Effect of drying method on oleuropein, total phenolic content, flavonoid content, and antioxidant activity of olive (Olea europaea) leaf

The purpose of this research was to study the effect of microwave drying (MD) on the major phenolic compound oleuropein, total phenolic and flavonoid contents, and antioxidant activity of the olive leaves. The results were compared to those obtained after using freeze drying (FD), vacuum drying (VD), oven drying (OD), and ambient air-drying (AAD). Response surface methodology, using a central composite face-centerd design, was used to optimize the effects of microwave irradiation power (300-500 W), solid mass (1.5-2.5 g), and drying time (4-6 min) on oleuropein and other olive leaves antioxidant compounds. The second-order models were found highly adequate with very low root mean squared deviation (RMSD) and high coeffecient of determination (R-2) values. The results of MD under optimum conditions (2.085 g sample at 459.257 W for 6 min of drying period) has demonstrated to be the best drying method followed by FD, VD, OD, and AAD.