Because it is solubilized. At neutral or acidic pH is not soluble. Alkaline conditions cause the deprotonation of some of the functional groups making the molecule "more polar". It makes sense giving the pKa values (pKa1, pKa2, and pKa3 value of 7.8, 8.5, and 9.0, respectively, for three acidic protons, see the publication below).

The color is frequently derived from alternating single and multiple bonds. The color change in alkaline conditions could be a derived effect from the deprotonation or due to complexation (check the patent linked)

http://en.wikipedia.org/wiki/Conjugated_system#Conjugated_systems_in_pigments

http://www.google.com/patents/EP2249852A2?cl=en

Article Stabilization of Curcumin by Complexation with Divalent Cati...

Because it is solubilized. At neutral or acidic pH is not soluble. Alkaline conditions cause the deprotonation of some of the functional groups making the molecule "more polar". It makes sense giving the pKa values (pKa1, pKa2, and pKa3 value of 7.8, 8.5, and 9.0, respectively, for three acidic protons, see the publication below).

The color is frequently derived from alternating single and multiple bonds. The color change in alkaline conditions could be a derived effect from the deprotonation or due to complexation (check the patent linked)

http://en.wikipedia.org/wiki/Conjugated_system#Conjugated_systems_in_pigments

http://www.google.com/patents/EP2249852A2?cl=en

Article Stabilization of Curcumin by Complexation with Divalent Cati...

What about color changes of curcumin in acidic conditions and when it's entrapped in liposomes? https://www.researchgate.net/post/Why_does_curcumin_changes_its_color_when_producing_a_thin_lipid_film_liposome_production

The simple answer of why it turns red in alkaline is this.  Active ingredient of turmeric is curcumin - chemically a biphenol and   1,3 diketone. Curcumin has an extended conjugation, and as you know, two aromatic phenolic rings. This tautomerism, extended conjugation & aromatic phenolic rings combine to make curcumin/ Tumeric yellow. Phenols like curcumin, are chemically acidic but easily drop their acidic proton when in alkaline environment. The loss of proton at any of the phenolic sites converts the phenolate ion from benzenoid structure-- into a quinonoid one. When it switches into quinonoid  the extended conjugation is altered & the tautomerism alters. When a  benzenoid form goes into a quinonoid form, a bathochromic shift arises in the optical properties and VOILA:  the quinonoid form now appears with a longer wavelength than its benzenoid form--Red and not Yellow!. Intriguingly, breakdown of curcumin is pH-dependent with it degrading much slowly in acidic environments and quickly in neutral-basic conditions.  Curcumin degrades rapidly with hydrolytic degradation at physiological pH or greater -- a worrisome disadvantage in  therapeutic use. 

"Curcumin is more stable in acidic conditions probably owing to its conjugated diene structure which is gradually destroyed as the proton is removed during dissociation of phenolic groups in the curcumin structure (H2A−, HA2− and A3−) at higher pH, thus causing curcumin to be more prone to degradation as detected using HPLC . In the pH range of 8.08 to 8.75, it is postulated that curcumin exists in equilibrium in three forms: H3A, H2A− and HA2" 

Why is it that when you do a PZC study of curcumin between pH 4 and pH 8 the surface is negative throughout. Can someone please explain, I'm trying to find out if curcumin can acquire a positive charge below pH 7.