Maillard reaction reactive intermediates

Glomb MA and Monnier VM (1995) Mechanism of protein modification by glyoxal and glycolaldehyde, reactive intermediates of the Maillard reaction. J Biol Chem 270, 10017-10026. 

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. 

It may be concluded that the mechanism of the "early phase" of the Maillard reaction is not dramatically changed by the addition of phosphate. It is therefore clear that the phosphate did not act as a nucleophile in the reaction, giving a reactive intermediate, in the rate-limiting step of a typical phosphate-dependent mechanism, but acts as a basic catalyst during the Amadori rearrangement. 

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. 

Heterocyclic compounds are dominant among the aroma compounds produced in the Maillard reaction, and sulfur-containing heterocyclics have been shown to be particularly important in meat-like flavors. In a recent review, MacLeod (6) listed 78 compounds which have been reported in the literature as possessing meaty aromas seven are aliphatic sulfur compounds, the other 71 are heterocyclic of which 65 contain sulfur. The Strecker degradation of cysteine by dicarbonyls is an extremely important route for the formation of many heterocyclic sulfur compounds hydrogen sulfide and mercaptoacetaldehyde are formed by the decarboxylation and deamination of cysteine and provide reactive intermediates for interaction with other Maillard products. 

The volatiles from cooked meat contain large numbers of aliphatic compounds including aldehydes, alcohols, ketones, hydrocarbons and acids. These are derived from lipids by thermal degradation and oxidation (J7) and many may contribute to desirable flavor. In addition, the aldehydes, unsaturated alcohols and ketones produced in these reactions, as well as the parent unsaturated fatty acids, are reactive species and under cooking conditions could be expected to interact with intermediates of the Maillard reaction to produce other flavor compounds. 

Lipids play an important part in the development of aroma in cooked foods, such as meat, by providing a source of reactive intermediates which participate in the Maillard reaction. Phospholipids appear to be more important than triglycerides. The addition of phospholipid to aqueous amino acid + ribose mixtures leads to reductions in the concentrations of heterocyclic compounds formed in the Maillard reaction. This effect could be due to lipid oxidation products reacting with simple Maillard intermediates, such as hydrogen sulfide and ammonia, to give compounds not normally found in the Maillard reaction. The precise nature of the odoriferous products obtained from lipid - Maillard interactions is dictated by the lipid structure and may depend on the fatty acid composition and the nature of any polar group attached to the lipid. 

At the Intermediate Stage, the Maillard reaction involves the loss of substantial amounts of water. Therefore, in an excess of water, in dilute solution, at high aw values, browning is hindered by the dilution of the reagents. On the contrary, at low aw values, the concentrations of reactants will have increased, but they will have begun to lose mobility. It is not surprising therefore that the Maillard reaction possesses an aw for maximum reactivity. This was realised by Wolfram and Rooney as early as 1953.574... 

This reaction has many implications for foodstuffs. For example, aroma components possessing a carbonyl group become involatile and do not contribute anymore to the overall flavor. Other nucleophilic reactions include the cleavage of S-S bonds in proteins and addition to C=C bonds of a,(l-unsaturated carbonyl compounds. Control of nonenzymatic browning is based on this latter reaction (McWeeny et al., 1974). A key intermediate of the Maillard reaction, i.e., 3,4-deoxyhexulos-3-ene, is efficiently blocked by a fast reaction with sulfite, leading to formation of 3,4-dideoxy-4-sulfohexosulose, which is much less reactive and in which sulfite is irreversibly bound. 

Interest in the structure and reactivity of D-fiuctosamine and its derivatives is determined by their practical significance for the food industry and for control/ treatment of diabetes-related pathologies, but also by a notable structural flexibility of this carbohydrate derivative, which may provide for some models that are unique for the field of carbohydrate chemistry. As an important and a major class of intermediates in the Maillard reaction, the fiuctosamine structure serves as a precursor of a broad variety of both low- and high-molecular-weight Maillard reaction products, hundreds of which have so far been identified. Here we consider only the most immediate transformation steps of D-fiuctosamine, while some of the more important Maillard reaction products whose origin can be traced to the D-fructosamine intermediates will be considered in Section IV. 

Amadori products are only intermediates formed in the course of the Maillard reaction. In spite of their limited stability, these products can be used under certain conditions as an analytical indicator of the extent of the heat treatment of food. Unlike the acidic (pH <3) and alkaline (pH >8) sugar degradation reactions, the Amadori compounds are degraded to 1-, 3-, and 4-deoxydicarbonyl compounds (deoxyosones) in the pH range 4-7. As reactive a-dicarbonyl compounds, they yield many secondary products. Formulas 4.54. 57 summarize the degradation reactions starting with the Amadori compound. 

Norfuraneol (I in Table 5.18) is under discussion as the precursor of MFT. As proposed in Formula 5.12, the addition of hydrogen sulfide leads to 4-mercapto-5-methyl-3(2H)-furanone, which yields MFT after reduction, e. g., by reductones from the Maillard reaction, and water elimination. MFT can also be formed in meat by the hydrolysis of thiamine (Fig. 5.19). The postulated intermediate is the very reactive 5-hy droxy-3 -mercaptopentan- 2-one. 

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