Curcuminoids confer tinctorial power to Curcuma longa L. and other Curcuma species. Srinivasan analyzed this as principally curcumin [1,7-bis-(4-hydroxy-3-methoxyphen l)hepta- 1,6-diene-3,5-dione] (1) mixed with related demethoxy curcumin [4-hydroxycinnamoyl(4- hydroxy-3-methoxy cinnamoyl)methane] (2) and bis-demethoxy curcumin [bis-(4-hydroxycinnamoyl) methane] (3) and probably other minor constituents. Synthesis as well as spectral data are in accord with these structures.
Numerous methods are available for isolating curcumin from Curcuma longa L. and from other species. Technical synthesis of this colorant is also known. Except by the chromatographic route, all others generally converge on curcuminoids with curcumin as the dominant constituent. Purification of curcumin via the lead salts is under study.
A recent recommendation resorts to mixed melting point determination with authentic specimen to check the purity of curcumin. However, firstly, the isolation of pure curcumin is very difficult and time-consuming and it is not. produced commercially. One has to accept with caution the statement that “the industry wants pure curcumin and this know-how is also available to Indian manufacturers." According to Govindarajan, “pure curcumin which was not generally available in the early years . . . is available now; apparently, the author had in mind the curcumin of Eastman Kodak Co. Secondly, melting point versus purity pattern is very confusing, For example, melting point 180-181” is recorded for Eastman Kodak Co. curcumin of 90.1% purity (Assay UV-VIS) (vide infra) as against melting point 172-175° for E. Merck curcumin of 97% purity (Assay Alkalimetric).