## 1 Introduction  
Antioxidants play important roles in nutrition by increasing the shelf life and preserving the nutritional aspect of lipid-containing food products. Antioxidants also prevent the oxidation of biological molecules in the body by chelating redox metals or quenching free radicals [1; 2]. Plants contain numerous natural antioxidants and can be a source of new antioxidant compounds given the various chemical structures and mechanisms of action of these natural antioxidants [3]. _Phyla nodiflora_ (L.) [PERSON], also known as _Lippia nodiflora_ and widely known as frog fruit, belongs to family Verbenaceae and is distributed in subtropical and tropical regions [4; 5]. Previous studies have confirmed the presence of antioxidants in _P. nodiflora_ extracts [6; 7], namely, stigmasterols, nodiforetin, nodiforin,lippiflorin, and batalilfilin [4]. Traditionally, the leaves are used to treat diarrhea, fever, cold, knee joints pain, and gonorrhea [5]. Fresh leaves are high in moisture and are perishable commodities [8]. The antioxidant compounds in the leaves may undergo reduction during the period between harvesting and processing. Thus, fresh medicinal plants must be effectively preserved by removing moisture to inactivate enzymatic, chemical and metabolic reactions, which may otherwise cause the loss of important antioxidants [9]. Moisture removal from food is an integral operation in food processing. Convective drying (CD) has remained the most common drying method to reduce moisture from food and biomaterials. In CD, heat is transferred convectively from the hot air or combustion gasses to the cooler moist boundaries of the drying material. Thereafter, heat diffuses into the drying material primarily through conduction. Evaporation of moisture at the surface of the material is efficient; however, the removal of moisture from the center regions to the surface is slow because it is driven by a moisture concentration gradient, rendering CD a low drying rate method [10]. In view of the limitations of CD, there is increasing interest in emerging drying technologies, such as microwave drying, which gives better drying performance. Microwave drying is gaining popularity because drying time can be reduced and a higher retention of active constituents can be achieved. Despite fast drying rates, the heterogenic structure of materials and the inhomogeneity of microwave field may result in hot spots within the material at the final stage of drying [11]. A solution is to use microwave drying in combination with vacuum drying. This combination has been proven effective in many previous studies on preserving active compounds in the drying of herbs and fruits [12; 13]. Furthermore, a lower pressure of vacuum-microwave drying (VMD) induces quicker water evaporation from food material at a lower temperature [14], thereby reducing chemical alterations [15]. However, high water evaporation from plant material at the initial stage of VMD can exceed the vacuum pump's capacity, so reducing the material load or a larger vacuum pump installation is required [11; 14]. Pre-drying through CD, hereafter denoted as CPD, reduces the load mass of raw material to be dried in VMD. This two-stage drying method of CPD followed by vacuum-microwave finish drying (VMFD), hereafter denoted as CPD-VMFD, reduces the total cost of dehydration and improves the quality of products [16]. Although the chemical composition of several herb species dried using vacuum-microwaves has already been well recognized [13; 17; 18], to date no studies have investigated the influences of VMD and CPD-VMFD on the antioxidant activity of _P. nodiflora_ and similar species. Therefore, it is of interest to evaluate the effectiveness of new vacuum-microwave drying methods compared to conventional CD in retaining bioactive constituents.