Solid-State Microwave Drying for Medical Cannabis Inflorescences: A Rapid and Controlled Alternative to Traditional Drying

Abstract

Introduction: As the medical use of Cannabis is evolving there is a greater demand for high-quality products for patients. One of the main steps in the manufacturing process of medical Cannabis is drying. Most current drying methods in the Cannabis industry are relatively slow and inefficient processes. Materials and Methods: This article presents a drying method based on solid-state microwave (MW) that provides fast and uniform drying, and examines its efficiency for drying Cannabis inflorescences compared with the traditional drying method. We assessed 67 cannabinoids and 36 terpenoids in the plant in a range of drying temperatures (40°C, 50°C, 60°C, and 80°C). The identification and quantification of these secondary metabolites were done by chromatography methods. Results: This method resulted in a considerable reduction of drying time, from several days to a few hours. The multiple frequency-phase combination states of the system allowed control and prediction of moisture levels during drying, thus preventing overdrying. A drying temperature of 50°C provided the most effective results in terms of both short drying time and preservation of the composition of the secondary metabolites compared with traditional drying. At 50°C, the chemical profile of phytocannabinoids and terpenoids was best kept to that of the original plant before drying, suggesting less degradation by chemical reactions such as decarboxylation. The fast-drying time also reduced the susceptibility of the plant to microbial contamination. Conclusion: Our results support solid-state MW drying as an effective postharvest step to quickly dry the plant material for improved downstream processing with a minimal negative impact on product quality.

Keywords: drying technology; medical Cannabis; microwave; phytocannabinoids; terpenoids.

FIG. 1.

Schematic diagram of the solid-state microwave drying method. The signal generation unit generates MW signals with controlled frequencies. Each signal is split into two channels that go through a phase shifter and from there to a power amplification or attenuation that prepares the signal for final amplification by an amplifier unit of a constant gain. The energy is transmitted to the antennas in the cavity through each channel, and the returning energy from the cavity is measured through each antenna using the measurement matrix. Then, the central processing unit calculates the appropriate MW parameters, based on which it controls further signal generation, phase shift, and amplification. The process is iterative, so the frequency, phase, and amplitude of MWs transmitted to the Cannabis are controlled in a closed-loop until the end-point. The module controls the phase between the antennas in the range of 0 to 360 with ∼1.5° step. Of the total available FPCs, the module algorithm selects 6 different phase levels for each frequency. FPCs, frequency-phase combinations; MW, microwave.

FIG. 2.

Forward-power envelope at various FPCs during the drying process. The forward-power envelope is displayed for a subset of the available frequencies at the beginning of the drying process (cycle 5, A) and after ∼25 min of drying (cycle 250, B).

FIG. 3.

Concentrations of major phytocannabinoids in Cannabis inflorescences before and after drying traditionally or with the MW oven at different temperatures. Fresh—immediately after harvest, traditional drying—after 10 days at 16°C and 50% relative humidity, and MW drying—at the indicated temperatures. Data are reported as mean±SD of phytocannabinoid concentration (n=3, %w/w). Statistically significant differences between groups were calculated by one-way ANOVA (*p≤0.05, **p≤0.01, ***p≤0.001, ****p≤0.0001). The mean of each drying treatment was compared with the mean of the fresh material.

FIG. 4.

Heat map of terpenoid profile in Cannabis inflorescences before and after drying traditionally or with MW drying. The concentration of terpenoids (parts per million) was evaluated using GC/MS/MS for Cannabis Infiniti strain immediately after harvesting (fresh), after traditional drying, and after MW drying at the indicated temperatures. Absolute concentrations were color coded relative to the maximum value of each compound (n=2). SUM value represents the total terpenoid content in Cannabis inflorescences (%w/w). GC/MS, gas chromatography/mass spectrometry; LOQ, limit of quantification; na, not applicable.

FIG. 5.

Prediction of drying process termination. (A) SEM images of Cannabis inflorescences dried either traditionally (left) or by MW drying at 50°C (right) and their corresponding magnified images (bottom). (B) Moisture content versus FPC number of Cannabis inflorescences dried with the MW oven at 50°C. The dotted line represents the linear regression. (C) Moisture content and drying time for Cannabis inflorescences dried with the MW oven at 50°C. (A–D) Four independent experiments. (D) FPC (normalized units) versus drying time in minutes. The dotted rectangle represents FPC numbers in the range of 0.50–0.53. Each FPC is characterized by an absorption efficiency value, and when the number of FPCs characterized by a value higher than a threshold crosses a predetermined number, the drying is stopped. SEM, scanning electron microscopy.

FIG. 6.

Microbiological assay of Cannabis inflorescences. TYMC of Cannabis Infiniti strain immediately after harvesting (fresh), after traditional drying, and after MW drying at 50°C. Microbial counts are expressed as colony-forming units per gram of inflorescence (CFU/g). TYMC, total combined yeast and mold count.