Reductone, Maillard reaction

Fig. 12.—Carbonyl-amine reactions leading to Maillard reaction products and reductones (adapted from Ref. 51a).

The most practical method for preventing WOF in meat products is to add antioxidants prepared from natural precursors such as sugars and amino adds by heating them to produce constituents that not only act as antioxidants but serve to enhance meaty flavor as well. The resulting Maillard products have been known to have antioxidant activity in lipid systems (6-8). It is assumed that the antioxidative property of the Maillard reaction is assodated with the formation of low molecular weight reductones and high molecular weight melanoidins (6, 7, 9-13). 

As shown above, glycine is formed again in the course of the Maillard reaction, during the conversion of the ARP into the reductones. Eventually it may react with these reductones or other electrophiles. Therefore the rate of the decrease of the concentration of the amino acid is not similar to the rate of the reaction with the sugar (Figure 1). The rate of consumption of monosaccharide, however, is a good measure of the reaction rate. 

The chemical degradation of carbohydrates, particularly under acidic conditions, produces reductones, furan derivatives, pyruvaldehyde, and so on, which can condense, either among themselves or with amino compounds (Maillard reactions), to produce dark-colored, amorphous products, similar to humic substances. Pyruvaldehyde, which has been held to be an intermediate in Maillard reactions, has been identified in many soils. Such con-... 

Its formation from rhamnose heated with piperidine acetate in ethanol, under the same conditions that produced amino-hexose-reductones from glucose and other hexoses, was described as early as 1963 by Hodge et al., who confirmed the structure by IR and NMR data and proposed a formation pathway. The formation from Amadori intermediates was been reviewed by Vernin (1981). Numerous model systems have confirmed that it is one of the main Maillard-reaction products. For instance we will mention the formation from L-rhamnose and ethylamine (Kato et al., 1972) and from pentose/glycine or alanine, whose mechanism was proposed by Blank and Fay (1996) and Blank et al. (1998), from the intermediate Amadori compound, /V-(l-deoxy-D-pentos-l-yl)glycine. Furaneol is also formed by recombination of... 

The chemistry of the browning reaction has been reviewed periodically (1-7). The carbohydrate-amino acid browning reaction produces literally hundreds of reaction products. Despite the fact that the Maillard reaction has been investigated for many years, we cannot as yet identify all the reactant compounds. The first steps are, however, clearly established. The aldose or ketose reacts with amine to produce N-substituted glycosyl amine (Fig. 1). This rearranges, as illustrated, to produce a 1-amino-desoxy-2-ketosyl amine. If it is blocked, the overall reaction is blocked. This key compound or compounds can then continue to react (Fig. 2). The desoxy-ketose or amadori rearrangement product can dehydrate to produce furfural-like compounds or, through the loss of water, produce reductones. All of these compounds can react with one another or with other amine compounds to produce a wide variety of reaction products. 

Fig. 8.27. Maillard reaction involved in the non-enzymic oxidative browning of plant tissues, (a) Formation of an imine by an amino acid reacting with an aldose (Ri = H) or ketose (Ri H). (b) Enolization of the imine to enaminol, then to an Amadori (Ri = H) or Heyns (Ri H) intermediate, (c) Breaking of the preceding intermediates, with the appearance of a reductone in redox equilibrium with an a-dicarbonylated compound, responsible for the non-enzymic oxidation phenomenon...

As shown in Table III no radicals could be detected clearly demonstrating that the CROSSPY formation was still in the induction period. In order to check the influence of reductones on radical formation, this ftiermally pre-treated mixture was incubated in the presence of ascorbic acid at room temperature. Analysis of the mixture by EPR spectroscopy revealed that instanftmeously after reductone addition the radical cation was generated (Table III). To investigate the effectivity of carbohydrate-derived reductones in CROSSPY formation, in comparative experiments, acetylformoin as well as methylene reductinic acid, both well-known to be formed during thermal treatment of hexoses (19), were added to the thermally pre-treated mixture. Both the Maillard reaction products were found to rapidly induce radical formation, however, in somewhat lower effectivity when compared to ascorbic acid (Table III). 

It was also demonstrated that vanillin, in food items where its aroma is desired, plays an important role as an antioxidant. Finally, some of the Maillard reaction products, such as reductones (cf. 4.2.4.4), should be considered as naturally active antioxidants. 

In the course of the Maillard reaction, de-oxyosones and reductones, e. g., acetylformoin (cf. Ill, Formula 4.67), are formed. They cttn react to give enol and triketo compounds via an addition with disproportionation (Formula 4.86). Redox reactions of this type can explain the formation of products which are not possible according to the reactions described till now. In fact, it has recently been found that, for example, glucose 6-phosphate and fructose-1,6-diphosphate, which occur in baker s yeast and muscle, form 4-hydroxy-2,5-dimethyl-3(2H)-furanone to a large extent. Since the formation from hexoses (or hexose phosphates) is not explainable, reduction of the intermediate acetylformoin (Formula 4.87) must have occurred. As shown, this reduction can proceed through acetylformoin itself or other reductones, e. g., ascorbic acid. Such re-... 

Reduction of disulfides by reductones from the Maillard reaction... 

In the isolation of aroma substances, foods which owe their aroma to the Maillard reaction should not be exposed to ten eratures of more than 50 °C. At higher tenqteratures, odorants are additionally formed, i. e., thiols in the reduction of disulfides by reductones. Fats and oils contain volatile and non-volatile hydroperoxides which fragment even at tenqteratures around 40 °C. 

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. 

The chemical principle behind the antioxidant properties of Maillard reaction products is currently not well understood. It is assumed that these properties show both low molecular weight products and high molecular weight melanoidins. As the structure of melanoidins has not been clarified satisfactorily, it is difficult to explain the chemical nature of their antioxidant activity. The active structures are probably reductones and aminoreductones, bound in melanoidin molecules that reduce the products of autoxidation. One of a few identified reductone structures in real food melanoidins is 2,4-dihydroxy-2,5-dimethyl-l-(5-acetamino-5-methoxycarbonylpentyl)-3-oxo-2H-pyrrole bound by a peptide bond. This so-called pronyl-L-lysine (pyrrolinone... 

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