Ascorbic browning reaction

Another function of ascorbic add is to generate Fe(II) from Fe (III), which is part of the Haber-Weiss-Fenton Reaction (77). In that reaction, peroxide is reduced to the hydroxyl radical, HO, an extremely powerful oxidizing agent. It should also be noted that ascorbic acid can chelate metals 18 and will promote the carbohydrate-amine browning reaction (79-20). 

Amino acids are quickly deaminated by L-ascorbic acid, leading to browning reactions (66). In the presence of oxygen, iron, and ascorbic acid or DHA, the amino acids gave ammonia, carbon dioxide, and an aldehyde with one carbon less than the original acid (67,68). The aldehydes are isolated as dimedone derivatives and are useful for identification of the amino acids. In the presence of copper and UV light, the deamination is increased. The red color 69-73) formed upon reaction of DHA with amino acids was used for their detection. Recent studies (74-78) of the reaction of DHA with amino acids led to the isolation of a product that changes readily to a novel, stable, free radical species... 

Pyrroles are common products in the reaction of different lipid oxidation products with primary amino groups of amines, amino acid, and protein (Zamora et al., 2000). For example, when squid microsomes were oxidized with iron and ascorbate, TEARS increased simultaneously with the -value (yellowness) and pyrrole compounds, concomitantly with a decrease in free amines. Off-color formation in squid muscle could be due to the nonenzymatic browning reactions occurring between aldehydic lipid oxidation products and the amines on phospholipids head groups... 

Schulz, A. et al. Electrospray ionization mass spectrometric investigations of a-dicarbonyl compounds— Probing intermediates formed in the course of the nonenzymatic browning reaction of L-ascorbic acid, Int. J. Mass Spectrom., 262, 169, 2007. 

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). 

Lattanzio V, Linsalata V, Pahnieri S, Van Sumeie CF (1989) The beneficial effect of citric and ascorbic acid on the phenohc browning reaction in stored artichoke Cynara scolymus L.) heads. Food Chem 33 93-106... 

Kurata, T., and Fujimaki, M., 1976, Roles of L-xylosone in the browning reaction of dehydro-L-ascorbic acid, Agric. Biol. Chem. 40 1429-1430. 

The loss of vitamin C may also occur during reactions of ascorbic acid with some of the reactive food components. In particular, reactions of ascorbic acid with quinones generated by enzymatic browning reactions, reactions with nitrites and haem pigments in meat and meat products are technologically significant. 

A large number of furan derivatives are produced in non-enzymatic browning reactions as dehydration products of carbohydrates and related compounds, such as ascorbic acid (y-lactones can also be considered furan derivatives). Most furans are common to many thermally processed foods. Higher levels of these compounds are also present in acid protein hydrolysates. Furan-2-carbaldehyde has the characteristic pleasant woody aroma resembling nuts, which arises from pentoses and ascorbic acid, and 5-methylfuran-2-carbaldehyde, which is a product of 6-deoxyhexoses. A fatty herbal type flavour is provided by 5-hydroxymethylfuran-2-carbaldehyde arising from hexoses. 2-Acetylfuran arising from hexoses and pentoses is an important component of a number... 

Pyrans occurring in foods are hypothetically derived from a-pyran or y-pyran, or from a- or y-pyrones, respectively. 8-Lactones can also be considered derivatives of a-pyran. An important a-pyrone is 3-hydroxypyran-2-one, also known as 3-hydroxy-2-pyrone or 3-hydroxy-2ff-pyran-2-one, which is a 8-lactone of 2,5-dihydroxypenta-2,4-dienoic acid with a caramel-Hke flavour, which arises from ascorbic acid (see Section 5.14.6.2.2). Undoubtedly, the most important y-pyrone is 3-hydroxy-2-methylpyran-4-one, which is known as maltol (8-167), featuring a caramel (or malt) smell and taste. A large number of other y-pyrones are formed in non-enzymatic browning reactions, but as flavourings they are of Httle consequence. 

Sulfur dioxide (bisulfites) also reacts with thiamine yielding inactive compounds (see Section 5.6.6), forms adducts with riboflavin, nicotinamide, vitamin K, inhibits ascorbic acid oxidation (see Section 5.14.6.1.6) and reacts with ascorbic acid degradation products, reduces o-quinones produced in enzymatic browning reactions back to 1,2-diphenols (see Section 9.12.4), causes decolourisation of fruit anthocyanins (see Section 9.4.1.5.7) and reacts with synthetic azo dyes to form coloured or colourless products (see Section 11.4.1.3.2). Sulfur dioxide also reacts with pyrimidine bases in vitro, specifically with cytosine and 5-methylcytosine. Important reactions of sulfur dioxide are shown in Figure 11.4. 

---