Maillard melanoidin

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... 

L.C. Maillard and M.A. Gautier, Action des acides amines sur les sucres formation des melanoidines par voie methodique, Compt. Rend. Acad. Sci., 1912, 154, 66-68. 

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 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). 

The final polymerization products (melanoidins) are brown and hence dairy products which have undergone Maillard browning are discoloured and aesthetically unacceptable. 

The last report of Maillard (21) is rather a review in nature, with more than 50 references. The chemical natures not only of humus in soil but also of mineral fuel (coal) and browning in food material were discussed, especially in relation to the presence of nitrogen in browned products, which was inferred to be derived from amino acids (and related nitrogenous materials) used for synthetic "melanoidins". 

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. 

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)... 

The brown product or melanoidin which results from the Maillard reaction has been isolated from the products from various mixtures of reactants by numerous workers. A selection of these is given below. Maillard72, studied the interaction of D-glucose and other reducing sugars with glycine and several other amino acids at 34°, 40°, 100°, and 150°. 

Figure 1. Sugar-amine Maillard browning reactions Two pathways to melanoidins and byproducts. (Reprinted with permission from ref. 12. Copyright 1967 Avi Publishing.)...

It is worth noting that Mauron7 calls the three stages Early, Advanced, and Final Maillard reactions, respectively. The way these reactions fit together is outlined in Scheme 1.1. The final products of nonenzymic browning are called melanoidins to distinguish them from the melanins produced by enzymic browning. Theoretically, the distinction is clear however, in practice, it is very difficult to classify the dark-brown products formed in foods, since they tend to be very complex mixtures and are chemically relatively intractable. 

Scheme 2.3 Maillard reactions the two major pathways from Amadori compounds to melanoidins (based on Hodge213)...

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. 

Lipid oxidation products react with proteins and other amino compounds to form brown substances, similar to melanoidins. The formation of such brown substances was reviewed already at the first Maillard Symposium.150 The pigments formed are partly soluble in chloroform-methanol and partly insoluble, whereas true melanoidins are largely water-soluble. As most brown pigments of fish muscle are soluble in benzene-methanol and only to a lesser extent in water, the implication is that here oxidised lipid-protein interactions are more important than Maillard browning due to ribose-amino acid interactions. 

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. 

Much attention has recently been paid to arginine as a melanoidin precursor. In a model system, lV -acetylarginine was first allowed to react with glyoxal in aqueous solution at pH 7, followed by furfural. Repeated chromatography of the aqueous residue after ethyl acetate extraction led to a 1% yield of the deep red 29.180 Such a compound provides an example of a potentially coloured crosslink in proteins derived by the Maillard reaction (see Chapter 8). 

Casein heated in aqueous solution with furfural rapidly turns orange-brown. Melanoidins (> 10 kDa), isolated by ultracentrifugation, were enzymically hydrolysed and the product was separated by HPLC. Two peaks led to red compounds in the ratio 1 7, which were unequivocally identified by Hofmann191 as the lysine analogues of 24a and 24b (MM = 0.476 kDa), respectively. This is the first demonstration of the attachment of a specific type of coloured Maillard product to a protein... 

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. 

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