Ascorbic acid irradiation

The product analysis of L-ascorbic acid irradiated in deaerated solution is restricted to the measurement of dehydro-L-ascorbic acid, hydrogen, and the decrease in L-ascorbic acid.263 To account for the fact that N20 has no effect on G(L-ascorbic acid consumption), a rather complex mechanism has been put forward that also allows the formation of a reduction product (L-gulono-1,4-lactone, suggested but not measured). 

Chemical Properties. The most significant chemical property of L-ascorbic acid is its reversible oxidation to dehydro-L-ascorbic acid. Dehydro-L-ascorbic acid has been prepared by uv irradiation and by oxidation with air and charcoal, halogens, ferric chloride, hydrogen peroxide, 2,6-dichlorophenolindophenol, neutral potassium permanganate, selenium oxide, and many other compounds. Dehydro-L-ascorbic acid has been reduced to L-ascorbic acid by hydrogen iodide, hydrogen sulfide, 1,4-dithiothreitol (l,4-dimercapto-2,3-butanediol), and the like (33). 

As metal ions catalyze peiroxidation reactions, glass-distilled water should be used and chelating agents can be added to the medium. (5) The dispersions should not be exposed to y irradiation. (6) Antioxidant can be added to the system. a-Tocopherol, buty-lated hydroxytoluene, butyl hydroxyanisole, and ascorbic acid have been proposed as antioxidants. 

FIGURE 10.13 The TLC profiles of labeled peaks isolated from [U- C]ascorbic-acid-modified calf lens protein obtained from Bio-Gel P-2 chromatography. Peaks 2 to 7 were spotted on a preparative silica gel TLC plate and developed with ethanol/ammonia (7 3, v/v). The fluorescence in each lane was detected by irradiation with a Wood s lamp at 360 nm, and the pattern of radioactivity was determined by scanning the plate with AMBIS imaging system. (Reprinted with permission from Cheng, R. et al., Biochim. Biophys. Acta, 1537, 14-26, 2001. Copyright (2001) Elsevier.)... 

Thomas P and Janave MT. 1975. Effects of gamma irradiation and storage temperature on arotenoids and ascorbic acid content of mangoes on ripening. J Sci Food Agric 26 1503-1512. 

Figure 11.4. Effect of UV-C irradiation time (0, 1, 3, 5, 10 min) on (I) ascorbic acid (mg/100 g FW), (II) p-carotene (mg/100 g FW), and (III) total phenols (mg/100 g FW) of fresh-cut mango (cv. Tommy Atkins) stored at 5°C. Bars show the final values after treatments. Different letters on top of the bars indicate statistical differences among treatments (p < 0.05).

Irradiation of fresh-cut mangoes improved the antioxidant capacity of the product (Fig. 11.5, II), even when a long exposure could reduce the ascorbic acid and 3-carotene content (Gonzalez-Aguilar and others 2007). This was not proportional to the antioxidant capacity, which was influenced more by total phenols and flavonoid content. It seems that the antioxidant capacity improvement could be an additional factor to give added value in the fresh-cut mango industry. 

Literature data on cytotoxic effects of photoexcited fullerene C60 are controversial. In the studies on transformed B-lymphocytes of Raji fine, phototoxic action of water-soluble carboxy-C60 was not revealed even upon its concentration of 5 x 10 5 M (Irie et al., 1996). In the study (Kamat et al., 2000) damaging effect of fullerenes C60 in dependence on intensity of irradiation toward CHO cells has been demonstrated. Using microsomal fraction of rat liver that was treated with C -cyclodextrin complex, it was shown that already in 5-30 min after UV-irradiation the accumulation of LPO products occurs that is suppressed by antioxidants like ascorbic acid and a-tocopherol. Similar effect of fullerenes C60 has been revealed in microsomal fraction of the cells of ascitic sarcoma 180 (Kamat et al., 2000). 

Because potatoes are good source of vitamin C, it is important to point out that irradiation does not adversely affect the vitamin C levels [23,30]. Although some ascorbic acid is converted into dehydroascorbic acid on irradiation, the latter is also biologically active. 

At low and medium doses, it is well established that the nutritional value of proteins, carbohydrates, and fats as macronutrients are not significantly impaired by irradiation, and neither the mineral bioavailability is impacted. Like all other energy depositing process, the application of ionizing radiation treatment can reduce the levels of certain sensitive vitamins. Nutrient loss can be minimized by irradiating food in a cold or frozen state and under reduced levels of oxygen. Thiamin and ascorbic acid are the most radiation sensitive, water-soluble vitamins, whereas the most sensitive, fat-soluble vitamin is vitamin E. In chilled pork cuts at the 3 kGy maximum at 0-10°C, one may expect about 35 0% loss of thiamin in frozen, uncooked pork meat irradiated at a 7 kGy maximum at —20°C approx., 35 % loss of it can be expected [122]. 

The same authors (G8, G7) also found very substantial decreases in riboflavin (approx. 80%), and niacin (P9) fared little better. When mixtures were irradiated unusual events occurred. Riboflavin and ascorbic acid were each protected by niacin. Addition of cystine or cysteine apparently sensitized the niacin (P10). Since initial rates were not given, and the doses were considerably above the oxygen breakpoint (Sec. IIIA2), no mechanistic interpretation is possible. There also appears to be some doubt about the reliability of the colormetric assay used by these workers. 

Oster [174] proposed the second hypothesis to explain his results on the photopolymerization of acrylonitrile in aqueous solution, buffered at pH 7.0, and sensitized by xanthene dyes and riboflavin using ascorbic acid as the reducing agent. Whereas the monomer is efficiently polymerized when the solution is illuminated in the presence of oxygen, irradiation in its absence leads to photoreduction of the dye to its leuco form but no polymer is formed. Therefore, the author suggests that the leuco dye reacts with atmospheric... 

The photochromic effect was observed after both UV-irradiation and thermal ageing, but it was not dependent on the storage temperature. Extraction of the pulp had a slight effect. The most pronounced decrease was obtained if the pulp was impregnated with ascorbic acid, which probably works as a radical scavenger. The brightness increase follows the decrease in radicals in the pulp upon storage as monitored by ESR. 

Ascorbic acid inhibits light-induced yellowing for a finite time, 1 to 2 hours when irradiated with near-uv light with an intensity of 9.2 mW/cm2 (6). This limitation has been attributed in part to photooxidation of ascorbic acid. In addition to air oxidation, ascorbic acid is oxidized by photochemically produced peroxyl radicals, superoxide radical anion and singlet oxygen (27,28). If ascorbic acid is to be an effective inhibitor of light-induced yellowing it s oxidation must be slowed. 

Agnemo (29) in his patent application describes the effect of treatment of bleached aspen chemithermomechanical pulp with ascorbic acid and sodium sulphite. Figure 3 is a plot of the specific absorption at 457 nm against the time of irradiation. It appears that relative to the initial specific absorption, adding NaaSC and ascorbic... 

Ascorbic acid (0.8% w/v) in aqueous solution degraded according to apparent first order kinetics, with a rate constant of 2.34 x 10 2/hour, when irradiated by artificial sunlight [40]. The presence of 5% aspartame in the solution decreased the rate constant to 1.48 x 10 2/hour, thus stabilizing ascorbic acid to photochemical degradation by about 37%. Similar effects were also seen with some carbohydrate sweeteners. 

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