Modelling the reactions between free phenols, vitamin C, apple PPO and O2 during a thermal treatment.

International audienc

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, oxidatio

Apple polyphenoloxidase inactivation during heating in the presence of ascorbic acid and chlorogenic acid

Publication Inra prise en compte dans l'analyse bibliométrique des publications scientifiques mondiales sur les Fruits, les Légumes et la Pomme de terre. Période 2000-2012. http://prodinra.inra.fr/record/256699It was recently reported that during osmotic dehydration of ascorbic acid (AA)-treated apple cubes, losses in AA and phenolics could partly arise from enzymatic oxidation, provided polyphenoloxidase (PPO) was still active under the processing conditions. To determine the impact of dehydration temperatures on PPO action, as well as chemical and enzymatic oxidation reactions, apple PPO inactivation alone or with AA (1 mM) and/or chlorogenic acid (CG, 3 mM), as well as AA and CG levels evolution, during heating of the model solutions at 45 and 60 degrees C were investigated. At pH 3.8, PPO was still functional, keeping 61 and 4% residual activity after 2 h of heating at 45 and 60 degrees C, respectively. The combined treatment of heating and AA was more effective in reducing PPO activity, while incubation at 60 degrees C with AA and CG minimised the losses of PPO activity. CG remained stable during heating, even in the presence of AA which, in turn, was more affected by heating. Thus, during heating, provided PPO remained active with enough available O(2) in the model systems, CG oxidation and coupled oxidoreduction with AA could readily develop

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

Times Cited: 1The kinetics of degradation of chlorogenic acid (CG), (-) epicatechin (EPI), L-ascorbic acid (AA) and polyphenoloxidase (PPO) activity from Marie-Menard apple in pH 3.8 solutions at 20 and 50 degrees 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 degrees C were elaborated before modelling the interaction among reactants, by fitting the reaction curves to predictive model. Apple PPO was thermolabile, denaturing after 10 min at 70 degrees 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. (C) 2012 Elsevier Ltd. All rights reserved

Purification, characterization and subunits identification of the diol dehydratase of Lactobacillus collinoides

International audienceThe three genes pduCDE encoding the diol dehydratase of Lactobacillus collinoides, have been cloned for overexpression in the pQE30 vector. Although the three subunits of the protein were highly induced, no activity was detected in cell extracts. The enzyme was therefore purified to near homogeneity by ammonium sulfate precipitation and gel filtration chromatography. In fractions showing diol dehydratase activity, three main bands were present after SDS/PAGE with molecular masses of 63, 28 and 22 kDa, respectively. They were identified by mass spectrometry to correspond to the large, medium and small subunits of the dehydratase encoded by the pduC, pduD and pduE genes, respectively. The molecular mass of the native complex was estimated to 207 kDa in accordance with the calculated molecular masses deduced from the pduC, D, E genes (61, 24.7 and 19,1 kDa, respectively) and a α2β2γ2 composition. The Km for the three main substrates were 1.6 mm for 1,2‐propanediol, 5.5 mm for 1,2‐ethanediol and 8.3 mm for glycerol. The enzyme required the adenosylcobalamin coenzyme for catalytic activity and the Km for the cofactor was 8 µm. Inactivation of the enzyme was observed by both glycerol and cyanocobalamin. The optimal reaction conditions of the enzyme were pH 8.75 and 37 °C. Activity was inhibited by sodium and calcium ions and to a lesser extent by magnesium. A fourth band at 59 kDa copurified with the diol dehydratase and was identified as the propionaldehyde dehydrogenase enzyme, another protein involved in the 1,2‐propanediol metabolism pathway