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 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

Revealing the main factors and two-way interactions contributing to food discolouration caused by iron-catechol complexation

Fortification of food with iron is considered to be an effective approach to counter the global health problem caused by iron deficiency. However, reactivity of iron with the catechol moiety of food phenolics leads to discolouration and impairs bioavailability. In this study, we investigated the interplay between intrinsic and extrinsic factors on food discolouration caused by iron-catechol complexation. To this end, a three-level fractional factorial design was implemented. Absorbance spectra were analysed using statistical methods, including PCA, HCA, and ANOVA. Furthermore, a direct link between absorbance spectra and stoichiometry of the iron-catechol complexes was confirmed by ESI-Q-TOF-MS. All statistical methods confirm that the main effects affecting discolouration were type of iron salt, pH, and temperature. Additionally, several two-way interactions, such as type of iron salt × pH, pH × temperature, and type of iron salt × concentration significantly affected iron-catechol complexation. Our findings provide insight into iron-phenolic complexation-mediated discolouration, and facilitate the design of iron-fortified foods.</p

Design and characterization of Ca-Fe(III) pyrophosphate salts with tunable pH-dependent solubility for dual-fortification of foods

Food-fortification using poorly water-soluble mineral-containing compounds is a common approach to deliver iron. However, it comes with the challenge of ensuring iron bio-accessibility and limiting iron-phenolic interactions that can change organoleptic properties. Mixed Ca-Fe(III) pyrophosphate salts with the general formula Ca2(1-x)Fe4x(P2O7)(1+2x) were designed as a system for simultaneous delivery of iron and calcium. The salts were synthesized via a co-precipitation method and characterized by TEM-EDX, XRD, and FT-IR. All mixed salts with 0.14 ≤ x ≤ 0.35 yielded homogenous amorphous particles. The iron dissolution from these mixed salts showed a fourfold increase at gastric pH compared to Fe(III) pyrophosphate. In the food-relevant pH range, the salts with x ≤ 0.15 showed up to an eight-fold decrease in iron solubility. Despite this, reactivity of the mixed salts in tea was similar to that of FePP. Our results indicate that these mixed salts are potential dual-fortificants with tunable iron content and solubility

Legume and cereal defence metabolites as lead compounds for novel antimicrobials : production, analysis, and structural modification

Secondary metabolites from plants include those that function as defence compounds, which often possess antimicrobial activity. These metabolites can be valuable for the food and pharmaceutical industries, as natural food preservatives and leads for new antimicrobial compounds, respectively. The main aim of this research was to explore the antimicrobial potential of defence metabolites from peanut and cereals. Prenylated stilbenoids can be produced by germination of peanuts with simultaneous fungal elicitation. We demonstrated that some of these compounds possess good antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA). The results also indicated that dimeric prenylated stilbenoids could be even more potent than their monomeric precursors. A systematic approach to the mass spectrometric characterisation of defence metabolites in wheat and oat led to a more complete overview of the chemical diversity in these species. This facilitates more accurate quantification of the total content of benzoxazinoids and avenanthramides in wheat and oat, respectively. Contrary to peanuts and other legumes, fungal elicitation did not enhance the quantity or diversity of metabolites formed in cereals during germination. A literature review revealed that monomeric natural benzoxazinoids lack antimicrobial potency. Synthetic derivatives with a 1,4-benzoxazin-3-one backbone were much more potent than natural analogues. QSAR studies on the antimicrobial activity of 1,4-benzoxazin-3-ones and in silico drug design showed the potential of this backbone for the development of novel antimicrobial drugs.</p

Structure and biosynthesis of benzoxazinoids : Plant defence metabolites with potential as antimicrobial scaffolds

