Melanoidins structure

Kato, H. Tsuchida, H. Estimation of melanoidin structure by hydrolysis and oxidation. Prog. Food Nutr. Sci. 1981, 5, 147-56. 

For many situations, a simple total anthocyanin determination is inappropriate because of interference from polymeric anthocyanins, anthocyanin degradation products, or melanoidins from browning reactions. In those cases, the approach has been to measure the absorbance at two different pH values. The differential method measures the absorbance at two pH valnes and rehes on structural transformations of the anthocyanin chromophore as a function of pH. Anthocyanins switch from a saturated bright red-bluish color at pH 1 to colorless at pH 4.5. Conversely, polymeric anthocyanins and others retain their color at pH 4.5. Thus, measurement of anthocyanin samples at pH 1 and 4.5 can remove the interference of other materials that may show absorbance at the A is-max-... 

An effort has also been made to determine the structure of products providing coloration in the Maillard reaction prior to melanoidin formation. The reaction between D-xylose and isopropylamine in dilute acetic acid produced 2-(2-furfurylidene)-4-hydroxy-5-methyl-3(2/f)-furanone (116). This highly chromophoric product can be produced by the combination of 2-furaldehyde and 4-hydroxy-5-methyl-3(2//)-furanone (111) in an aqueous solution containing isopropylammonium acetate. The reaction between o-xylose and glycine at pH 6, under reflux conditions, also pro-duces " 116. Other chromophoric analogs may be present, including 117,... 

Melanoidins (36, 48-50) are different from melanins, humins, and caramels, but similar to humus (37), according to Maillard (4, 1 1, 19-21). Kato and Tsuchida (51) studied the possible chemical structure of melanoidins. 

A number of studies have detected Maillard reaction products (melanoidins) in refractory organic matter from natural environments—for example, from sediments from a west African upwelling (Zegouagh et al., 1999) and archeological plant remains (Evershed et al., 1997). Poirer et al. (2000,2002) and Quenea et al. (2006a) showed that the refractory organic matter isolated from different soils consists in part of cross-linked melanoidins poorly resolved by 13C NMR spectroscopy. The presence of amide structures in soil as elucidated by 15N NMR is ascribed to either preserved proteinacious structures or melanoidin-type macromolecules (Derenne and Largeau, 2001). 

Aldehydes, particularly a,/Tunsaturatcd ones, react readily at low temperatures with amines to give polymeric high molecular mass, coloured products of unknown structure, called melanoidins. Heterocyclic ring systems, such as pyridines, pyrazines, pyrroles, and imidazoles, have been shown to be present. Melanoidins usually contain 3 4% nitrogen. 

Tressl et al7r J1 designated the linear polymers as Type I and the branched ones as Type II. In most melanoidins, they would represent domains (or substructures), unsubstituted pyrroles and Strecker aldehydes, for example, being integrated into the melanoidin backbone, giving a complex macromolecular structure overall. Tressl et aV1 consider the oligomerisation/polycondensation reactions described as the only experimentally established pathways by which simple Maillard products generated from hexoses and pentoses are easily and irreversibly converted into macromolecules. 

Structures 4.1 show some low-molecular-mass coloured compounds formed in model Maillard systems. The organic chemistry underlying colour in melanoidins was not clear until 1972, when a specific compound, 4a, was isolated by Severin and Kronig167 in 0.07% yield from a heated aqueous mixture of xylose and isopropylamine acetate. It is yellow with Alliax = 365 nm. When the amine was replaced by glycine or lysine, the yield dropped further. Similar results were obtained when ara-binose was substituted for xylose. Subsequent work led to the identification of related compounds (see 4b and 4c), in some of which the ring oxygen of the furfural is replaced by NR. 

Clearly, Class III and IV caramels are close to melanoidins produced by the Maillard reaction, but detailed structures for the coloured components cannot be given for any of these commercial caramels. 

Superoxide radicals are another factor in oxidative damage. They can be determined with nitrobluetetrazolium (NBT), which then forms the colourless formazan. When melanoidins scavenge the superoxide radicals, the colour of the NBT persists.490,491 The activity of a glucose-glycine melanoidin on superoxide radicals is equivalent to the effect of 16 units of superoxide dismutase. The effect of the HMM and LMM fractions of this melanoidin is almost the same. The reaction rate constant of the melanoidin was markedly higher than that of ascorbic acid. If this were due to the reductone structures embedded in the melanodin, it is difficult to explain why the reducing power of the melanoidins is only 0.7 that of ascorbic acid.490... 

The relationship between humic substances and melanoidins has been reviewed by Dean et al.,15 who provide an entry to the earlier literature. Both humic substances and melanoidins present enigmas, their structures still being largely undefined. This makes comparing and contrasting them doubly difficult. 

U. Lessig and W. Baltes, Model experiments on the Maillard reaction. VI. Structural studies on selected melanoidins, Z. Lebensm. Unters. Forsch., 1981, 173, 435 144 via Food Sci. Technol. Abstr., 1982, 14, 6A528. 

M. Lindenmeier, V. Faist, and T. Hofmann, Structural and functional characterization of pronyl-lysine, a novel protein modification in bread crust melanoidins showing in vitro antioxidative and Phase Eli enzyme modulating activity, J. Agric. Food Chem., 2002, 50, 6997-7006. 

By examination of the recent literature as reviewed by Tissot and Welte (1978), one gets the impression that the diagenesis of humic substances to form kerogen is well understood. In effect, many consider kerogen to be formed via condensation mechanisms in which aquatic plant substances are microbially degraded to form soluble monomers that condense to form humic polymers that eventually condense to form kerogen. The melanoidin pathway (sugar-amino acid condensation products) has been invoked by some to explain the structures formed (Nissenbaum and Kaplan, 1972 Hue and Durand, 1973, 1977 Welte, 1973 Nissenbaum, 1974 Stuermer et al. 1978 Tissot and Welte, 1978). 

A great deal of work has been devoted to recognition of the structure of melanoidins. A wide variety of methods has been employed for this purpose. First, melanoidin shows a hyperfine structure in the e.s.r. spectrum and that means that stable free-radicals are present in caramel. Amino acids and ammonia were also detected in nondialyzable melanoidin after acid hydrol-ysis, indicating that amides are present in melanoidin. 

Structural studies on melanoidins are in their infancy, but it does appear that they can contain intact carbohydrate fragments, particularly if oligosaccharides are involved. Pyrolysis yields a range of compounds containing aromatic heterocycles, which contribute to the odour of cooked food, with many having furan rings. ... 

Fig. 4.1 Reactions and structures involved in melanoidin formation (a) example of sugar rearrangement (b) typical units in the carbohydrate backbone of melanoidin (c) common initial condensation reaction between a sugar residue and an amino acid (after Rubinsztain et al. 1984).

Fig. 7.6 Melanins, melanoidins and caramels (a) proposed structure for eumalanin -repeating unit of large polymer of brown melanins (b) isosacchrosan proposed formula ...

The effect of spectral changes, and evidence of the consequent increase in conjugation and resonance structures during the formation of melanoidins is shown in the data presented by MacDougall and Granor (1998). They showed that as the sugar-amino acid mixtures were heated the maximum absorption peak (Amax) increased from around 200 nm at the start (due to the amino acid), after... 

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