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Stability of turmeric constituents in natural soaps

Natcha Wongthongdee^a,b, Pranee Inprakhon^a,*

 
ABSTRACT:     The suitability of using turmeric powder in natural soaps was assessed by evaluating its stability and antioxidant activity in models and actual soaps. Proper storage conditions should be considered prior to use because 63% of curcumin was lost after 3 months of storage at room temperature. Among physical properties influencing soaps, pH exerts the most detrimental effect on curcumin stability. Only 8% of curcumin remained 7 days after its addition into liquid soap of pH 9.32, whereas it was degraded completely after 21 days of the soap-bar curing process, in which pHs varied from 13 to 10. Vanillin was detected as a major by-product of degradation and might make curcumin still valuable as an antioxidant in soaps. However, the antioxidant activities of turmeric extracts treated in the model system for soaps indicated that the contribution of vanillin to antioxidant activity was not significant since its occurrence in µg was too low compared to the loss of mg of curcumin. In vitro assessment of the antioxidant activity of turmeric powder when extracted from a buffer of pH 10.6 confirmed that there was no vanillin contribution in the 2,2-diphenyl-1-picrylhydrazyl (DPPH^.) radical-scavenging activity, since the IC₅₀ increased as a function of the decreased residual curcumin content, despite the occurrence of vanillin. In the ferric ion reducing antioxidant power (FRAP) assay, vanillin contributed slightly to the ferric-reducing ability, since increasing EC₁ was slowed down by gradually increased vanillin in the turmeric extract. The curcumin degradation demonstrated clearly in this study indicates that turmeric powder is not a suitable antioxidant additive and/or skin lightening agent in soap due to its physico-chemical properties.

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* Corresponding author, E-mail: pranee.inp@mahidol.ac.th

Received 8 Jul 2012, Accepted 20 May 2013