## 1 Introduction
The introduction and application of hybrid drying technologies are necessary to overcome the limitations of conventional drying methods whilst considering the sensitivity of bioactive constituents in medicinal herbs to thermal degradation and oxidation. Combined drying methods are perceived as more versatile because the advantages of two or more drying methods may offer better drying outcomes than an individual approach [9]. Vacuum microwave drying (VMD) is a combined microwave heating and vacuum drying method. This combination is deemed appropriate for drying process to occur in a reduced pressure environment, thereby lowering oxidation reactions and lowering the boiling point of water. During the process of VMD, microwaves will penetrate the plant material and cause water to be heated up via dipole rotation and ionic conduction, thereby causing water vaporisation at a low temperature under a reduced pressure condition [11]. In addition to increasing the drying rate with VMD's volumetric heating, the lower pressures used can induce rapid water evaporation from plant materials at a low drying temperature, thereby minimising possible chemical alterations in plant material by high temperatures [15]. The problem with VMD is that high water evaporation from the plant material at the initial stage of drying may exceed the vacuum pump's capacity; therefore, the material load has to be reduced, or a large vacuum pump installation is often required [11; 16]. Installing a large-scale vacuum pump requires a large capital and can potentially incur a large processing cost. Convective pre-drying followed by vacuum microwave finish drying (CPD-VMFD) is a two-stage combined drying method in which the raw material is pre-dried using CD and finish dried with VMD. This method has been used in numerous studies being a cheaper alternative of VMD in terms of capital expenditures and operating costs. The high retention of bioactive constituents in various biomaterials has also been reported with this two-stage method [17]. The present study aims to evaluate the influences of various drying methods, conventional drying method, CD, new drying technologies (namely, VMD and CPD-VMFD) and the respective drying intensities on the total phenolic content, antioxidant activity, volatiles and phytosterol content. The fatty acid profile of the plants was ascertained to understand the active constituents contained in _C. alata_. The drying kinetics was modelled with three common thin-layer drying models to integrate the experimentally acquired data into industrial applications, thereby providing the basis for the understanding of the unique drying characteristics of _C. alata_. Drying is considered the most energy-intensive operation in the food industry. Thus, this study also determined the relationship of the drying kinetics of _C. alata_ with the corresponding energy input of a drying operation. The specific energy consumptions were used to provide an estimate of the cost-effectiveness of the drying of _C. alata_, which has yet to be reported. Colour properties and water activities of fresh and dried samples, which serve as quality indicators, were also determined.