers. A carefully monitored and controlled drying cycle that is between 7-14 days is also thought to aid terpene preservation by reducing their rate of evaporation/degradation and thus improving the taste and smell of dried flower.
The Drying Process Explained
Drying cannabis involves the removal of moisture from harvested material until the desired moisture content is reached. This can vary depending on the specific local legal specifications that cover different cannabis products. However, one international specification applied to dried cannabis flower stipulates a range of 0.55 to 0.65aw (water activity) for dried cannabis flowers. Water activity is distinct from moisture content and is a measure of the energy status of water in a system. An aw of 0.40 indicates that water in a system has 40% of the energy that pure water would have in the same system. It is used to define the minimal water activity that various bacteria, moulds and yeasts require to grow and therefore the maximum level dried product must contain prior to long term storage (Fig.1).
Fig.1. Source: European Pharmaceutical Review: The minimal water activity required for the growth of various microorganisms.
During the drying process, chlorophyll and starches degrade and enzymatic processes continue including THCA synthase converting CBGA to THCA. Put simply, the concentration of THC can continue to rise in flowers after they are cut and are put to dry. Due to the role of enzymes in this important chemical process drying is a delicate process that impacts the quality of dried flower.
Managing the drying process is a balancing act between decreasing water activity to <0.65 and preventing microbial growth and preserving desirable volatile organic compounds by preventing their evaporation via closely controlling temperature, relative humidity and vapour pressure deficit (VPD) so as to not dry too quickly. Flowers that are over-dried i.e <0.55aw can become brittle and are at risk of fragmenting during bulk packaging and storage. Aside from losing aroma and terpenes, over-dried cannabis also burns hotter and faster, a less than ideal scenario for the end medical user.
The Ideal Dry Room Environment
An ideal dry room environment would maintain temperatures between 12- 20°C, relative humidity between 50-60% and have excellent ventilation and air circulation. Drying should ideally take place in the dark since UV light can also degrade cannabinoids and terpenes. Proper airflow mapping is necessary in order to identify and eliminate any microclimates and ensure a homogenous drying climate is maintained throughout to ensure flowers dry evenly and a whole batch is ready at the same time. Precise control of environmental parameters is best maintained with a high spec HVAC system capable of ensuring accurate control of temperature, relative humidity and VPD.
The target environmental conditions of a dry room are ultimately a cultivator's choice and depend on the type of material introduced i.e. whole plants that are hung to dry or wet trimmed flowers arranged on trays and racks. In either case, proper drying should aim to reduce the moisture of harvested product by at least 50% in the first 24-48 hours to eliminate any risk of infection with mould, yeast and bacteria.
Importantly, terpenes are a type of volatile organic compound which means they are highly sensitive to low relative humidity, heat, UV light and exposure to oxygen. Cannabis flowers contain an array of different terpenes which depending on their structure are volatile under different conditions, some have been shown to degrade in temperatures as cool as 15°C and many monoterpenes start to degrade as soon as plants are harvested.
Managing Risk During Drying
Whilst growing plants have the ability to manage stress using their natural defence mechanisms, once harvested they are at their most vulnerable to infection by various microorganisms. This means harvested material must be handled carefully and dry rooms should be fully sealed environments containing anti-microbial surfaces that can be easily cleaned and disinfected between batches.
Mitigating risks during drying actually begins further upstream in the cultivation process and includes keeping a close eye on harvesting workflows, techniques and environmental conditions and minimising the difference in temperature and relative humidity between trimming and bucking rooms and the final dry room environment.
The Difference Between Drying and Curing
One of the most useful parallels for explaining the contribution of a preservation process in influencing the final quality of a product is the curing of chewing and smoking tobacco and the ageing of wine in barrels. Although curing is essential for tobacco and ageing wine improves the maturity and intensity of its flavours, curing cannabis can be considered an "artisanal" process that is not entirely essential for its preservation especially if it has been dried slowly and effectively.
It is difficult to establish the value of curing for medical cannabis producers especially since quantifiable measures of quality are difficult to establish using subjective consumer experience - any purported benefit linked to user experience i.e. taste and smoothness of smoke is not a reliable indicator of quality.
The Curing Process Explained
It's important to note that there is currently no peer-reviewed published research investigating the effects of drying and curing on the most important secondary metabolites of interest in medical cannabis cultivation, cannabinoids and terpenes. Some of the most commonly reported benefits of curing circulating in the cannabis community include:
- Additional breakdown of chlorophyll
- Break down of sugars by aerobic bacteria
- Preservation and enhancement of terpenes
- Increased cannabinoid concentration due to the preservation of THCA
- Better tasting and smoother smoking dried flower
However, some of the only publicised (note: not published) research on curing was conducted by OutCo, a California based cannabis producer. In one experiment researchers compared a control group of "unburped" cannabis with two groups of "burped" product. In the first group containers were burped once a day for three days for 15-20 minutes and the flower was turned. In the second group termed the "industry burp" containers were opened for one hour once per day and the flower was turned. They found:
- CO2 built-up in all containers - burping was a release of this CO2 and introduction of fresh 02, the industry burp most drastically increased CO2.
- The containers used were not gas-tight. Burping was not a release of moisture, rather the relative humidity of burped containers immediately followed the environmental trends of the curing room.
- Temperature followed the trend of the curing room.
- Between burps, the relative humidity in the buckets of all groups remained constant.
