| Abstract
| - The kinetics of the thermal degradation of the natural cis carotenoid bixin in a water/ethanol (8:2)solution was studied as a function of temperature (70−125 °C), using high-performance liquidchromatography. The curves for the decay of bixin and formation of products (e.g., di-cis and all-trans isomers and a C17 degradation compound) did not adjust well to a first-order rate law, but verygood fits were obtained using a biexponential model. This mathematical modeling gave the rateconstant values for the formation of the primary products from bixin, and the energy barrier for eachstep was obtained. The di-cis isomers were formed immediately (15 kcal/mol) together with the decayof bixin, followed by a slow consumption, indicating their role as reaction intermediates. In fact, thedi-cis isomers could easily revert to bixin (Ea ≈ 3 kcal/mol) or yield the primary C17 degradationproduct, with an energy barrier of 6.5 kcal/mol. In turn, 24 kcal/mol was necessary for the Bix →all-trans step, explaining its slower formation. Keywords: Bixin; annatto; Bixa orellana; heating; kinetics; degradation mechanism
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