Reductone, formation

At the First International Bleaching Conference at Appleton, Wisconsin, in 1955, in the final panel discussion on bleaching problems, fourteen theories were put forward to account for brightness reversion. They are listed here for reference purposes (a) residual lignin (b) furfural (c) reductone formation from carbohydrates (d) residue resin (e) poor washing (f) pH (g) metallic catalysts (h) metallic resinates (i) diffusion (j) carbonyl groups (k) water impurities (1) microorganisms (m) low bleach residual and (n) UV radiation. 

It must be stressed again, that the reaction sequence given here proceeds quantitatively, without formation of free iodine, only if the concentration of triose reductone is less than 10 3M and if the pH is above 3. 

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). 

In the advanced stage, subsequently the ARP opens, via 1,2- or 1,3-enollsation, depending on the pH of the medium, leaving dicarbonyl-compounds, the so called reductones (10) after elimination of the glycine residue. These highly functionalized compounds are very reactive towards nucleophiles, like the amino acids, or may give rise to the formation of condensation products. 

The a-aminoketones are important for the formation of heterocyclic compounds via condensations with the reductones or with themselves. The aldehydes mentioned, and the heterocyclic compounds are the actual flavour components. 

Oxidation of the reductone functionality of ascorbic acid is certainly its single most important reaction and results in the formation of its most biologically important derivative, dehydroascorbic acid, 28. As chemistry and biochemistry of dehydroascorbic acid will be covered in a separate section of this volume, only a few of its reactions will be covered here. 

The formation of reductones and rearrangements of the carbon chain are possible side reactions which should be considered. 

Lipides, 238, 239, 241-243, 261 Lipidosis, 239, 242, 243 Lobry de Bruyn-Alberda van Ekenstein transformation, 63, 291 acid catalysis of, 79 aldolization in, 77 base catalysis of, 79-81 catalysis of, by metal ions, 81 dealdolization in, 77 dehydration reactions in, 73 enzyme-catalyzed, 66, 70 formation of reductones in, 79 of or-hydroxy aldehydes, 71 mechanism of, 84 of noncarbohydrate a-ketols, 71 non-enzymic, 66, 67, 83 in paper chromatography, 81 rearrangement of carbon chain, 79 scope of, 65 of steroids, 72 use of, for synthesis, 82 Lyxonic acid, 3-deoxy-D-, 300 Lyxose, D-, condensation of, with urea, 218... 

Quite similar results have been described for the action of ultrasound on D-fructose solutions. After two hours, an absorption maximum was observed at 283 mu, probably showing the formation of reductone, as mentioned by Phillips. An effect on the specific rotation was also noted, but this was much less than that observed during some irradiation studies. An x-ray investigation proved that at least 15% of the parent sugar is changed to other products, but isolation of the products has not yet been attempted. ... 

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... 

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...

A similar route has been suggested to account for the formation of glyoxylic acid (18, R = H) and methyl glyoxylate (18, R = Me) during the periodate oxidation of inositols and (9-methylinositols, respectively. Formation of these carboxy derivatives was depicted by sequences involving enolization to such reductones as 16, which are hydroxylated to 17 and then cleaved oxidatively. 

In acidic conditions, at pH 7 or below, it undergoes mainly 1,2-enolization with the formation of furfural (when pentoses are involved) or HMF (when hexoses are involved). In alkaline medium, at pH > 7, the degradation of the Amadori compound is thought to involve mainly 2,3-enolization, where reductones, such as 4-hydroxy-5-methyl-2,3-dihydrofuran-3-one, and a variety of fission products, including acetol, pyruvaldehyde, and diacetyl are formed. 

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