ANTIOXIDANT ACTIVITY OF LICORICE EXTRACT 557 lOO 90 80 70 60 50 40 30 20 lO o o A i i i i i 4 6 8 10 12 Time (Week) B lOO 9o 8o 70 50 30 20 10 0 2 4 6 8 10 12 Time (week) C CB .--•Ext. 2% C CB .--•Ext. 2% .& SM2% ,( BHT2% .& SM2% ,( BHT2% Figure 5. Formulation stability study of 2% w/w hydroquinone cream containing 2.0% extract and commercial antioxidants incubated at 25 ø _+ 0.5øC (A) and 45 ø _+ 0.5øC (B) for three months. mercial antioxidants. According to our procedure, by incorporating hydroquinone after the cream was formed, and together with the solubility property of hydroquinone that is freely soluble in propylene glycol and slightly soluble in oil and water, we had expected that hydroquinone had been absorbed in both the oil and the water phases of the formulation. It is also evident that hydroquinone can be incorporated in the for- mulation in different ways. Besides our procedure, hydroquinone was used in the oil phase with laevo-ascorbic acid as antioxidant, and in the water phase with sodium metabisulfite and ascorbic and citric acids as antioxidants (1). This meant that the types of antioxidants selected for use in the formulation depended on the method of the incorporation of hydroquinone into the system. In our hydroquinone systems containing extract, the licorice acted as both water- and oil-soluble antioxidant agents and dem- onstrated significant protection from oxidative degradation in hydroquinone for three months, in comparison with sodium metabisulfite and BHT that were water-soluble and oil-soluble, respectively. CONCLUSION The licorice extract at 0.5% and 1.0% can be used as a double-action (both water- and oil-soluble) antioxidant, having 76% and 78% (at 25øC) and 55% and 60% (at 45øC) of hydroquinone remaining, respectively, after three months. These results indicate that

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