Thermodynamics and kinetics of the Fenton reaction in foods
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A major cause of food oxidation is free-radical generation via the Fenton reaction. Calorimetric measurement of heat rate during the reaction between food antioxidants and H2O2 with added Fe(II), the catalyst in the Fenton reaction, is used to ascertain the reactions occurring and evaluate stoichiometric, thermodynamic and kinetic parameters for the reactions at 25 °C. The reaction of ascorbic acid with H2O2 at varying pH and concentrations of ascorbic acid, Fe(II), and H2O2 was used as a model system. Measurements were also made on antioxidants, fruit juices, fruit purees, tea, and coffee. Enthalpies of formation of ascorbate monoanion (-1124 ± 3 kJ/mol) and dehydroascorbate anion (-1069 ± 2 kJ/mol) are estimated from literature data and Thornton's Rule to give ΔrH = −325 kJ/mol for H2O2(aq) + C6H7O6 −(aq) = C6H5O6 −(aq) + 2H2O(l). The experimental results show an initial very rapid reaction of ascorbate monoanion to dehydroascorbate anion followed by an undefined series of reactions with variable stoichiometry (4 ± 2 H2O2/ascorbate) that are overall described by first order kinetics. An empirical linear relation between the maximum heat rate observed and ascorbate concentration was found, but no definitive relation was found between total heat and amount of ascorbate. However, the model directly and rapidly provides stoichiometric, thermodynamic and kinetic properties of the radical scavenging activity of food antioxidants and commercial food products. The total heat measured under specified conditions is related to the radical scavenging capacity of the sample; higher values correspond to higher capacity for radical scavenging and vice versa.