Unravelling discolouration caused by iron-flavonoid interactions: Complexation, oxidation, and formation of networks

Iron-flavonoid interactions in iron-fortified foods lead to undesirable discolouration. This study aimed to investigate iron-mediated complexation, oxidation, and resulting discolouration of flavonoids by spectrophotometric and mass spectrometric techniques. At pH 6.5, iron complexation to the 3-4 or 4-5 site instantly resulted in bathochromic shifting of the π → π* transition bands, and complexation to the 3'-4' site (i.e. catechol moiety) induced a π → d_π transition band. Over time, iron-mediated oxidative degradation and coupling reactions led to the formation of hydroxybenzoic acid derivatives and dehydrodimers, respectively resulting in a decrease or increase in discolouration. Additionally, we employed XRD, SEM, and TEM to reveal the formation of insoluble black metal-phenolic networks (MPNs). This integrated study on iron-mediated complexation and oxidation of flavonoids showed that the presence of the C2-C3 double bond in combination with the catechol moiety and either the 4-carbonyl or 3-hydroxyl increased the intensity of discolouration, extent of oxidation, and formation of MPNs.