## 1 Introduction  
The majority of the world population has used medicinal plants in some ways as their first source of treatment as remedy for infectious and non-infectious diseases. Many of these medicinal plants have been researched and have garnered scientific evidence to be considered and used in general medical practice. A large number of diseases are caused by oxidative stress that can elicit cellular and tissue damage--a result of increased levels of free radicals or reactive oxygen species (ROS). Deficiency of antioxidants or overproduction of ROS in the body can cause the disturbance in the equilibrium between ROS formation and elimination, favouring the upsurge of ROS, which results in oxidative stress [1]. Hence, discovering plants that exhibit antioxidant effect that can be a potential source of antioxidants is necessary. _Cassia alata_ (Leguminosae) is a shrub native to Southeast Asia, Northern Australia, Latin America, Fiji and Africa and can be found widespread in tropical regions, cultivated for medicinal purposes [2; 3]. _C. alata_ is known as gelenggang in Malaysia, ringworm bush in Australia, mbai ni thangi in Fiji and te'elango in Tango [2]. This shrub can grow up to 3 m in height. The leaves are 30-40 cm long, pinnately compound, have 6-12 pairs of broad leaflets, blunt at the tip portion, with unequal base and are 15 cm long and 8 cm wide. The flowers are golden yellow, roundish in axillary racemes that are 3-4 cm wide and 20-30 cm long [4]. _C. alata_ is widely used in folk medicine and home remedies for treating common ailments, such as ring worm, skin diseases, rheumatism, diabetes, stomach pain, constipation and gonorrhoeae [3; 5]. Numerous past reports on the promising antioxidant effect shown by _C. alata_ extract have prompted the investigation of the influence of drying methods on the antioxidant activity and chemical constituents [6; 7; 8]. The leaf extract of _C. alata_ is reported to contain phenols, flavonoids and anthraquinones, which is believed to contribute to the strong antioxidant effect of the plant extract [6]. Freshly harvested medicinal plants have high water content and are thus categorised as perishable commodities. Unfavourable enzymatic and metabolic activities are likely to occur due to high water content, in addition to the increased susceptibility to microbial growth [9]. Bioactive content may reduce considerably in the raw material between the period of harvesting and processing. Effective preservation via the removal of moisture is needed to inactivate enzymatic degradation and chemical and metabolic reactions [10]. Drying operations have been used in the food processing industry for the preservation of final product. The primary aim of drying is to reduce moisture in food to a level that can avoid microbial spoilage. However, drying can also be seen as a process that retains important constituents in raw herbs with nutraceuticalal properties [11]. Convective drying (CD) remains the most widely used drying method for reducing moisture from fruits, vegetables and medicinal plants. However, CD methods have several limitations; for example, they are a low energy efficiency drying method and may not necessarily produce dried medicinal herbs with high retention of bioactive compounds [9]. In addition, CD is a surface heating method. Firstly, water evaporates from the surface of food material, and the subsequent moisture removal from the inner regions of raw material is driven by a moisture concentration gradient. Diffusion of moisture from the centre areas of the material to the surface is slow and rate limiting, thereby resulting in low drying rate at the final stage of drying [12]. Prolonged drying durations can result in extensive quality deterioration in the final dried product [13]. At the same time, bioactive constituent loss may also be contributed by the long exposure of plants to oxygen during the CD process [14].