Top 5 Drying techniques for pharmaceutical Cannabis

Hang drying or hot air drying is one of the earliest drying methods [77], to dry either whole medicinal Cannabis, branches with flowers or trimmed flowers [75]. Firstly, remove the excess stems from the plants, transferring them to drying racks and hanging on either string lines, wire cages, or static wires upside-down to allow for air circulation and uniform drying by slowly transferring water from the stem, slowly migrating into the buds as the water evaporated [75,76]. Place the Cannabis material in a well-ventilated room. This room should be equipped with an environmental temperature control system set between 18–21 °C, relative humidity at 50–55% and air circulation using a small fan [76]. Under these controlled conditions, branches with flowers take 5–6 days to reach the desired moisture level, and trimmed flowers take only 4–5 days, but the whole plant takes 14 days to obtain 11% w/w moisture content [75]. Although this technique is mainly used, hot air drying takes a long time to process [76,77]. A study by Chandra, Lata, ElSohly, Walker and Potter [59] reported using a forced-air dryer for large scale drying. However, the disadvantage of this method is the requirements of additional curing time. Furthermore, the process requires daily inspection and proper maintenance of the conditions. Another disadvantage is the production of a product with a harsher taste due to the removing of buds from stems [92]. Another issue is often encountered with the traditional current method of drying is mould growth due to uncontrolled conditions [75]. Coffman and Gentner [78] evaluated the effect of drying conditions on the cannabinoid profile. It was found that the loss of the percentage of total cannabinoids and degradation of terpene increased from 7.5 to 11% when changing the process from 65 °C for 1 h to 105 °C for 64 h [76].

Oven drying is one of the fast methods to dry medicinal Cannabis [87]. Many types of oven drying like a vacuum chamber, vacuum desiccator or drying oven with or without air circulation have been used to dry medicinal Cannabis [75]. The oven must be preheated at 37 °C for 24 h as optimal conditions to prevent decarboxylation for Phyto cannabinoids [79]. At any temperature higher than 37 °C, the sample contains both acid and neutral cannabinoids [79], and the total cannabinoids yield will decrease [78]. The oven had to be operated as a batch system, with buds hanging upside down. The benefit of this process is simple and low cost [93], and moisture accumulates in the flower until the capillaries begin to harden. Another advantage of using this method is ensuring the moisture will keep circulated inside the plant and prevent drying buds so quickly [75]. The disadvantage of this method is that it is only the best option for a small manufacturer of medicinal Cannabis [87]. This method will not cause any degradation or change of cannabinoids [79].

Microwaves are non-ionising electromagnetic waves found between the radio and infrared wavelengths on the electromagnetic spectrum [94]. Microwave-assisted hot air drying is based on volumetric heating and creating a temperature gradient. The first stages of the drying process encourage the movement of the flow of moisture from the inner part to the surface [95,96], while the final stages of the drying process help with the removal of the bound water present in the material [97]. For most industrial applications, microwaves frequency of 915 MHz is considered the most valuable, due to its greater penetration depth. The significant advantages of microwave-assisted hot air drying are preserving nutrient contents, microstructure and colour of the dried sample [80,98]. The disadvantage is this process produces high heat, so it is only used in the final filling stage of the drying process. This high heat also causes an off taste [99]. Micro-wave-assisted hot air-drying preserves terpenes and cannabinoids [81,94]. A study by Kwaśnica, Pachura, Masztalerz, Figiel, Zimmer, Kupczyński, Wujcikowska, Carbonell-Barrachina, Szumny and Różański [82] found that drying with 240 W microwave-assisted hot air-drying cause maintained high quality 93 volatile compounds, predominantly β-myrcene, limonene and β-(E)-caryophyllene, as well as α-humulene similar to the chemical composition of fresh medicinal Cannabis.

The best method for drying medicinal Cannabis is freeze-drying the bud. Freeze-drying is carried out in three steps: freezing, primary drying and secondary drying [83,84]. During freezing, sample temperature is reduced to approximately −40 °C, thus converting most of the water present into ice. Freezing the product before drying is critical, as it determines the final quality. This step prevents the formation of water foam when the vacuum is applied [85]. In general, drying is complete when the sample’s temperature is the same as the system temperature [86]. After that, it must run secondary drying to remove residual moisture present in the sample without damaging proteins and lipids [83]. The preservation action is because of the structure of frozen materials, preventing the solid matrix’s degradation and resulting in a porous, unaltered structure [87].

In contrast, the glands and terpenes remain without damage. The main advantage of using vacuum freeze-drying in the Cannabis industry is that the low temperature inhibits microbial enzymatic activities and preserves the end product quality [100]. Tang and Pikal [83] show that secondary drying eliminates residual moisture. However, the main disadvantages of vacuum freeze-drying are the high initial capital and operational cost. It only permits drying in batches, and requires higher energy [101,102]. The best method for drying medicinal Cannabis is vacuum freeze-drying because it retains a maximal number of active compounds like the aroma and full flavour, and preserves the volatile compounds and acidic form of cannabinoids [84].

Another method used to dry medicinal Cannabis is microwave-assisted freeze drying. This process simply circulates cold, dry air over the frozen material at a temperature below −40 °C to −45 °C, pressure at 100 Pa, and microwave frequency of 2450 MHz [88,89,90]; this is to condense the vapors, maintain the frozen nature of the Cannabis and improve the mass transfer of water [101,102,103].

There are many types of atmospheric freeze-drying, such as a tunnel dryer, fluidised bed dryer and spray freeze dryer. Compared to tunnel dryers, the heat and mass transfer rates are better in fluidised bed dryers, while tunnel dryers can solve size reduction problems caused by mechanical shaking. The advantage of microwave freeze-drying (MFD) is 50–75% less time than freeze-drying [88]. Furthermore, Spray freeze-drying consists of a combination of spray drying and atomisation freeze-drying of the material, freezing and drying [103]. A study by Ishwarya, et al. [104] found that spray freeze-drying has higher volatile retention (93%) compared to freeze-drying (77%) and hot air spray drying (57%). However, there are many disadvantages, such as non-uniform heating of the dry zones in the product, impacting the product quality [105]. Its price is about four to ten times higher than other drying methods [89], and needs to be used in a limited amount to preserve high-quality hemp, and further research is required for testing and optimised CBD and terpenes contents [76].