Modelling the reactions between free phenols, vitamin C, apple polyphenoloxidase and oxygen during a thermal treatment

The kinetics of degradation of chlorogenic acid, epicatechin, L-ascorbic acid and ofpolyphenoloxidase activity from some apple in malate buffer (pH 3.8) solutions at 20 and 50 °Cwith oxygen were investigated to provide information on the impact of the presence ofchlorogenic acid, epicatechin and/or L-ascorbic acid on polyphenoloxidase thermostability butalso the effect of the heat treatment on their degradation both by enzymatic and nonenzymaticways. Stoechiokinetic reactions on the basis of experimental data and literature anddetermination of the kinetic constants at 20 and 50 °C were elaborated before modelling theinteraction among reactants, by fitting the reaction curves to predictive model. Applepolyphenoloxidase was thermolabile, as total denaturation occurred after 10 min at 70 °C andonly 3 min at 80 °C. Losses of polyphenoloxidase activity at 20 and 50 °C were favored by thepresence of epicatechin in model solutions, compared with chlorogenic acid, due to theformation of quinones of epicatechin that diminished polyphenoloxidase stability. Temperaturequickened both enzymatic phenol oxidation before polyphenoloxidase deteriorated and thewhole set of the chemical reactions, including the production of secondary oxidation productsand chlorogenic acid or epicatechin regeneration. The results obtained by simulation of themodel fittings to experimental data also confirmed that L-ascorbic acid in excess induced a fastregeneration of chlorogenic acid and epicatechin from quinones of chlorogenic acid andquinones of epicatechin formed enzymatically via redox chemical reactions.Keywords: modelling, reactions, thermal treatment, oxidation