Cannabis drying and curing

Drying

For many applications, the dried version of the cannabis herb is required; however, like many plants, cannabis contains approximately 80% water. For this reason, drying is considered an essential step for product development (Hawes and Cohen ). Drying the plant not only prevents the growth of microorganisms that would otherwise rot plant tissue (based on ASTM D8196-18 which is a standard practice for determination of water activity (aw) in cannabis flower), it would also enable long term storage while maintaining potency, taste, medicinal properties, and efficacy (Hawes and Cohen ). This is done by maintaining the water activity level between 0.55 and 0.65 aw, minimizing the risk of mold or fungal infection while preserving the quality of the flower (ASTM D8196-18).

Air-drying, also known as hang-drying

Hang-drying or air-drying is considered the oldest way of drying cannabis plants after harvest (Fig. 1) that requires no dedicated equipment (Ross and ElSohly ). Slow-drying includes placing whole plants or separated inflorescence in a cool dark room with a temperature between 18 and 25 °C and humidity between 45 and 55%, either hung from a string or laid out on drying screens (Hawes and Cohen). Ross and ElSohly () applied four treatments for air-drying to evaluate the efficacy of each condition in producing the highest yield of cannabis products. The treatments were extracted immediately, after the flower harvest at room temperature (0.29% yield, w/v) (A), after 1 week of air-drying at room temperature (0.20% yield based on wet material, v/w) (B), after 1 week of air-drying followed by storage for 1 month at room temperature (0.16% yield based on wet material, w/v) (C), and air-drying for 1 week and stored in paper bags for 3 months at room temperature (0.13% yield based on wet material, v/w) (D). From this experiment, it was found that the yield from treatments A to D decreased from 29 to 13%, respectively (Ross and ElSohly ). Inconveniences of this method include the manual removal of leaves and buds from the stem as well as the time taken to complete the overall process. The separation is crucial as different parts dry at different rates; therefore, a lack of completing this step may result in uneven drying. Consequently, a disadvantage of removing buds from stems is the possibility of producing a product with a harsher taste. Another detriment of this method is the involvement of gravity. The water from the top part of the plant will absorb into the lower parts leading to a slower and uneven drying process. To speed up the procedure, heaters, fans, and dehumidifiers can be used. However, fast-drying can lead to a harsher taste as opposed to slow-drying which produces smoother tasting products. It is also believed that speeding up the drying process can prevent the plant from reaching peak potency in the curing phase (Hawes and Cohen ). Coffman and Gentner () evaluated the effect of drying conditions on the cannabinoid profile. They stored the cannabis hang dried leaves in 65, 85, and 105 °C for 1, 4, 16, and 64 h to compare the mean percentage of total cannabinoid content. The results were shown that the percentage of total cannabinoids was decreased by increasing time and temperature. To be precise, the percentage mean weight loss of total cannabinoids increased from 7.5 to 11% in 65 °C after 1 h and 105 °C after 64 h, respectively.

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Air-drying (hang-drying) of the cannabis plant

Oven-drying

A faster direct method of drying is the oven-drying approach (Mujumdar ). This method can be carried out in either a vacuum chamber, vacuum desiccator, or in a drying oven with or without air circulation (Hawes and Cohen ). To illustrate the outcomes of the process, an early study tested out four different oven conditions to compare the end products. Inflorescences were dried for 1, 4, 16, and 64 h at 65, 85, and 105 °C. After extraction with ethanol, gas chromatography showed that the yield of CBD and THC decreased as the temperature and time of drying increased. It was also observed that at temperature 105 °C, the thermal degradation of THC increased the CBN content (Coffman and Gentner ). CBN is considered a less potent psychoactive and mild analgesic; therefore, conversion of THC to CBN will decrease the therapeutic potential (Citti et al. ).

Additionally, using high temperatures and excessive drying can result in the loss of key components (Hawes and Cohen ). This statement can be the reason for the lack of information about using oven dying in the cannabis industry. This was highlighted in a study that compared the ratio of cannabinoid and by-product produced during vaporization. The cannabis material was placed in the desiccator for 5 days to dry out, while the smoke condensate and vaporized condensate trapped in the organic solvent were dried with a rotary evaporator at 40 °C. These approaches had produced intense fragrance which is indicative of the loss of terpenoids and other volatile components (Pomahacova et al. ).

Freeze-drying

Freeze-drying (also known as lyophilization) has become a popular option due to the increasing demand for high-quality medicinal cannabis. The freeze-drying method holds the cannabis plant at temperatures far below those of air or oven, while removing the water content, in the form of vapor, via sublimation in a vacuum chamber (Mujumdar ). The nascent legal cannabis industry claims that freeze-drying preserves the volatile compounds and acidic form of cannabinoids (Tambunan et al. ). It is generally agreed that the end products of freeze-drying are considered high quality compared to other methods of drying. This is due to the structural rigidity found on the surface of frozen materials where sublimation occurs, preventing the disintegration of the solid matrix and resulting in a porous, unaltered structure (Mujumdar ). When assessing the end product produced by freeze-drying, it was found that the composition is largely unaffected from that found in the plant (Tambunan et al. ). A disadvantage of freeze-drying is the cost of operation. This procedure requires an intense amount of energy to maintain such temperatures, vacuum, and long-running time (Mujumdar ).

Comparing the different drying methods, we can safely state that the approach elected will affect the yield and cannabinoid profiles in the extracts. Therefore, the selection of a drying procedure will largely alter the outcomes (Coffman and Gentner ). The process of hang-drying cannabis was found to be time-consuming as it can take several days, while the main factors that increase the rate of drying were determined to be moving air and low humidity (Ross and ElSohly ). In contrast, the oven-drying method was observed to be faster, but readily volatile compounds and neutral forms of cannabinoids decreased in extracts to almost non-detectable concentrations, affecting therapeutic potential (Coffman and Gentner ). To address this issue, freeze-drying is thought to be the preferred method. Freeze-drying enables the preservation of flavor qualities in many foods, themselves often due to the presence of volatile compounds (Tambunan et al. ).

In all the drying methods mentioned above, humidity, temperature, ventilation rate, and time are the most important parameters to be optimized. Incorrect drying conditions may cause decarboxylation of acidic cannabinoids and loss of terpenes. The presence of light, oxygen, and heat may also cause degradation in cannabinoids and terpenes and can affect the taste (Jin and Chen ).

Curing

Curing is the final post-harvest procedure that allows for the development of the maximum flavor in the cannabis plant (Vogel ). Jin et al. () believed that the best temperature and humidity for curing are at 18 °C and 60% RH for 14 days. Green et al. () suggested keeping the trimmed flowers in a can for up to 4 weeks in a dark cupboard while opening the lid every day for about 6 h is the best method for curing (Jin and Chen ). At temperatures between 15–21 °C and 45–55% humidity, enzymes and aerobic bacteria will be in the optimum condition to breakdown undesired sugars and degrade minerals. Curing can reduce the harsh smell and the sense of throat burning during smoking or vaping as well as increasing the shelf life by minimizing mold growth. It is also believed that curing can increase cannabis potency as the number of cannabinoids such as THC and CBN will increase by curing. Although curing is one of the most significant post-harvest stages for the cannabis plant, there are not enough academic investigations around this area.

Continue at:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8290527/

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