Abstract

This research aimed to determine efficient aqueous conditions for extracting the anthocyanin dye from Thai dark purple glutinous rice (Oryza sativa L.; Luem Pua) and then quantitatively evaluate its potential application in silk dyeing. It showed that the highest anthocyanin content depended on the dye extraction method, extraction time, and extraction temperature. The results revealed that the ultrasound-assisted extraction under optimal conditions at 45 ^oC for 20 min gave three times more anthocyanin than the traditional method, which involved soaking in water overnight at room temperature without sonication. Thermal degradation of the anthocyanin dye in water can also be a problem, so this was assessed quantitatively with the dye in the flavylium ion form (pH of 2.8±0.1) at 50, 60, 70, and 80 ^oC. The degradation rate profiles closely matched the first-order kinetic model with a high correlation coefficient. Dye adsorption kinetics were investigated at different temperatures. An activation energy (E<inf>a</inf>) of 45.6 kJ mol⁻¹ was determined, indicating that a physisorption process was involved in the silk dyeing. Thermodynamic parameters for adsorption were also estimated, with the enthalpy change (∆H^o, -6.52 kJ mol⁻¹) and entropy change (∆S^o, -18.24 J mol⁻¹ K⁻¹) values being consistent with an exothermic and spontaneous process.