Thermal Stability of the Pigments Hydroxysafflor Yellow A, Safflor Yellow B, and Precarthamin from Safflower ( <i>Carthamus tinctorius</i> )

Abstract

ABSTRACT Three major yellow pigments were isolated from safflower petals ( Carthamus tinctorius ) and characterized as hydroxysafflor yellow A (1), safflor yellow B (2), and precarthamin (3) by 1 H‐NMR, 1 H‐ 1 H COSY, 13 C‐NMR, HMQC, and HMBC spectral analysis. Thermal degradation reactions of 3 yellow pigments 1 to 3 at temperatures of 70 to 90 °C were carried out at different pH levels within the range of 3.0 to 10.0 by UV‐vis spectral measurements. First‐order reaction kinetics was observed for the degradation of safflower yellow pigments (1 to 3) at pH 3.0 and 5.0. Activation energies of thermal degradation of 1‐3 at pH 5.0 were calculated as 17.0, 15.4, and 20.1 kcal/mol, respectively. At neutral and alkaline conditions, yellow pigments 1‐3 did not follow simple first‐order kinetics.