“The brown color is formed as a result of an enzymatic reaction, primarily driven by a group of enzymes called polyphenol oxidases (PPOs). These are endogenous native enzymes in plant tissues which are found all over the vegetal kingdom. PPOs have a special capacity of catalyzing the oxidation of phenols in the presence of oxygen. Thus, a rich source of compounds with phenolic groups in its structure (known as phenolic compounds), fruits and vegetables are a common attack target of these enzymes.
PPOs oxidation of phenolic compounds result in the formation of a molecule called quinone, that further undergoes through self-coupling reactions by forming a brown-color macromolecule (polymer). Moreover, quinones might also react with other compounds, such as amino acids, proteins, phenols, and sugars, by yielding organoleptic (e.g. color, flavor, and texture) and nutritional alterations that may be unappealing to consumers.”