Effect of different drying methods on antioxidant properties, stevioside and rebaudioside A contents of stevia (Stevia rebaudiana bertoni) leaves

The effect of different drying treatments of stevia leaves on antioxidant activity, ferric reducing power (FRAP), total phenolic content as well as stevioside and rebaudioside A content were evaluated. Drying treatments that were applied were oven drying (80°C and 60°C), sun drying, microwave drying and freeze drying. Results of all samples were compared with fresh leaves. Antioxidant activity of dried leaves was evaluated using DPPH radical scavenging activity and ferric reducing power (FRAP) by spectrophotometer. Stevioside and rebaudioside A content were evaluated using HPLC. Among all drying treatments, microwave was found to be the highest in scavenging the DPPH radical activity with no significant different with freeze dried and fresh leaves (P> 0.05). Inhibitory concentration at 50% of microwaved leaves was the lowest compared to other dried leaves. In addition to that, microwave dried leaves exhibit highest total phenolic content at 53.95 ± 2.83 mg/g gallic acid equivalent. As for stevioside and Rebaudioside A, no degradation happened in comparison with fresh leaves after drying treatment. Stevioside appeared to be higher in content than rebaudioside A. This indicate that microwave can be good drying method, without altering the stevioside and rebaudioside A content inside the leaves, thus maintaining the sweetening properties of the leaves.