Ammonia melanoidin

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. 

Caramel in this context means a brown colour that is produced either traditionally by heating sugar or as a very intense product that is made by heating carbohydrate, usually glucose syrup, with ammonia. Caramel colour is the product of the Maillard reaction, i.e. the reaction of a reducing sugar with an amino group. Chemically the colour is a melanoidin - these substances are extremely stable and can be used in any type of confectionery. 

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. 

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. 

The properties of the melanoidin complexes depend upon the nature of the amino compound and sugar from which they are prepared. They have characteristic flavours no doubt due, in part, to the relevant aldehydes. For example, the melanoidin from ammonia has been described as bitter [24], that from glycine reminiscent of bread crust [24] or beers [25] and that from... 

The importance of Strecker degradation is mainly in the formation of reactive and often sensory active aldehydes and ammonia (free or bound in reactive a-amino carbonyl compounds). Strecker aldehydes, together with a-amino carbonyl compounds, contribute significantly to the formation of many heterocyclic compounds (which are important flavour-active components of processed foods) and the brown pigments melanoidins. Aldoses simultaneously produce the corresponding 2-deoxyaldoses or 1-amino-1-deoxyalditols and 2-oxoacids. 

The possibility that carbohydrate-ammonia reactions may participate in non-biological processes of humus formation under natural soil conditions received much attention during the period 1950-1960. Unlike the Mai Hard or "browning reaction, in which melanoidin pigments are formed in mixtures of sugars ana ammo acias in acidic media the ammonia reaction was... 

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