Maillard reactions nitrogen compounds

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-... 

The reaction of dextrose with a nitrogen-containing compound, eg, amino acids or proteins, yields a series of intermediates which form pigments of varied molecular weight (Maillard reaction). The type of pigments produced is dependent on reaction conditions such as pH, temperature, and concentration of reactants. 

If a nitrogen source is added, or if residual nitrogen-containing compounds are present, classical non-enzymatic browning (MaiHard) along with caramel ization can occur. Although the Maillard reaction is not the major emphasis of this review, it is difficult not to include it in discussions of carbohydrates and their degradation products. 

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. 

Biologically, proteins have had most of the attention with regard to glycation, but, in principle, it has always been realised that any amino compound bearing at least one hydrogen atom on its nitrogen is able to participate in the Maillard reaction. 

The Maillard reaction plays an important role in flavor development, especially in meat and savory flavor (Buckholz, 1988). Products of the Maillard reaction are aldehydes, acids, sulfur compounds (e.g., hydrogen sulfide and methanethiol), nitrogen compounds (e.g., ammonia and amines), and heterocyclic compounds such as furans, pyrazines, pyrroles, pyridines, imidazoles, oxazoles, thiazoles, thiophenes, di- and trithiolanes, di- and trithianes, and furanthiols (Martins et al., 2001). Higher temperature results in production of more heterocyclic compounds, among which many have a roasty, toasty, or caramel-like aroma. 

Additional minor volatile compounds are found in both frying oils and in fried foods, including cis,trans- and trans,trans-2,4-decaderived from oxidized linoleate and 2,4-heptadienal derived from linolenate. The isomers of 2,4-decadienal impart a desirable fried food flavor in fried potatoes when present in small amounts, but excessive amounts of this aldehyde would be expected to cause undesirable rancid flavors. Furfural compounds may be derived from interactions between food sugars and proteins. Minor amounts of sulfur compounds and nitrogen-containing heterocyclic compounds (methyl pyrazine and 2,5-dimethylpyrazine in potatoes) may originate from Maillard reactions (Chapter 11). 

An example of a radical fragmentation reaction discussed in this volume is the metal ion catalyzed degradation of Amadori compounds. This reaction provides an alternate path to osones, which are key intermediates in the formation of nitrogen heterocycles via the Maillard reaction. 

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