Effects of different drying methods on the antioxidant properties of leaves and tea of ginger species

Effects of five different drying methods on the antioxidant properties (AOP) of leaves of Alpinia zerumbet, Etlingera elatior, Curcuma longa, and Kaempferia galanga were assessed. All methods of thermal drying (microwave-, oven-, and sun-drying) resulted in drastic declines in total phenolic content (TPC), ascorbic acid equivalent antioxidant capacity (AEAC), and ferric-reducing power (FRP), with minimal effects on ferrous ion-chelating ability and lipid peroxidation inhibition activity. Of the non-thermal drying methods, significant losses were observed in air-dried leaves. Freeze-drying resulted in significant gains in TPC, AEAC, and FRP for A. zerumbet and E. elatior leaves. After one week storage, AOP of freeze-dried E. elatior leaves remained significantly higher than those of fresh control leaves. Freeze-dried tea of A. zerumbet was superior to the commercial tea for all AOP studied.