Benzoxazinoids, comprising the classes of benzoxazinones and benzoxazolinones, are a set of specialised metabolites produced by the plant family Poaceae (formerly Gramineae), and some dicots. The family Poaceae in particular contains several important crops like maize and wheat. Benzoxazinoids play a role in allelopathy and as defence compounds against (micro)biological threats. The effectivity of benzoxazinones in these functionalities is largely imposed by the subclasses (determined by N substituent). In this review, we provide an overview of all currently known natural benzoxazinoids and a summary of the current state of knowledge of their biosynthesis. We also evaluated their antimicrobial activity based on minimum inhibitory concentration (MIC) values reported in literature. Monomeric natural benzoxazinoids seem to lack potency as antimicrobial agents. The 1,4-benzoxazin-3-one backbone, however, has been shown to be a potential scaffold for designing new antimicrobial compounds. This has been demonstrated by a number of studies that report potent activity of synthetic derivatives of 1,4-benzoxazin-3-one, which possess MIC values down to 6.25 μg mL−1 against pathogenic fungi (e.g. C. albicans) and 16 μg mL−1 against bacteria (e.g. S. aureus and E. coli). Observations on the structural requirements for allelopathy, insecticidal, and antimicrobial activity suggest that they are not necessarily conferred by similar mechanisms.</p

Tea phenolics as prebiotics

Background: Due to the relatively low bioavailability of tea phenolics in the small intestine, their reciprocal interaction with gut microbiota in the colon may contribute largely to their beneficial health effects. This implies that tea phenolics may be considered as prebiotics. Scope and approach: This review summarizes the current knowledge on the metabolic fates of phenolics from green and black tea, the health benefits of tea phenolics and their microbial metabolites, and the potential underlying mechanisms. Additionally, the prebiotic effects of tea phenolics and conventional oligosaccharide prebiotics are compared. Key findings and conclusions: Phenolics from green tea are promptly metabolised into a series of (hydroxylated) phenylcarboxylic acids by gut microbiota, whereas lower degradation rates and metabolite yields are documented for black tea phenolics. Despite these differences, phenolics from green and black tea exhibit comparable gut microbiota modulatory effects. Moreover, intact green and black tea phenolics and their microbial metabolites provide various health benefits upon consumption, most likely due to their anti-oxidation, anti-inflammatory, gut barrier protection, and bile acid metabolism regulatory effects. Overall, the health benefits conferred by tea phenolics via modulation of gut microbiota composition and via formation of health-promoting metabolites is in many ways analogous to the prebiotic action of conventional oligosaccharides prebiotics. Therefore, we conclude that phenolics from green and black tea have the potential to be considered as prebiotics. Gaining better insights in the prebiotic effects of phenolics from green and black tea may pave the way for their high-value application in food and pharmaceutical industries

Facile Amidation of Non-Protected Hydroxycinnamic Acids for the Synthesis of Natural Phenol Amides

Phenol amides are bioactive compounds naturally present in many plants. This class of compounds is known for antioxidant, anti-inflammatory, and anticancer activities. To better under-stand the reactivity and structure–bioactivity relationships of phenol amides, a large set of structurally diverse pure compounds are needed, however purification from plants is inefficient and labo-rious. Existing syntheses require multiple steps, including protection of functional groups and are generally overly complicated and only suitable for specific combinations of hydroxycinnamic acid and amine. Thus, to facilitate further studies on these promising compounds, we aimed to develop a facile general synthetic route to obtain phenol amides with a wide structural diversity. The result is a protocol for straightforward one-pot synthesis of phenol amides at room temperature within 25 h using equimolar amounts of N,N′-dicyclohexylcarbodiimide (DCC), amine, hydroxycinnamic acid, and sodium bicarbonate in aqueous acetone. Eight structurally diverse phenol amides were synthesized and fully chemically characterized. The facile synthetic route described in this work is suitable for a wide variety of biologically relevant phenol amides, consisting of different hy-droxycinnamic acid subunits (coumaric acid, ferulic acid, and sinapic acid) and amine subunits (ag-matine, anthranilic acid, putrescine, serotonin, tyramine, and tryptamine) with yields ranging between 14% and 24%

Structure and biosynthesis of benzoxazinoids : Plant defence metabolites with potential as antimicrobial scaffolds

