Alkaline degradation of the pigments curcumin (CC), demethoxycurcumin (DMC) and bis-demethoxycurcumin (BDMC) in aqueous solutions of purified and commercial oleoresin sources was investigated. Alkaline degradation of the compounds corresponded to pseudo-first-order kinetics. Responses were similar for pigments combined or alone in solutions. Profiles of rate constants were closely associated to polynominal curves. Degradation rate constants rapidly increased from pH 7.45 to a maximum at about 10.2 and then tended to decline at higher pH. Rates of alkaline degradation were lower for BDMC than for CC and DMC. Halflives of CC, DMC and BDMC at pH 7.45 were 900, 1700 and 2200 hr, respectively. In contrast, half-lives of CC, DMC and BDMC at pH 10.2 were 0.4, 1.0 and 5.0 hr, respectively. Use of BDMC instead of CC in alkaline foods would be expected to improve color stability.

Turmeric curcuminoids are important natural pigments but their instability limits their use in many foods. This study was conducted to examine light‐induced degradation of turmeric curcumin as affected by solvent system and oxygen. The rate constants and half‐lives were determined for purified curcumin, demethoxycurcumin and bis‐demethoxycurcumin exposed to 1450 lux using reversed‐phase HPLC. Rates of degradation of each pigment followed first‐order kinetics. Differences in rate constants and half‐lives between the individual pigments were minor in an acetic acid‐NaCl solution; however, in methanol, large differences in stability to light of the individual pigment were observed. Stability of all the pigments to light was several fold greater in methanol than in acid brine. In methanol sparged with air, stability to photooxidation was curcumin > demethoxycurcumin > bis‐demethoxycurcumin. In methanol, curcumin was more stable than the other curcuminoids when sparged with air than when sparged with nitrogen. In methanol sparged with nitrogen, demethoxycurcumin was the most stable pigment. All of the pigments were stable in the dark.