A stoichio-kinetic model for a DPPH∙ -ferulic acid reaction.

Estimates of the activity of antioxidants in the literature often appear inconsistent. In the specific case of the DPPH∙ test, the diversity of measurements may arise from variations in the protocols followed. This paper proposes an unbiased method which models the reduction mechanism. This method is applied to the reduction of DPPH∙ by ferulic acid. A scheme with eight coupled reactions is proposed and has been validated on different solvents and using a wide range of DPPH ̇, ferulic acid, and 5,5'-diferulic acid concentrations, and verified using data from the literature on ferulic acid activity. This modeling approach permits a correction to the bias of the 8th reaction (spontaneous reduction of DPPH ̇), because of its sensitivity to solvent, which in most cases is not taken into account. The best experimental strategy to determine the Efficient Concentration of ferulic acid to reduce 20% (EC20) and 50% (EC50) of DPPH∙ is then detailed in terms of initial DPPH∙ concentrations and the duration of the experiment.

Kinetic modelling of ascorbic and dehydroascorbic acids concentrations in a model solution at different temperatures and oxygen contents.

The degradation kinetics of vitamin C (ascorbic and dehydroascorbic acids, AA and DHA) were determined under controlled conditions of temperature (50-90 °C) and oxygen concentrations in the gas phase (10-30% mol/mol) using a specific reactor. The degradation of vitamin C in malate buffer (20 mM, pH 3.8), mimetic of an apple puree, was assessed by sampling at regular intervals and spectrophotometric quantification of AA and DHA levels at 243 nm. The results showed that AA degradation increased with temperature and oxygen concentration, while DHA exhibited the behaviour of an intermediate species, appearing then disappearing. A kinetic model was successfully developed to simulate the experimental data by two first order consecutive reactions. The first one represented AA degradation as a function of temperature and concentration in dissolved oxygen, and the second reflected DHA degradation as a function of temperature only, both adequately following Arrhenius' law.

Spectrophotometric method for fast quantification of ascorbic acid and dehydroascorbic acid in simple matrix for kinetics measurements.

A simple, rapid and reliable method was developed for quantifying ascorbic (AA) and dehydroascorbic (DHAA) acids and validated in 20mM malate buffer (pH 3.8). It consists in a spectrophotometric measurement of AA, either directly on the solution added with metaphosphoric acid or after reduction of DHAA into AA by dithiothreitol. This method was developed with real time measurement of reactions kinetics in bulk reactors in mind, and was checked in terms of linearity, limits of detection and quantification, fidelity and accuracy. The linearity was found satisfactory on the range of 0-6.95mM with limits of detection and quantification of 0.236mM and 0.467mM, respectively. The method was found acceptable in terms of fidelity and accuracy with a coefficient of variation for repeatability and reproducibility below 6% for AA and below 15% for DHAA, and with a recovery range of 97-102% for AA and 88-112% for DHAA.

Thermal degradation of folates under varying oxygen conditions.

Folate losses in thermally treated foods are mainly due to oxidation. Other mechanisms and folate vitamers behaviour are poorly described. Our study evaluated oxygen impact on total folate degradation and derivatives' evolution during thermal treatments. Spinach and green bean purees were heated, in an instrumented reactor, in anaerobic conditions, under an oxygen partial pressure of 40 kPa. Folates were stable in the absence of oxygen, whilst they were degraded under 40 kPa of oxygen. Total folate showed a sharp decrease in the first hour driven by the degradation of 5-CH3-H4folate, followed by a plateau due to the formyl derivatives and minor compounds stability. The different evolution of the main derivatives was confirmed by the degradation of 5-CH3-H4folate and folic acid in solution, under the same conditions of oxygen concentrations. The stability of folic acid and the high susceptibility of 5-CH3-H4folate to degradation in the presence of oxygen were confirmed.

Modelling the interactions between free phenols, L-ascorbic acid, apple polyphenoloxidase and oxygen during a thermal treatment.

The kinetics of degradation of chlorogenic acid (CG), (-) epicatechin (EPI), L-ascorbic acid (AA) and polyphenoloxidase (PPO) activity from Marie-Ménard apple in pH 3.8 solutions at 20 and 50°C were investigated to provide information on the impact of the presence of CG, EPI and/or AA on PPO thermostability. The effect of the heat treatment on their degradation by enzymatic and/or nonenzymatic ways was also studied. Stoechiokinetic reactions on the basis of experimental data and literature and determination of the kinetic constants (k) at 20 and 50°C were elaborated before modelling the interaction among reactants, by fitting the reaction curves to predictive model. Apple PPO was thermolabile, denaturing after 10min at 70°C. Losses of PPO activity were favoured by the presence of EPI in model solutions, compared with CG, due to the formation of o-quinones of EPI (QEPI) lowering PPO stability. Temperature quickened both enzymatic phenol oxidations before PPO deteriorated and the whole set of the chemical reactions, including the production of secondary oxidation products and CG or EPI regeneration. Results also confirmed that AA in excess induced a fast regeneration of CG and EPI from the corresponding o-quinones formed enzymatically via redox chemical reactions.