Benzoxazinoids, comprising the classes of benzoxazinones and benzoxazolinones, are a set of specialised metabolites produced by the plant family Poaceae (formerly Gramineae), and some dicots. The family Poaceae in particular contains several important crops like maize and wheat. Benzoxazinoids play a role in allelopathy and as defence compounds against (micro)biological threats. The effectivity of benzoxazinones in these functionalities is largely imposed by the subclasses (determined by N substituent). In this review, we provide an overview of all currently known natural benzoxazinoids and a summary of the current state of knowledge of their biosynthesis. We also evaluated their antimicrobial activity based on minimum inhibitory concentration (MIC) values reported in literature. Monomeric natural benzoxazinoids seem to lack potency as antimicrobial agents. The 1,4-benzoxazin-3-one backbone, however, has been shown to be a potential scaffold for designing new antimicrobial compounds. This has been demonstrated by a number of studies that report potent activity of synthetic derivatives of 1,4-benzoxazin-3-one, which possess MIC values down to 6.25 μg mL−1 against pathogenic fungi (e.g. C. albicans) and 16 μg mL−1 against bacteria (e.g. S. aureus and E. coli). Observations on the structural requirements for allelopathy, insecticidal, and antimicrobial activity suggest that they are not necessarily conferred by similar mechanisms.</p

Fatty acids attached to all-trans-astaxanthin alter its cis-trans equilibrium, and consequently its stability, upon light-accelerated autoxidation

Fatty acid esterification, common in naturally occurring astaxanthin, has been suggested to influence both colour stability and degradation of all-trans-astaxanthin. Therefore, astaxanthin stability was studied as influenced by monoesterification and diesterification with palmitate. Increased esterification decelerated degradation of all-trans-astaxanthin (RP-UHPLC-PDA), whereas, it had no influence on colour loss over time (spectrophotometry). This difference might be explained by the observation that palmitate esterification influenced the cis-trans equilibrium. Free astaxanthin produced larger amounts of 9-cis isomer whereas monopalmitate esterification resulted in increased 13-cis isomerization. The molar ratios of 9-cis:13-cis after 60 min were 1:1.7 (free), 1:4.8 (monopalmitate) and 1:2.6 (dipalmitate). The formation of 9-cis astaxanthin, with its higher molar extinction coefficient than that of all-trans-astaxanthin, might compensate for colour loss induced by conjugated double bond cleavage. As such, it was concluded that spectrophotometry is not an accurate measure of the degradation of the all-trans-astaxanthin molecule.</p

Separation of flavonoid isomers by cyclic ion mobility mass spectrometry

Analytical techniques, such as liquid chromatography coupled to mass spectrometry (LC-MS) or nuclear magnetic resonance (NMR), are widely used for characterization of complex mixtures of (isomeric) proteins, carbohydrates, lipids, and phytochemicals in food. Food can contain isomers that are challenging to separate, but can possess different reactivity and bioactivity. Catechins are the main phenolic compounds in tea; they can be present as various stereoisomers, which differ in their chemical properties. Currently, there is a lack of fast and direct methods to monitor interconversion and individual reactivity of these epimers (e.g. epicatechin (EC) and catechin (C)). In this study, cyclic ion mobility mass spectrometry (cIMS-MS) was explored as a potential tool for the separation of catechin epimers. Formation of sodium and lithium adducts enhanced IMS separation of catechin epimers, compared to deprotonation and protonation. Baseline separation of the sodium adducts of catechin epimers was achieved. Moreover, we developed a fast method for the identification and semi-quantification of cIMS-MS separated catechin epimers. With this method, it is possible to semi-quantify the ratio between EC and C (1:5 to 5:1, within 50–1200 ng mL−1) in food samples, such as tea. Finally, the newly developed approach for cIMS-MS separation of flavonoids was demonstrated to be successful in separation of two sets of positional isomers (i.e. morin, tricetin, and quercetin; and kaempferol, fisetin, luteolin, and scutellarein). To conclude, we showed that both epimers and positional isomers of flavonoids can be separated using cIMS-MS, and established the potential of this method for challenging flavonoid separations