Phenolic compounds limit or promote oxidative degradation of pectin related to iron-H₂O₂ ratio

During fleshy fruit consumption and processing, metallic cations, cell wall pectin and vacuolar phenolic components come into contact. This mixing is at the origin of new interactions and reactions catalysed by reactive oxygen species arising from fruit metabolism or the presence of trace transition metal cations. Such oxidative reactions may induce cell wall deconstruction through pectin degradation or structural change of phenolic compounds. The consequences of this oxidative degradation will affect the organoleptic properties of the fruit and its product as well as its processability. To better understand these complex oxidative reactions, model solutions simplifying the fruit matrix rich in phenolic compounds such as found in apple and grape, were used to study the impact of (+)-catechin, p-coumaric acid, cyanidin, calcium, iron and hydrogen peroxide on pectin molecular weight. Results showed that H2O2 and Fe2+ are both required to depolymerize pectin. The antioxidant action of phenolic compounds limiting pectin degradation was shown to depend on iron concentration. At low iron concentration, (+)-catechin was revealed to act as a pro-oxidant. The results are discussed with regard to possible mechanisms involving the redox potential of iron and H2O2, and interactions between pectin, phenolic compounds and iron.