Curcumin is a yellow color polyphenolic compound derived from the turmeric of the herb Curcuma longa L. Its chemical name is bis-α,β-unsaturated β-diketone or commonly called diferuloylmethane , which exhibits keto-enol tautomerism having a predominant keto form in acidic and neutral media and stable enol form in alkaline medium. It is insoluble in water under acidic or neutral conditions but dissolves in  alkaline conditions, . This discoloration is due to changes in pH. Curcumin is an oil soluble pigment, practically insoluble in water at acidic pH. It is stable at high temperatures and in acids, but unstable in alkaline conditions and  its color will change to red . In storage alkaline conditions it loses its hue .

Adicionalmente, en condiciones alcalinas, la estabilidad del pigmento disminuye rápidamente respecto a condiciones alcalinas

La contribución relativa de los tautómeros ceto – enol, así, como su forma cis o trans dependerá de factores como las características del disolvente, la temperatura, la polaridad y el grado de sustitución de la curcumina (Khopde, et al, 2000).

Cuando la curcumina interactúa con proteínas, tiene un buen comportamiento en el sistema.

Existe una fuerte correlación entre la deslocalización π del sistema y el fortalecimiento del enlace intramolecular  de Hidrógeno. Además, en solución, la cúrcuma puede formar enlaces intermoleculares de Hidrógeno con las moléculas del disolvente y este influye mucho en sus propiedades fisicoquímicas, tanto en el estado fundamental como excitado

Curcumin is a natural product  and exhibits keto-enol tautomerism with keto form is predominant in acidic  and stable enol form in alkaline medium. The color  is not restored  as it is pH dependent.

Dear Sephideh

Thanks for sharing your research activities in "Researchgate". The orange-yellow crystalline powder curcumin as the most active component of turmeric is insoluble in water while it is soluble in cooking oil or fat. Since curcuminoids compounds having specific anticancer activities loading and converting the above compound with useful carrier may help us to use it as medicine. The instability of curcumin in alkaline condition may be due to polyphenolic chemical structure and ketone functional group as the pH changes you can observe strong red color.

Hello everyone, what do you think about this? https://www.researchgate.net/post/Why_does_curcumin_changes_its_color_when_producing_a_thin_lipid_film_liposome_production 

I would be glad if you can answer my question there.