Maillard reaction structure

Electron-spin resonance (e.s.r.) spectra with characteristic hyperfine structure have been recorded during the initial stages of the Maillard reaction between various sugar and amino compounds. The products responsible for the spectra appear to be IV, Af -disubstituted pyrazine radical cations. The pyrazine derivatives are assumed to be formed by the bimolecular condensation of two- and three-carbon enaminol compo-... 

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,... 

As part of extensive studies lasting over 30 years on the structures of chromophores involved in nonenzymatic browning reactions, two intensely orange, previously unknown, compounds have been identified (2R,8aR)-l and ZS, 8aR)-4-(2-furyl)-7-[(2-furyl)methylidene]-2-hydroxy-2//,7//,8a//-pyrano[2,3-3]pyran-3-one <1998CAR215>. Additional studies on the single Maillard reaction products of these compounds have also been reported <1998JFA3912>. 

Human perception of flavor occurs from the combined sensory responses elicited by the proteins, lipids, carbohydrates, and Maillard reaction products in the food. Proteins Chapters 6, 10, 11, 12) and their constituents and sugars Chapter 12) are the primary effects of taste, whereas the lipids Chapters 5, 9) and Maillard products Chapter 4) effect primarily the sense of smell (olfaction). Therefore, when studying a particular food or when designing a new food, it is important to understand the structure-activity relationship of all the variables in the food. To this end, several powerful multivariate statistical techniques have been developed such as factor analysis Chapter 6) and partial least squares regression analysis Chapter 7), to relate a set of independent or "causative" variables to a set of dependent or "effect" variables. Statistical results obtained via these methods are valuable, since they will permit the food... 

Figure 4. N,N -Disubstituted pyrazine cation radicals with assignments for hyperfine structures of the ESR spectra in the Maillard reaction mixtures.

Formation of novel free radical products at an early stage of the Maillard reaction was demonstrated by use of ESR spectrometry. Analyses of the hyperfine structures for various sugar-amino compound systems led to the conclusion that the radical products are N,N -disubstituted pyrazine cation radicals. These new pyrazine derivatives are assumed to be formed by bimolecular condensation of a two-carbon enaminol compound involving the amino reactant residue. The presence of such a two-carbon product in an early stage reaction mixture of sugar with amine was demonstrated by isolation and identification of glyoxal dialkylimine by use of TLC, GLC, NMR, MS and IR. 

Literature information about the sensory properties for nearly 450 Maillard reaction products has been compiled in a survey. It includes qualitative aroma and flavor descriptions as well as sensory threshold values in different media for the compounds, classified according to their chemical structure. 

A constant observation when the MRP were separated by various methods was that antioxidative effect was found in many different fractions. Both the dialysates and the retentates from dialysis were antioxidative to some extent. All the electrophoresis fractions exhibited some antioxidative effect. Attempts to separate the MRP by column chromatography on Sephadex G-50 have resulted in several fractions with some antioxidative effect, and so on. This indicates that several antioxidative products are formed by the Maillard reaction, possibly differing in molecular size and chemical structure, but perhaps with one single antioxidative functional group in common, such as a free radical function. However, it can not be excluded that the MRP contain a few entirely different antioxidants with different modes of action. Various mechanisms have also been suggested. Eichner (6) claimed MRP to inactivate the hydroperoxides formed by the lipid oxidation. There are also reports on the complex binding of metals by MRP (18, 19). 

A number of studies have detected Maillard reaction products (melanoidins) in refractory organic matter from natural environments—for example, from sediments from a west African upwelling (Zegouagh et al., 1999) and archeological plant remains (Evershed et al., 1997). Poirer et al. (2000,2002) and Quenea et al. (2006a) showed that the refractory organic matter isolated from different soils consists in part of cross-linked melanoidins poorly resolved by 13C NMR spectroscopy. The presence of amide structures in soil as elucidated by 15N NMR is ascribed to either preserved proteinacious structures or melanoidin-type macromolecules (Derenne and Largeau, 2001). 

The sensory properties of nearly 450 volatile Maillard reaction products and related compounds have been compiled (45). The review includes quantitative aroma and flavor descriptions, as veil as sensory threshold values for different media, classified according to chemical structure. 

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. 

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. 

Because of the importance of dicarbonyl compounds in the Maillard reaction, Meade and Gerrard58 are trying to elucidate their structure-activity relationships, using linear and cyclic dicarbonyls and RNase A, crosslinking being assessed by SDS PAGE. 

The fluorescence of some molecules, derived from crosslinks and related structures formed in Maillard reactions, are given in Table 3.4. 

The red pigment, obtained from the interaction of amino acids and dehydro-ascorbic acid and shown to have structure 31 (p. 54) by Kurata et al.,194 provides another model chromophore for Maillard reaction products. 

Clearly, Class III and IV caramels are close to melanoidins produced by the Maillard reaction, but detailed structures for the coloured components cannot be given for any of these commercial caramels. 

Burdon542 has surveyed the current hypotheses for the structure of humic substances and has concluded that the various products from chemical degradations and NMR data are all consistent with their being mixtures of plant and microbial materials and their microbial degradation products. The examination of soil carbohydrates, proteins, lipids, and aromatics supported this view the presence of colour, fluorescence, ESR signals, mellitic acid, and other features do not contradict it. Regarding the Maillard reaction, some free monosaccharides and the necessary amino species are present in soil, so it may proceed, but only to a small extent it is not a major process. However, in marine environments, the relative abundance of carbohydrates and proteins makes them more probable precursors of humic substances than lignin or polyphenols. 

S. J. Meade and J. A. Gerrard, The structure-activity relationships of dicarbonyl compounds and their role in the Maillard reaction, in G, 2002, 455-456. 

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