Thermal degradation kinetics of chlorophyll pigments in virgin olive oils. 1. Compounds of series a.

Virgin olive oils (VOO) collected at three maturation stages were thermodegraded to determine the degradation kinetics of series a chlorophyll pigments. The proposed degradation mechanism involves reactions that alter the structure of the isocyclic ring of pheophytin, originating intermediary products such as pyropheophytin, 13(2)-OH-pheophytin, and 15(1)-OH-lactone-pheophytin, and reactions that affect the porphyrin ring, producing colorless compounds. The marked effect of temperature has been pointed out in these competitive processes with the formation of pyropheophytin and the significantly higher value of its kinetic constant. No significant effect of the oily medium on the reaction mechanisms of pyropheophytin and 15(1)-OH-lactone-pheophytin has been found, comparing kinetic and thermodynamic parameters determined in the three VOO matrices of different pigment contents (high, medium, and low). The reaction mechanism of 13(2)-OH-pheophytin, by contrast, was affected by the medium; the reaction rate was the same for all of the matrices only at the isokinetic temperature (51 degrees C).