Characterization of biologically active compounds from different herbs: Influence of drying and extraction methods

The hot air (HA) method is the most widely implemented drying method for plants (herbs, vegetables, and fruits). This method has a few drawbacks that include long drying time, limited heat transfer, and limited thermal conductivity. This study investigated the effects of HA and infra-red (IR) heating method on biologically active compounds from different herbs (Khaffir lime, Lemongrass, Prai, Tamarind, and Turmeric). The efficiency of the drying methods was evaluated by considering (a) moisture ratio (MR), (b) specific energy consumption (SEC, MJ/kg.H₂O), (c) moisture diffusivity (D_eff), and (d) activation energy (E_a, kJ/mol). The active compounds were extracted from HA and IR dried herbs using different solvents (hexane, water, and ethanol) through Solid-Liquid Extraction (SLE) and Soxhlet Solvent Extraction (SSE) methods. The moisture removal in the IR drying process increased 10–11% for the herb samples. Specific energy consumption (SEC) increase during the IR drying process is attributed to the rapid evaporation of water at shorter time intervals than HA. Activation energy (E_a) values decreased by 1.66, 1.48, 2.24, 3.13, and 2.07 fold times for IR dried prai, turmeric, lemongrass, tamarind, and kaffir lime, respectively. The higher yields of herbal extracts and the abundance of bioactive terpene derivatives in hexane extracts were obtained from HA herbs compared to IR samples. Therefore, it is concluded that the IR method and SSE process using hexane was suitable to dry and retain the bioactive active compounds within herbs. Further, the IR method over the HA method was considered based on energy consumption, processing time, yield, and active compounds.