Nitrogen Maillard reactions

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-... 

Several other non-nitrogenous products have been identified as products of the Maillard reaction. These include butanol, butanone, butane-dione, and pentane-2,3-dione as well as dihydroxyacetone, glycer-aldehyde, and D-erythrose. Obviously, the same products are present after mild acidic or basic degradation of carbohydrates. Thus, the necessity of an amine or amino acid in the mechanism of their formation is uncertain. 

Many nitrogen- and sulfur-containing heterocycles have been identified in the aroma fractions of foods [214]. In roasted products (e.g., coffee) and heat-treated foods (e.g., baked bread or fried meat), these heterocycles are formed from reducing sugars and simple or sulfur-containing amino acids by means of Maillard reactions [215, 216]. Their odor threshold values are often extremely low and even minute amounts may significantly contribute to the aroma quality of many products [217, 218]. Therefore, N- and N,S-heterocyclic fragrance and flavor substances are produced in far smaller quantities than most of the products previously described. 

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. 

When promoted by sodium hydroxide, the Maillard reaction, as evidenced by a decrease in amino nitrogen, is reversible on addition of acid. However, if the reaction proceeds near neutrality in the presence of phosphate, this is not so.168... 

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. 

A starch-glycine mixture, heated at 290°C, was significantly different from the starch control in its composition of alkoxyphenols and imidazoles in the involatile phase, and pyrazine, pyridine, methylpyridine, and dimeth-ylpyrroles in the volatile phase (Umano and Shibamoto, 1984). The neurotoxin 4(5)-methylimidazole appeared in the vapor phase when ammonia, but not amino acids, was the nitrogen source (Tomasik et al., 1989b). A corn starch-sucrose combination inhibited the Maillard reaction (Lee and Woo, 1988). 

Decolorization and Finishing. Decolor-ization is sometimes required for certain applications, mostly as an aesthetic preference. It is always desirable to solve these issues upstream. For example, color can be minimized by choice of fermentation medium components and control of the sterilization cycle so as to lessen the Maillard reactions between nitrogen and sugars and carameliza-tion. Color can also be reduced by treatment with activated carbon, use of antioxidants, and by diafiltration with membranes. Carbon-impregnated filter pads can be used to combine polish filtration with a decolorization step. 

El Khadem, HASSAN, Synthesis of Nitrogen Heterocycles from Saccharide Derivatives, 25, 351-405 Ellis, G. P., The Maillard Reaction, 14, 63-134... 

The Mai Hard reaction involves attack of the nitrogen of the amino group on the carbon atom of the carbonyl, sometimes followed by removal of water to produce the Schiff base (17.) (Figure 7). Detailed coverage of the Maillard reaction is given elsewhere in this volume by Hodges (18), so only a few examples, particularly those with which the author has had some relation-... 

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