Melanoidins

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... 

Steinhart, H., Packert, A., Melanoidins in coffee. Separation and characterization by different chromatographic procedures, Colloq. Sci. Int. Cafe 15th(Vol.2), 593, 1993. (CA121 33720e)... 

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. 

Compiex Substances Formed from Breakdown Intermediates, e.g, Phenols + quinones + amino compounds —h Melanins, melanoidin, gelbstoffe... 

Oxidation—reduction reaction MELANOIDIN-TYPE POLYMERS... 

A mthenium-containing melanoidin (a condensation product of amino acids and carbohydrates) was found to photocatalyze hydrogen production from water under light irradiation with 1. < 320 nm, in the presence of EDTA as an electron donor and methyl viologen as an electron relay. The Ru-melanoidin photocatalyst is stable nnder prolonged irradiation times and, due to its insolubility in water, is suitable for... 

Serban, A., Nissenbaum, A. 2000. Light induced production of hydrogen from water by catalysis with ruthenium melanoidins. Int J Hydrogen Energy 25 733-737. 

The kinetics of the Amadori-compound formation from D-glucose and l-lysine, as well as melanoidin formation, has been examined. The so-called Amadori compounds are formed at an exponential rate, while showing pseudo-first-order disappearance of L-lysine in the presence of... 

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. 

Dreizen S, Gilley EJ, Mosny JJ and Spies TD (1957) Experimental observations on melanoidin formation in human carious teeth. J Dent Res 36, 233-236. 

However, coffee beverages are complex mixtures of several hundred chemicals that either occur naturally or are later induced in coffee by the roasting process in the form of nicotinic acid or melanoidins. 

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 Amadori compound may be degraded via either of two pathways, depending on pH, to a variety of active alcohol, carbonyl and dicarbonyl compounds and ultimately to brown-coloured polymers called melanoidins (Figure 2.31). Many of the intermediates are (off-) flavoured. The dicarbonyls can react with amino acids via the Strecker degradation pathway (Figure 2.32) to yield another family of highly flavoured compounds. 

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

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. 

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