Formation in Nonenzymic Browning

Colour formation is the primary characteristic of the Maillard reaction, yet even now our knowledge of the coloured moieties responsible for the coloration is only rudimentary. 

In many ways, colour, which is a sensation, should be assessed visually. On analogy with the treatment of odorants, Hofmann73 has taken a crucial step in this area by defining a colour dilution factor (CD) and a colour activity value (CAV). CD is the factor required for any solution of a colorant x to be diluted to its colour threshold. CAVX is the ratio of the concentration of x (jug kg-1) to its threshold concentration (jug kg-1)- The colour contribution of a component colorant to the colour of a mixture can then be defined as 

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. 

Even though 4a and its analogues are relatively simple compounds, they come to assume an importance which is quite disproportionate, when their origin is considered. Their two parts are derived differently the furfural resulting from 1,2-enolisa-tion of the Amadori compound and the furanone from 2,3-enolisation (see Chapter 2), the former being favoured by low pH and the latter by moderate pH. This means that for both components to be available for interaction, the reaction conditions, including pH, need to be carefully balanced. pH optimisation of the reaction will also be affected by the acid/base properties of the amino acid moiety of the Amadori compound. 

Structures 4.1 The structures of some low-molecular-mass coloured compounds formed in model Maillard systems 

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Oxygen plays an essential role in enzymic browning, but is not essential for nonenzymic browning. It may help in fact, for example, in the formation of... 

Figure 3-8 Reversible Formation of Glycosylamines in the Browning Reaction. Source From D.T. Hurst, Recent Development in the Study of Nonenzymic Browning and Its Inhibition by Sulpher Dioxide, BFMIRA Scientific and Technical Surveys, No. 75, Leatherhead, England, 1972.

Studies have been made of the influence of y-radiation on starch, and it was found that depolymerization takes place, and water-soluble dextrins are formed, but the aim of those studies was not connected with the preparation of syrups for caramelization. "However, nonenzymic browning can be induced by y-radiation. In such cases, the formation of melanoi-dins from sugars and amino acids competes with reaction of both types of reagent with hydroxyl radicals produced from water, the reaction medium. ... 

Rizzi G.P. (1974) Formation of V-alkyl-2-acylpyrroles and aliphatic aldimines in model nonenzymic browning reactions. J. Agric. Food Chem. 22, 279-82. 

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