Melanoidin reaction

Other possible theories of stream humic substance formation are the browning reactions (the black substances produced by the action of acids on sugars) (Haworth, 1971) and the melanoidin reaction (the dark pigments produced when glucose, alanine or ammonia, and phenolic substances react with one another) (Hoering, 1973 Filip et al., 1974 Ertel and Hedges, 1983). These theories are discussed in detail by Stevenson in Chapter 2 of this book. 

The relationship of condensation products of sugars and amino acids to labile complexes of carbohydrates and amino acids and to the melanoidin reaction has stimulated the study of the simplest systems. The amino acids... 

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-... 

Monosaccharides are probably involved in the browning reactions that occur during the roasting of coffee. Caramelization involving the sugars alone, and Maillard reactions, between sugars and free amino acids, produce polymeric yellow to dark brown substances, known as melanoidins. These melanoidins can be extracted into hot water, separated and characterized.105... 

Oxidation—reduction reaction MELANOIDIN-TYPE POLYMERS... 

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,... 

The initial products of the reactions between sugars and proteins may enter a cascade of reactions yielding fluorescence, browning, and polymerization of proteins ("cross-linking"). The brown pigments, so-called melanoidins, are polymers whose composition has not yet been established completely. Melanoidins bind calcium and may thus interfere with de- and remineralization in caries. 

The most practical method for preventing WOF in meat products is to add antioxidants prepared from natural precursors such as sugars and amino adds by heating them to produce constituents that not only act as antioxidants but serve to enhance meaty flavor as well. The resulting Maillard products have been known to have antioxidant activity in lipid systems (6-8). It is assumed that the antioxidative property of the Maillard reaction is assodated with the formation of low molecular weight reductones and high molecular weight melanoidins (6, 7, 9-13). 

In summary, it may be stated that the reaction of acyl esters of aldoses and aldobioses (see Section III, p. 92) with ammonia consists of a set of competitive pathways, including intramolecular O —> N migrations of acyl groups, deacylations, and transesterifications, with formation of aldose amides and variable proportions of the free sugar as the principal products. Significant proportions of basic or insoluble polymeric substances were not observed with aldose acetates or benzoates, although occurrence of extensive browning indicates the probable formation of soluble melanoidins. 

On the other hand, the acyl esters of 2-ketoses follow a completely different pathway, as heterocyclic, nitrogenated compounds and melanoidins are the principal products of their reaction with ammonia. The free 2-ketoses show similar behavior, but they present a significant difference in the extent of formation of these compounds, the presence of the esterifying acyl groups in the molecule being decisive in increasing their yields. 

The formation of melanoidins in yields exceeding 20% (on the basis of the free sugar involved in the reaction) is a noteworthy feature of this reaction in the 2-ketose field. Direct reaction of L-sorbose with ammonia gave only 2-3% of melanoidins, showing the special influ-... 

The formation of melanoidins amounted to a 20-25% yield (based on free sugar), whereas prolonged reaction (eight days) of ammonia with D-fructose gave only a 2.5% yield of these polymers. 

The pigments of soy sauce, first studied by Kurono and Katsume (100), are melanoidins, which were reported by Omata, et al. (101) to be produced from the reaction of sugars and amino acids. Kato, e a l. (102) found 3-deoxyglucosone as an intermediate in browning of soy sauce (cf. 103). Oxidative browning (104,105) was again reviewed (44) with emphasis on the browning of Amadori compounds and interaction between melanoidins and iron. 

With the single exception of the stable free radical observed in melanoidin prepared from the glycine-glucose reaction ( 7 ) > there have been no reports of free radical formation in the early stages of the Maillard reaction. We have observed the... 

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). 

Absence of catalysis Pioneering work on condensation reactions between sugars and amino acids to form melanoidins in the absence of catalysts Maillard (1913)... 

Hedges, J. I. (1978). The formation of clay mineral reactions of melanoidins. Geochim. Cos-mochim. Acta 42, 69-79. 

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