Modelling the reactions between free phenols, vitamin C, apple polyphenoloxidase and oxygen during a thermal treatment
Résumé
polyphenoloxidase activity from some apple in malate buffer (pH 3.8) solutions at 20 and 50 °C
with oxygen were investigated to provide information on the impact of the presence of
chlorogenic acid, epicatechin and/or L-ascorbic acid on polyphenoloxidase thermostability but
also the effect of the heat treatment on their degradation both by enzymatic and nonenzymatic
ways. Stoechiokinetic reactions on the basis of experimental data and literature and
determination of the kinetic constants at 20 and 50 °C were elaborated before modelling the
interaction among reactants, by fitting the reaction curves to predictive model. Apple
polyphenoloxidase was thermolabile, as total denaturation occurred after 10 min at 70 °C and
only 3 min at 80 °C. Losses of polyphenoloxidase activity at 20 and 50 °C were favored by the
presence of epicatechin in model solutions, compared with chlorogenic acid, due to the
formation of quinones of epicatechin that diminished polyphenoloxidase stability. Temperature
quickened both enzymatic phenol oxidation before polyphenoloxidase deteriorated and the
whole set of the chemical reactions, including the production of secondary oxidation products
and chlorogenic acid or epicatechin regeneration. The results obtained by simulation of the
model fittings to experimental data also confirmed that L-ascorbic acid in excess induced a fast
regeneration of chlorogenic acid and epicatechin from quinones of chlorogenic acid and
quinones of epicatechin formed enzymatically via redox chemical reactions.
Keywords: modelling, reactions, thermal treatment, oxidation