Ascorbic acid as antioxidant

Antioxidants should be labelled on the retail package with the specific chemical name or with the EC number. The legislation of member states of the EU is influenced by the decision taken within the EC. Some food standards are fully based on EC Directives and some are still based on national considerations. There may be differences between European states, for instance, the utilisation of ascorbic acid as antioxidant for egg products is permitted in France but prohibited in Germany. These differences concern usually the utilisation of antioxidants in various food commodities. The specification of antioxidants mentioned in EC Directives are respected by all member states. But it is still generally required that individual countries of the European Union as well as the central organisation should be approached. The requirements appearing in the EC Directives on additives must be applied by the member states. This means in the first place that for those categories of additives for which a Community positive list exists, member states may not authorise any additives which do not appear on the positive list. 

Interest has been shown in using derivatives of ascorbic acid as antioxidants. One such compound is ascorbyl palmitate. In the structure of ascorbyl palmitate, the 2- and 3-positions are occupied by hydroxyl groups the 6-position contains the substituted fatty acid. Other derivatives, synthesized by Seib and associates were ascorbate-2 phosphate and ascorbate 2-triphosphate (27). Both of these compounds were reported to inhibit lipid oxidation in ground meat as measured by chemical means (2). Ascorbate-2-phosphate was also found to inhibit MFD in beef as measured by sensory means (2). The ascorbate-2-triphosphate was not tested as an inhibitor of MFD in this study, but... 

Takahashi, M., Komuro, F., Niki, E. and Tanaka, K. (1992) Action of fatty acid esters of L-ascorbic acid as antioxidants in phosphatidylcholine liposomal membranes. Bull. Chem. Soc. Jpn, 65, 679-684. 

Reduced scavenger capacity is deduced from studies demonstrating low plasma and cellular levels of antioxidants such as glutathione, vitamin E, thiols, magnesium and ascorbic acid, as well as reduced levels of scavenger enzymes such as neutrophil glutathione peroxidase and red cell superoxide dismutase (Lyons, 1991 Sinclair /., 1992). 

Chemically carotenoids have conjugated double bonds that render them liable to oxidation. This tendency to oxidation can be diminished by adding antioxidants to the product. In the sort of product where natural colours are used suitable antioxidants would be tocopherols or ascorbic acid. Chemical antioxidants such butylated hydroxytoluene might be suitable technically but would not fit the image of an allnatural product. Ascorbic acid could be declared as vitamin C rather than as an antioxidant. Oxidation can be started by exposure to light and so this is best avoided. Carotenoids are generally stable to heat. The levels required can be as low as 10 ppm. P-Carotene is available as a nature identical form. 

Early syntheses7 16 employed ascorbic acid as the antioxidant to protect the newly formed tetrahydro ring. But Roth et al.9 found that ascorbic acid had no effect on leucovorin yield (40-50%) as long as incubation was performed under anaerobic conditions. More recent syntheses have used 2-mercaptoethanol21 or a stream of nitrogen22 to to protect the tetrahydro system. 

In its biochemical functions, ascorbic acid acts as a regulator in tissue respiration and tends to serve as an antioxidant in vitro by reducing oxidizing chemicals. The effectiveness of ascorbic acid as an antioxidant when added to various processed food products, such as meats, is described in entry on Antioxidants. In plant tissues, the related glutathione system of oxidation and reduction is fairly widely distributed and there is evidence that election transfer reactions involving ascorbic acid are characteristic of animal systems. Peroxidase systems also may involve reactions with ascorbic acid In plants, either of two copper-protein enzymes are commonly involved in the oxidation of ascorbic acid. 

Core-shell polystyrene-polyimide high performance particles have been successfully prepared by the dispersion copolymerization of styrene with vinyl-benzyltrimethyl ammonium chloride (VBAC) in an ethanol-water medium using an aromatic poly(amic acid) as stabilizer, followed by imidization with acetic anhydride [63]. Micron-sized monodisperse polystyrene spheres impregnated with polyimide prepolymer have also been prepared by the conventional dispersion polymerization of styrene in a mixed solvent of isopropanol/2-methoxyethanol in the presence of L-ascorbic acid as an antioxidant [64]. 

By contrast, at concentrations above 0.2 mM, it protects against LPO. Normal cytosolic concentrations would favour GSH over ascorbic acid as a cytosolic antioxidant in most tissues (McCay,... 

As shown in Fig. 1.16a, upon addition of CATl the fluorescence of P6 is efficiently quenched. The ( )-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (trolox) is added and allowed to equilibrate for 35 min, which reverses the quenching due to the scavenging of nitroxide radicals via hydrogen transfer from trolox to CATl. The fluorescence recovery depends on the concentration of trolox (Fig. 1.16b). The trolox can be detected in the range of 10 100 pM. The same probe can also be used to detect the capabilities of a variety of antioxidants. Sensing for ascorbic acid can be accomplished with high sensitivity and selectivity because of its excellent antioxidant capabilities. The ascorbic acid concentration can be determined in the 50 nM to 200 pM range (Fig. 1.17a). Control experiments were also done with a nonspecific quencher. A , A -dimethyl-d, 4 -bipyridinium (MV +) for ascorbic acid. As shown in Fig. 1.17b, upon addition of MV +, the fluorescence of P6 is... 

Since antioxidants may also under special conditions be pro-oxidants (they are oxidizable), it is not surprising that ascorbic acid, as well as cysteine, promotes hemolysis of red blood cells obtained from vitamin E-deficient rats (Rose and Gyorgy, 1950). 

Chipault et at. (1945) concluded that the keeping qualities of hog fats are dependent primarily upon the linoleic acid and tocopherol contents, that tocopherols are the only natural direct inhibitors of oxidation in hogs, and that these tocopherols are likely derived entirely from the diet. Larger amounts of tocopherol were deposited in the adipose of rats when receiving 0,01 % a-tocopherol plus 0.5% ascorbic acid as supplement to a basal diet than when any of a number of other antioxidants and synergists were fed (Prange, 1949). 

The eye is one of several organs in the body that maintains a high concentration of ascorbic acid. As in other tissues, ascorbic acid is thought to function as an antioxidant, as a factor involved in collagen synthesis, as a redox coupler involved... 

FIGURE 5. Ascorbic acid as an antioxidant. The free radical form of ascorbic acid, A , is the primary product of ascorbic acid oxidation, observed during catalytic, enzymatic, photooxidative, and free radical oxidation. It is relatively stable and, thus, can detected by electron spin resonance in ischemic reperfusion of the heart and iron overloaded blood plasma. Adapted from Bendich et al, (1986). 

L-ascorbic acid is the naturally occurring form of ascorbic acid and has the most biological activity. The o-ascorbic acid as well as d- and L-isoascorbic acids (erythorbic acid, lAA) have only marginal vitamin C activity. However, lAA is used in the food industry as an antioxidant, even though it has only about 5%... 

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