Effects of olive leaves on chemical and phenolic constituents of refined olive oils

Olive oils obtained from partly damaged olive fruits are negatively affected by biochemical changes and thus need to be refined for consumption purposes. However, the refining process precipitates losses of some bioactive compounds naturally found in olive oil. The primary purpose of this study is to evaluate the chemical properties of refined olive oils containing different rates of olive leaves (washed, dried, milled and sieved) in comparison with virgin olive oil. Relative to virgin olive oil, the total phenols, bitter index (K225) and antioxidant activity values of olive oils enriched with olive leaves at concentrations of 10 % and 15 % were found to be higher, whereas the extinction coefficient values (K232 and K270) were found to be lower. The phenolic compound levels, i.e. the oleuropein, luteolin, tyrosol and hydroxytyrosol levels of refined olive oils containing 10 % and 15 % olive leaves increased significantly with the amount of leaves added. The results obtained emphasize the great potential of dried olive leaves for enhancing the chemical and phenolic properties of refined olive oils as a phenolic compound source.