Abstract
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 3ʹ-4ʹ site (i.e. catechol moiety) induced a π → dπ 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.
Highlights
Food fortified with iron can effectively reduce the global prevalence of iron deficiency (Allen, De Benoist, Dary, & Hurrell, 2006)
Two characteristic π → π* bands were observed in absorbance spectra of all flavonoids possessing a 4-keto group in conjugation with the C2–C3 double bond, namely the benzoyl (A-ring) and cinnamoyl (B-ring) bands with λmax values ranging from 260 to 290 nm and 320–380 nm, respec tively (Fig. 2B)
Upon adjusting the pH to 6.5, a bathochromic shift to wave lengths in the visible spectrum was observed (Fig. 2C-E). These findings indicate that deprotonation of the hydroxyl substituents is a prerequisite for complexation, which subsequently leads to fast discolouration
Summary
Food fortified with iron can effectively reduce the global prevalence of iron deficiency (Allen, De Benoist, Dary, & Hurrell, 2006). Flavonoids are secondary plant metabolites that are ubiquitous in vegetables, herbs, and fruits. These compounds affect food colour and flavour, and possess a broad range of biological activities. The class of flavonoids includes several subclasses that share the same flavan (2-phenylchromen-4-one) backbone but possess different structural features based on the degree of oxidation of the pyran C-ring (Fig. 1A). Flavonoids possess several structural moieties that can bind metal ions via coordinate bonds: (i) the 3-hydroxy-4-ketone moiety in the C-ring (3–4 site), (ii) 5-hydroxy-4-ke tone moiety (4–5 site), and (iii) the 3ʹ-4ʹ-dihydroxy moiety located in the B-ring (3ʹ-4ʹ site) (Fig. 1B) (Uivarosi, Munteanu, Sharma, & Singh Tuli, 2019).
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