Maillard reaction, heterocyclic compounds

Maillard reaction. We are sure, however, they do not represent artefacts, formed upon work up or during the analytical procedure. Several of the heterocycles have been described earlier as volatile constituents of mother s milk (Pellizari et al. 1982 cf. Figure 31.2 compounds 5, 6, 9, 13, 17, 19 among other unknown oxygen con-... 

The formation of oxygen-containing heterocyclic compounds is also a consequence of the Maillard reaction. Amines and amino acids have a catalytic effect upon the formation of 2-furaldehyde (5), 5-(hydroxy-methyl)-2-furaldehyde (11),2-(2-hydroxyacetyl)furan (44),2 and 4-hy-droxy-5-methyl-3(2//)-furanone (111) (see Ref. 214). This catalytic effect can be observed with several other non-nitrogenous products, including maltol. The amino acid or amine catalysis has been attributed to the transient formation of enamines or immonium ions, or the 1,2-2,3 eno-lization of carbohydrates. Of interest is the detection of A -(2-furoyl-... 

Heterocyclic compounds in Maillard reaction, 36-54 acetoin-aldehyde-nammonium sulfide, 37,48... 

G. Vernin, Mechanism of Formation of Heterocyclic Compounds in Maillard and Pyrolysis Reactions, Chem. Heterocycl. Compd. Flavours Aromas 1982, 151-207. 

Hydrogen sulphide is a key intermediate in the formation of many heterocyclic sulphur compounds. It is produced from cysteine by hydrolysis or by Strecker degradation ammonia, acetaldehyde and mercaptoacetaldehyde are also formed (Scheme 12.4). All of these are reactive compounds, providing an important source of reactants for a wide range of flavour compounds. Scheme 12.6 summarises the reactions between hydrogen sulphide and other simple intermediates formed in other parts of the Maillard reaction. 

Scheme 12.6 The formation of heterocyclic aroma compounds from the reaction of hydrogen sulphide with intermediates of the Maillard reaction...

Many desirable meat flavor volatiles are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These latter constituents interact to form intermediates which are converted to meat flavor compounds by oxidation, decarboxylation, condensation and cyclization. 0-, N-, and S-heterocyclics including furans, furanones, pyrazines, thiophenes, thiazoles, thiazolines and cyclic polysulfides contribute significantly to the overall desirable aroma impression of meat. The Maillard reaction, including formation of Strecker aldehydes, hydrogen sulfide and ammonia, is important in the mechanism of formation of these compounds. 

Meat aroma is not the result of one chemical constituent but the sum of the sensory effects of many of these volatiles. Over 90% of the volume of volatile constituents from freshly roasted beef is from lipid, but approximately 40 percent of the volatiles from the aqueous fraction are thought to be heterocyclic compounds, many resulting from Maillard reaction products or their interactions with other ingredients. 

Figure 2.10. Strecker degradation of amino acids and a-dicarbonyls to form heterocyclic compounds. For glycine, R = H. Reprinted with permission from Wong, J. W., and Shib-amoto, T. (1996). Geotoxicity of the Maillard reaction products. In The Maillard Reaction. Consequences for the Chemical and Life Sciences, Ikan, R., ed., John Wiley Sons, Chichester, UK, 129-159.

The majority of heterocyclic compounds are formed through thermal interactions of reducing sugars and amino acids, known as Maillard reactions. Other thermal reactions such as hydrolytic and pyrolytic degradation of food components (e.g. sugars, amino acids and vitamins) and the oxidation of lipids also contribute to the formation of heterocyclic compounds responsible for the complex flavor of many foodstuffs. 

Recent studies in our laboratory show that lipids may be directly associated with the Maillard reaction in the formation of some heterocyclic compounds. The effect of lipids on the formation of heterocyclic compounds in a model Maillard reaction has also been reported by Mottram and Whitfield (2). 

This paper discusses model studies which indicate that lipid decomposition products such as 2,4-decadienal and hexanal may react with Maillard reaction intermediates to form heterocyclic compounds... 

The Maillard reaction has received much attention since the 1950 s as the source of flavor chemicals in cooked foods. Numerous compounds produced by this reaction have been reported in the last two decades. The major flavor chemicals are nitrogen- and sulfur-containing heterocyclic compounds. For example, nitrogen-containing pyrazines contribute a characteristic roasted or toasted flavor to cooked foods. Sulfur-containing thiophenes and thiazoles give a characteristic cooked meat flavor. A striking property of these compounds is their extremely low odor thresholds. 

Pyrrole may have been the first individual heterocyclic compound isolated from the Maillard reaction systems. Some pyrroles, such as 1-acetonylpyrrole... 

Figure 1. Number of heterocyclic compounds reported in the Maillard reaction mixtures.

Formation mechanisms of imidazoles in the Maillard reaction are not as well understood as those of other heterocyclic compounds. The role of a-amino carbonyl fragments as intermediates in imidazole formation was suggested in the reaction of sucrose and ammonia (43). In a study of a L-rhamnose/ammonia model system, which produced fifty-two imidazoles, it was proposed that an amino-hydroxy fragment was responsible for imidazole... 

Thermal aromas result from the Maillard reaction. By heating carbohydrates with amino acids degradation is accelerated yielding reactive compounds which, by new reactions with amino acids, are converted to heterocyclic products. Results of model investigations of glucose or its degradation compounds with the amino acids serine and phenylalanine are discussed. 

N, S and 0 heterocyclic compounds, along with noncyclic sulfur compounds and hydrocarbons, are predominant in "meaty" flavor volatiles. The mechanisms of heterocyclic formation by Maillard and pyrolysis reactions have been reviewed by Vemin and Parkanyi (57) and the Maillard reaction itself is a recurring subject of review (58). Since other speakers contributing to this volume will discuss these aspects of meat flavor, they will not be repeated in this presentation. 

Heterocyclic aroma compounds found in meat primarily arise from interactions between mono- and dicarbonyl compounds, H2S and ammonia. The carbonyl compounds are derived from the Maillard reaction, including Strecker degradation of amino acids, oxidation of lipids and aldolization reactions. H2S is produced by thermal degradation of sulfur amino acids and ammonia by amino acid pyrolysis. 

In heated foods the main reactions by which flavors are formed are the Maillard reaction and the thermal degradation of lipids. These reactions follow complex pathways and produce reactive intermediates, both volatile and non-volatile. It has been demonstrated that lipids, in particular structural phospholipids, are essential for the characteristic flavor development in cooked meat and that the interaction of lipids with products of the Maillard reaction is an important route to flavor. When model systems containing amino acids and ribose were heated in aqueous buffer, the addition of phospholipids had a significant effect on the aroma and on the volatile products. In addition a number of heterocyclic compounds derived from lipid - Maillard interactions were found. The extent of the interaction depends on the lipid structure, with phospholipids reacting much more readily than triglycerides. 

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