Antioxidant activity of propolis in oil-in-water emulsion

In this work, it has been developed an appropriate soybean oil-in-water emulsion system to study the antioxidant activity of different type of propolis extracts. Emulsions were stored at room temperature and under magnetic stirring for the entire period of analysis (10 days). Lipid hydroperoxides were measured according to the method described by Shantha and Decker.1 The secondary oxidation products were determined as TBARS according to the method described by McDonald and Hultin.2 The surface of the emulsion droplets was varied by using different surfactants, sodium dodecyl sulphate (SDS) is an anionic emulsifier, whereas Tween 80 and Brij 35 are non ionic surfactants. The Tween 80-stabilized emulsion oxidation rate is lower than Brij 35 and SDS emulsions. Besides, the Tween 80 emulsion resulted to be more stable then the others and it was used to determine the effects of pH on lipid oxidation. The lipid oxidation rate of the Tween 80-stabilized emulsion was tested at pH 3.0 and 7.0. Oxidation rate increases with decreasing pH and the oxidation rate at pH 3.0 is more desirable since the oxidation rate at pH 7.0 is too much low. As a consequence, the Tween 80-stabilized emulsion at pH 3.0 has been chosen for our purpose. For each propolis extract (ethanolic, acetonic, chloroformic, etheric), 3 oil-in-water emulsions were prepared by adding 0.06 ml, 0.27 ml and 1 ml of extract into the oil phase. All the extracts show antioxidant activity which, in all cases except for the acetonic one, tends to increase with increasing the extract content. Instead, the ethanolic extract used at low level presents an evident pro-oxidant effect. The lowest content of acetonic extract is the most active as antioxidant.