Ascorbic acid Maillard reactions

The first, called the Maillard reaction,1 occurs between a carbonyl compound, which here is usually a reducing sugar, and an amine, which here is usually an amino acid, a peptide, or a protein. The second is caramelisation, a reaction where the sugars react on their own, but normally requires more drastic conditions. (Some discuss this under the heading of active aldehydes.) The third is ascorbic acid oxidation. The last, although it need not involve any enzyme at all, is nearest to enzymic browning, since it often does involve ascorbic acid oxidase, which, however, does not affect the phenols, which are the normal substrate in enzymic browning, but may involve other enzymes, e.g., laccase or peroxidase. 

Here, much attention will be given to the Maillard reaction, since one can consider caramelisation and ascorbic acid oxidation as special cases of it. Also, the Maillard reaction is the one of physiological significance. 

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. 

Although ascorbic acid and particularly dehydroascorbic acid can undergo Maillard-type reactions, the loss of vitamin C due to the Maillard reaction is rarely an issue. The involvement of ascorbic acid and related compounds in the Maillard reaction is considered in Chapter 11. 

At pH 7.0 and 37 °C, the degradation of ascorbic acid continues further, the main products being threose, glyceraldehyde, xylosone, and 3-deoxyxylosone.551 Threose is more reactive compared with an aldopentose or an aldohexose. At pH 7.0 and 37 °C, it has a half-life of about 3.5 d. It seems probable that threose is a major factor in Maillard reactions involving ascorbic acid. 

Sugar, ascorbic acid, amino acids, thiamine (de Ross, 1992 Ames and Hincelin, 1992 Guntert et al., 1992,1994 Yoo and Ho, 1997), and peptides (Ho et al., 1992 Izzo et al., 1992 de Kok and Rosing, 1994) are potential reactants of the Maillard reaction. They are present in most foods, so the Maillard reaction occurs commonly when these foods are cooked. The conditions of cooking determine the aroma of the cooked foods. For example, the major volatiles identified from water-boiled duck meat are the common... 

Enzymes known as polyphenol oxidases cause enzymatic browning. Other names of the enzyme include phenolases and tyrosinases. The enzymes catalyze the conversion of monophenols and diphenols to quinones. The quinones can undergo a series of non-enzymatic reactions to produce brown, gray and black colored pigments, collectively known as melanins (11). Maillard reactions, caramelizations and ascorbic acid oxidations can produce similar types of colored compounds (12). For some food processing... 

The synergism exhibited by the ternary mixture of a-tocopherol, ascorbic acid and phospholipids has been shown to be due to the stabilization of a-tocopherol, on the basis of ESR studies with methyl linolenate oxidized at 90°C to detect the free radicals of a-tocopherol and ascorbic acid. Evidence was obtained by this technique for the formation of nitroxide radicals (R-N-0 ) in the presence of phosphatidylserine or phosphatidylethanolamine or soybean lecithin and oxidized methyl linolenate. However, as pointed out earlier (Section C), the synergistic activity of this ternary mixture may be derived from antioxidant products formed from the phospholipids at elevated temperatures by the Maillard browning reaction (Chapter 11). 

Ortwerth, B. J., and Olesen, P. R., 1988a, Ascorbic acid-induced crosslinking of lens proteins Evidence supporting a Maillard reaction, Biochim. Biophys. Acta 956 10-22. 

For a Maillard reaction involving glucose the major dicarbonyl intermediate is 3-deoxy-D-erythro-hexose-ulose (also known as 3-deoxyglucosone). Ascorbic acid can also produce many dicarbonyl compounds (Taqui-Khan, 1967 Kurata et al., 1973 Kurata and Fujimaki, 1976 Martell, 1980) via the Maillard reaction including 3-deoxy-D-erythro-hexose-ulose (Hirsch et al., 1992). Ascorbic acid can also lead to 3-deoxy-D-glycero-pentose-2-ulose and D-glycero-pentose-2-ulose. Aldehydic products of the Maillard reaction are able to cross-link proteins. 

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). 

The major pathway leading to the formation of carbonyls is the Strecker degradation. This reaction occurs between dicarbonyls and free amino acids. The dicarbonyls involved have vicinal carbonyls (carbonyl groups separated by one double bond) or conjugated double bonds [41], While these carbonyls typically are intermediates in the Maillard reaction, they may also be normal constituents of the food (e.g., ascorbic acid), be end products of enzymatic browning (e.g., quinones), or be products of lipid oxidation[42]. 

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-... 

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