Reducing power was measured according to the method described by Duh and Yen (1997). One mL of oil or plant extract
of M. piperita was mixed with phosphate buffer (0.2 M, pH 6.6, 0.5 mL) and potassium hexacyanoferrate solution
(1% v/w) in a test tube and heated at 50 C for 20 min. After cooling the tube on ice, 0.5 mL 10% trichloroacetic acid was added.