Ascorbic Maillard reaction

Phosphate, ascorbate, NaCl, nitrite, Maillard reaction products, and other antioxidants or prooxidants have been reported to influence development of WOF. Their roles have been reviewed (5), but newer evidence is now available and will be covered herein. 

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. 

The efficient browning of proteins by 3-DG and MGO under anaerobic conditions emphasises that oxidation is not essential for Maillard reactions in vivo, although oxygen and oxidation appear to be rate-limiting for browning and crosslinking of proteins by aldo- and ketohexoses, as well as ascorbate. 

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. 

R.H. Nagaraj and V.M. Monnier, Isolation and characterization of a blue fluorophore from human eye lens crystallins in vitro formation from Maillard reaction with ascorbate and ribose, Biochim. Biophys. Acta, 1992, 1116, 34-42. 

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... 

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-... 

In carbohydrate-containing foods, furan-2-carbaIdehyde is often present, arising from pentoses and ascorbic acid as a dehydration product. Other common furan-derived aldehydes are 5-hydroxymethyIfuran-2-carbaIdehyde (from hexoses) and 5-methyIfuran-2-carbaIdehyde (resulting from 6-deoxyhexoses). Other heterocyclic aldehydes derived from pyrrole, thiophene, pyridine, pyrazine and other heterocyclic compounds are formed as products of the Maillard reaction. 

In heat treated or stored food products several amino acids are not fully available because of derivatization or crosslinking reactions. Since 30 years furosine is known as a useful indicator of early Maillard reaction which is applied in food science, nutrition and medical biochemistry. Recently more sensitive analytical methods for furosine determination are available which have again increased the attractivity of this important indicator. Lately, N -carboxymethyllysine (CML) became available as another marker of special interest, because CML is a more useful indicator of the advanced heat damage by Maillard reaction than furosine. In addition, CML has the advantage to indicate reactions of lysine with ascorbic acid or ketoses such as fructose. Indicators for protein oxidation of sulfur amino acids are methionine sulfoxide and cysteic acid. An established marker for cross-linking reactions is lysinoalanine, which also indicates protein damages due to processing under alkaline conditions. Other markers formed as a consequence of alkaline treatment are D-amino acids. 

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