Comparison of antioxidant activity of extract from roots of licorice (Glycyrrhiza glabra L.) to commercial antioxidants in 2% hydroquinone cream.

Powdered dry roots of licorice (Glycyrrhiza glabra L.) were extracted with methanol. Licorice extract was tested for antioxidative activity in comparison with antioxidants (sodium metabisulfite and BHT) at 0.1%, 0.5%, 1.0%, and 2.0% w/w in 2% w/w hydroquinone cream. The systems were incubated in a dark room at 25 degrees +/- 0.5 degrees C and 45 degrees +/- 0.5 degrees C for three months. The physical stability and the percentages of hydroquinone remaining after two weeks and one, two, and three months were determined by UV spectrophotometer at 294 nm according to official standard procedures. The experiment revealed that oxidation degradation of hydroquinone was accelerated by heat even with the existence of antioxidants. The higher percentages of remaining hydroquinone were observed for higher antioxidant concentration but showed lower physical stability in the formulation in the presence of commercial antioxidants, especially in the cases of 1.0% and 2.0% BHT. In the third month, at 25 degrees +/- 0.5 degrees C and 45 degrees +/- 0.5 degrees C, the extract demonstrated more antioxidant activity from two other commercial antioxidants at all concentrations, with about 43-53% and 34-46%, respectively, more hydroquinone remaining than in the control system (p<0.001). In the third month, the preparation containing 0.1%, 0.5%, 1.0%, and 2.0% extract gave good physical formulation stability with about 72%, 76%, 78%, and 81 % hydroquinone remaining at 25 degrees +/- 0.5 degrees C and 51%, 55%, 60%, and 63% hydroquinone remaining at 45 degrees +/- 0.5 degrees C, respectively. This suggested the possibility of using a licorice extract at 0.5% and 1.0% as an effective natural antioxidant for substances that are oxidation-susceptible.