- Curing led to all samples becoming more homogenous (decrease in the variance of all moisture data points for all groups).
- All the samples in all three groups became wetter due to the release of internal moisture from larger buds i.e. homogenisation.
In a separate experiment on the effects of a 30 day cure by other OutCo researchers:
- No statistically significant change in the concentration of THC or CBD.
- A significant decrease in the concentration of THCA indicating decarboxylation was occurring.
- Significant changes to the ratio of individual terpenes and their concentration but no change in total terpene concentration. Primary and secondary terpenes were differentially affected.
The results from the studies presented some interesting areas for further research and highlighted the importance of ensuring containers were air-tight in order to ensure some of the purported benefits of curing can effectively take place i.e a reduction in the rate of decarboxylation resulting in a conservation of potency.
Some of the areas for further research include, investigating what the addition of fresh O2 does to the activity of aerobic bacteria, looking at whether fermentation is taking place in the flower, testing the impact on the concentration of cannabinoids over a longer curing period and researching what is happening to other chemical compounds such as alcohols, ketones, flavanoids and other cannabinoids during a curing event.
Further research corroborating the OutCo findings with respect to the effect of curing on terpene concentration was published in a 2021 whitepaper by Eybna Technologies, an Israeli R&D company focusing on terpene manufacture. Their study found monoterpenes and sesquiterpenes were differentially affected by post-harvest processes. Monoterpenes such as beta-myrcene, alpha-pinene and beta-pinene all decreased in concentration following harvest and sesquiterpenes such as alpha-humulene and germacrene B all increased at the end of a one week dry and one week cure (Fig.2).
Fig.2. Source: Ebayna Technologies 2021.
Both studies provide interesting evidence in support of the theory that curing can impact the ratio of individual terpenes which are differentially affected by the process but that curing does not change the overall concentration of terpenes.
Summary: Should Your Facility Cure?
Ultimately, unless facilities are able to quantifiably and therefore demonstrably prove that curing improves the quality of their final product, a thorough understanding of both the pros and cons of curing needs to be carefully considered before deciding to include a curing step in your production process.
Where possible it is imperative to develop quality indicators that are not based on subjective user experiences such as taste or smoothness of the smoke. For EU medical cultivators proof that a curing process can reduce the microbial load of any bulk stored product, conserve potency by preventing degradation of THCA and thereby lengthen its shelf life are some of the most important quality indicators that merit further investigation.
Aside from the additional time curing adds to your production cycle, proper curing requires space, technical expertise and appropriate gas-tight storage containers. Importantly, the conditions of burped curing containers will mirror the conditions of where they are stored meaning proper environmental controls and ventilation must be factored into the design of a curing room.
Another challenge for GMP producers is in establishing process and control validation for curing given that the exact chemical and biological processes occurring in the dried flower are not well understood. This impacts a facility's quality management system in terms of determining in-process testing, defining environmental specifications and outlining validation steps for a curing process.
The fact remains that more peer-reviewed research is necessary to determine the optimal environmental conditions for the most effective drying process. This is one that maintains an equilibrium between reducing the available moisture for the proliferation of harmful microorganisms and preserving the most volatile organic compounds, especially terpenes. As well as this more research is necessary to determine the optimal conditions for an effective curing stage. Within this studies should aim to research how curing affects flavonoids, cannabinoids beyond THC and CBD and molecules such as alcohols and ketones as this would benefit our understanding of the curing process and provide additional and objective measures of quality. These in turn could be used to measure its effectiveness on the quality of a final product and determine its value as part of a facility's production cycle.
If you are interested in learning more about an effective dry room and cure room design and equipment, reach out to Big Leaf Consultants for a discussion on some of the best options available to European medical cannabis producers.
ASTM D8197, "Standard Specification for Maintaining Acceptable Water Activity (aw) Range (0.55 to 0.65) for Dry Cannabis Flower." May 2018. Available at: https://www.nen.nl/en/astm-d8197-18-en-6522824
Beuchat, L.R., 1983. Influence of water activity on growth, metabolic activities and survival of yeasts and molds. Journal of Food Protection, 46(2), pp.135-141.
Booth, J.K. and Bohlmann, J., 2019. Terpenes in Cannabis sativa–From plant genome to humans. Plant Science, 284, pp.67-72.
Eybna Technologies., May 2021. "Analytical Chemistry Evaluation of Terpene Change Throughout Pre and Post- Harvesting Processes." Available at: https://www.eybna.com/wp-content/uploads/2021/06/Eybna_Live-Line_WhitePaper.pdf
"Exploration and Optimisation of Drying and Curing." May 2018. Cannabis Industry Journal. Available at: https://cannabisindustryjournal.com/feature_article/exploration-and-optimization-of-drying-and-curing/
"Inhibition of Microbial Growth in Solid Dosages at ICH Stability Storage Conditions." August 2011. European Pharmaceutical Review. Available at: https://www.europeanpharmaceuticalreview.com/article/8876/inhibition-of-microbial-growth-in-solid-dosages-at-ich-stability-storage-conditions/
"Water Activity in Cannabis. The Role of Water Activity in the Cannabis Industry. Neutec Group Inc. Available at: https://neutecgroup.com/resource-library/water-activity/white-papers/222-water-activity-in-cannabis-the-role-of-water-activity-in-the-cannabis-industry