Microwave-assisted hot air drying of Cannabis sativa: Effect of vacuum and pre-freezing on drying kinetics and quality

Abstract

Drying is an important postharvest step preceding long term storage of medicinal plants, and optimized drying conditions are critical for secondary metabolite preservation. The objective of this research was to determine the effect of pre-freezing, vacuum, drying temperature, and microwave power on cannabis, including drying kinetics, color retention, and moisture diffusivity, and cannabinoid and terpene concentrations. Harvested inflorescence was dried under four drying conditions, namely untreated, dried without vacuum, untreated, dried with vacuum, pre-frozen, dried without vacuum, and pre-frozen, dried with vacuum. The untreated groups were dried and analysed immediately after harvest and the pre-frozen group was frozen at −20°C for 24 h prior to drying and analysis. Results showed that moisture diffusion was significantly affected by the temperature, vacuum and microwave power and reduced exponentially with time. For untreated samples dried at a microwave power of 2 W g−1, it took only 61 min (with no vacuum) and 57.3 min (under vacuum) at 65°C to reach the desired final moisture content of 11 % dry basis. Increasing drying temperature from 35°C to 50°C at 1 W g−1 under vacuum reduced the drying time by 60.1 % and 68 % for pre-frozen and untreated samples, respectively and reduced by 68 % and 65.5 % for pre-frozen and untreated cannabis samples, respectively for samples dried without vacuum. Pre-freezing prior to hot air drying with and without vacuum can be used to preserve the color of biomass and cannabinoids. The effect was prominent in the acidic cannabinoids where pre-freezing prior to hot air drying at 50°C without vacuum increased tetrahydrocannabinolic acid and cannabigerolic acid concentrations from 20.6 to 23.6 g 100 g dry matter−1 and 0.47–0.61 g 100 g dry matter−1. Results presented add important knowledge for the optimization of postharvest steps for cannabis.