Introduction. The aim of this review is to focus on the unique chemistry of curcumin to explain its dissimilar behaviour in different mediums. Materials and methods. The papers regarding structure-activity relationship studies of curcumin as antioxidant or pro-oxidant were examined. This review summarizes the achievements made in this field since 1980. Results and discussion. Curcumin is the principal curcuminoid found in turmeric and is generally considered as the most active constituent of turmeric compared to other curcuminoids. Enormous research has been carried out to explain the beneficial activities of curcumin. It has been the centre of attraction for potential treatment of an array of diseases such as cancer, Alzheimer, diabetes, allergies, arthritis and other chronic illnesses. It also possesses antioxidant activity at lower concentration. In contrast, the pro-oxidant effect has also been reported. The behaviour of curcumin to act as antioxidant or pro-oxidant depends on its structural form. Curcumin exists in two tautomeric forms, keto and enol. In keto form, curcumin exerts antioxidant activity. The enol form is prone to degradation. Hence, it is essential to maintain curcumin in keto form. In polar and acidic medium, curcumin exists in keto form whereas in non-polar and basic medium, it undergoes degradation. The mechanisms of degradation of curcumin under different mediums are discussed. Under basic conditions, nucleophilic attack of hydroxyl group is involved and under non-polar conditions, free radical mechanism is involved. Degradation under basic conditions leads to complete breaking of the molecule while under non-polar conditions, it proceeds via peroxide intermediate formation, clarifying the pro-oxidant effect of curcumin. In either of the cases, vanillin is the degradation product besides other degradation products. The discussion is further extended for the other two curcuminoids, viz. demethoxycurcumin and bisdemethoxycurcumin as well. The antioxidant activity of curcumin is the highest whereas bisdemethoxycurcumin exhibits least antioxidant activity amongst curcuminoids. However, the rate of degradation of curcumin is also maximum amongst curcuminoids followed by demethoxycurcumin and bisdemethoxycurcumin. This reveals that the electron donating methoxy group influences the activity of curcuminoids. Thus, structure of a constituent is responsible for its activity. Conclusion. The importance of a particular medium to achieve the beneficial activities of curcumin is confirmed.

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It is now agreed that food plays a major role in the maintenance of good health. The awareness of consumers about diet‐related health problems has increased. Hence, nowadays, preference is given to the natural food ingredients, which are expected to be safe and health promoting. Recently, many studies on health benefits associated with curcumin have been reported. In the present study, an attempt had been made to isolate curcumin from a mixture of curcuminoids by means of column chromatography technique. Further, the antioxidant activity of curcumin was studied in refined sunflower oil (RSFO). The activity was compared with that of mixture of curcuminoids in RSFO. The results showed that the other curcuminoids, i.e., demethoxycurcumin and bisdemethoxycurcumin had a profound effect on the activity of curcumin. Practical Applications Natural extracts have potential to stabilize unsaturated essential fatty acids effectively. It is well known that the presence of an active constituent in these natural extracts provides the property of antioxidant to the extract. The results revealed that the minor constituents had a profound effect on the activity of active constituent. The synergistic activity of all the constituents in an extract determines the resultant antioxidant activity of the extract.

Emulsions undergo an oxidation process because of the presence of water. The undesired phenomenon of autoxidation needs to be prevented as the lipidic fraction in the emulsion undergoes degradation. The surface active compounds so formed in the process disturb the stability of the emulsion. Accordingly, the characteristics of the emulsions are altered. This autoxidation process can be prevented by means of antioxidant. In the present study, a well known hydrophilic antioxidant, ascorbic acid, was examined at various concentrations to stabilize water-in-oil emulsion. The outcome showed that the concentration of a compound played an important role in determining its antioxidant or pro-oxidant effect.

Introduction. Curcumin, demethoxycurcumin and bisdemethoxycurcumin collectively constitute curcuminoids, exhibit different rates of degradation as well as therapeutic properties. Their remarkable dissimilar behaviour is affiliated to their tendency to undergo keto-enol tautomerism whereas their rates of degrading and imparting therapeutic properties depend on the ratio of keto and enol tautomers of individual curcuminoid. Materials and Methods. The aim of the current review is to focus on the ratio of keto and enol tautomers and to explain how the ratio of the two tautomers affects the rate of activities of the curcuminoids. The papers regarding variations in the rate of activities of curcuminoids were scrutinized. The factors affecting the ratio of keto-enol tautomerism were studied. Results and discussion. Structurally, curcuminoids are the compounds possessing β-diketone moiety. The characteristic property of compounds containing β-diketone moiety is to exhibit keto-enol tautomerism. Hence, each curcuminoid is a mixture of keto and enol tautomers. The keto tautomer of curcumioids is responsible for their therapeutic activities. On the other hand, enol tautomer of curcuminods has tendency to undergo degradation. Hence, the ratio of keto and enol tautomer needs to be considered while studying the activities of curcuminoids. Nevertheless, the ratio of keto and enol tautomer depends on the existence of other functional groups. Because of the presence of electron donating hydroxyl group, the proportion of enol tautomer predominates in curcuminoids. Further, due to the presence of two more electron donating methoxy groups in curcumin, the ratio of the enol tautomer is higher and hence, the susceptibility to undergo degradation is greater than demethoxycurcumin, which contains only one methoxy group. The ratio of the enol tautomer is minimum in bisdemethoxycurcumin amongst all the curcuminoids, which is devoid of methoxy group. Consequently, bisdemethoxycurcumin has the least rate of degradation amongst curcuminoids. Methoxy group enhances the activity of the functional sites in curcuminoids in keto form. Therefore, curcumin shows maximum beneficial activities in keto form amongst all the curcuminoids, followed by demethoxycurcumin and bisdemthoxycurcumin. Thus, the rate of the activities displayed by curcuminoids is governed by their keto and enol proportion. Conclusion. It can be concluded from this review article that besides β-diketone moiety, responsible for keto-enol tautomerism, other groups also play an important role in determining the ratio of keto and enol tautomers.