Curcumin exists in both keto and enol forms, where the enol form is the dominant one. keto-enol tautomerism is mainly because of intramolecular hydrogen transfer. the degradation of curcumin is mainly pH-dependent, where degradation is slow in acidic conditions and faster in basic and neutral conditions. In physiologic pH, the degradation is very rapid (hydrolytic degradation). In pH

First, the CH2 hydrogens between the C=O groups are acidic and deprotonate under basic conditions to yield enolates as you already experimented. In order to go back to the keto form, it needs protonation which can occur by the absorption of CO2 from air, therefore carbonic acid formation. You also need to check on possible degradation products in time by HPLC.

Dear Cho Sung Been,

It is a under grad level Chemistry. Firstly, the color of curcumin do not related to the keto-enol tautomerism. Secondly, in both acid and base, enol content of a 1,3-diketone increases. The color on different pH was observed due to the phenolic OH present in the curcumin. In high pH, the proton of phenolic OH deprotonated to give phenoxo ion which give reddish-brown color itself due to extended resonance. Whether in low pH protonation hamper resonance and show its actual color.