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important factor in uencing the benzene yield. Test solutions containing 0.25mM cupric sulfate showed a near-linear dependence from pH 2 to 6. The highest amount of benzene was observed at a pH of 2. Test solutions with and without H2O2 were found to form similar amounts of benzene. A simpler model was employed by McNeal et al. (11). In that study, test solutions prepared with 0.04% (2.8 mM) sodium or potassium benzoate and 0.025% (1.4 mM) ascorbic acid generated about 300 ppb benzene after 20 h at either 45 C or exposure to strong UV light (wavelength and power were not reported). The benzoate and ascorbic acid concentrations were reportedly comparable to the concentrations found in commercial beverages. Only 4 ppb benzene was found when these test solutions were stored in the dark at room temperature. However, the benzene concentrations in these test solutions increased to 266 ppb after 8 days at room temperature. No additional benzene was formed when the heated or irradiated test solutions were stored at room temperature for 8 days. Benzene was not found in control solutions containing either benzoate or ascorbic acid and subjected to the conditions described. Interestingly, when benzaldehyde was substituted for benzoate, 74 ppb benzene was formed. This nding may be important since benzaldehyde is often used to simulate cherry avor in food and beverages. The exact mechanism of benzene formation is unknown, but it is unlikely that benzene would form as a result of direct oxidation of benzaldehyde or by the Cannizzaro reaction, which forms the benzoic acid precursor. Potentially, a mechanism similar to the decomposition of benzoic acid might be involved, i.e., loss of carbon monoxide from a benzaldehyde radical. In a study on the oxidation of ascorbate in the absence of catalytic metals, 50-mM phosphate buffer solutions were found to contain 0.3- M iron and 0.13- M copper (90). Similar amounts of metal ions may have shown catalytic activity in studies on benzene formation from benzoate and ascorbic acid. In the study conducted by McNeal et al. (11), unbuffered test solutions were prepared and no additional iron and copper salts were added. Chang and Ku (12) also prepared unbuffered test solutions of 0.04% sodium benzoate and 0.025% ascorbic acid; no additional iron or copper salts were added. In both studies, benzene formed in the test solutions after 8 days at room temperature. These results suggest that trace metal ions in the laboratory water or reagents may be suf cient to mediate hydroxyl radical formation. Chang and Ku also showed that the addition of chelating agents, ethylenediamine-tetraacetic acid (EDTA) or diethylenetriamine pentaacetic acid (DTPA) at 0.1 mM, prevented the formation of benzene in the benzoate/ascorbic acid test solutions. Ethanol at 100 mM was shown by Chang and Ku to reduce the yield of benzene by approximately 90%. Many factors can affect the oxidation of ascorbic acid and the subsequent generation of hydroxyl radicals in aqueous solutions. These factors may include the type of metal ion, pH of the solution, exposure to heat and UV light, and the effects of different chelating agents. All of these factors interact in a complex manner. For example, the rate of oxidation of ascorbic acid by Cu2+
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is somewhat less rapid than the rate of oxidation by Fe3+ at a pH of 3.45. The addition of EDTA to sequester Cu2+ or Fe3+ will slow down the reaction approximately 100-fold (91). At a pH of 7, Cu2+ has a stronger catalytic activity than Fe3+. However, the Fe3+ EDTA complex remains a very ef cient catalyst of ascorbic acid oxidation, while the Cu2+ EDTA complex is completely inactive (92). Various iron chelates differ in their ability toward reduction and subsequent ability to react with H2O2. For example, the Fe3+ EDTA complex . is reduced quite rapidly by superoxide (O2 ) to Fe2+ EDTA. Subsequently, the 2+ Fe EDTA complex reacts readily with H2O2 to form hydroxyl radicals. Complexes of Fe3+ with DTPA or desferal are more resistant to the reduction. However, the Fe2+ DTPA complex can still catalyze the formation of hydroxyl radicals (93). Another important consideration is possible synergistic effects of copper and iron salts on hydroxyl radical formation (94). Differences in the rates of oxidation of Cu2+ and Fe3+ and associated complexes make it dif cult to compare results from various model systems. In summary, these studies suggest that the use of chelating agents to sequester metal ions may not necessarily reduce or eliminate hydroxyl radical formation. Sodium and potassium benzoates are effective antimicrobial agents in beverages at a pH range of 2.5 to 4.0. Under these conditions, benzoate is converted to free benzoic acid, and benzene formation can proceed according to the series of reactions proposed by Gardner and Lawrence (13). Beverages high in sugar that contain benzoate and ascorbic acid may have less potential to form benzene. Sugars can react with and inactivate hydroxyl radicals. This effect is suggested by the data observed in surveys conducted in Canada and the United States (27, 76). For example in the Canadian survey, a regular calorie beverage with 10 times as much sugar was found to contain one-sixth the amount of benzene as the equivalent diet beverage. The effects of temperature and light were not always obvious. For example in the Canadian survey, the highest benzene concentrations (19 and 23 ppb) were found in two low-sugar soft drinks packaged in thermally sealed transparent pouches. After a 10-h exposure to sunlight, no additional benzene formed in these low-sugar soft drinks (X.L. Cao and A. Becalski, unpublished, 2006). In comparison, McNeal et al. (11) observed about 300 ppb benzene formation when test solutions containing benzoate and ascorbic acid were exposed to UV light for 20 h. Unpublished studies conducted at the FDA showed that concentrations of benzene increased in some beverages containing benzoate and ascorbic acid after heating for 24 h in a 60 C oven. Unpublished studies conducted by Health Canada showed that benzene increased in certain beverages after heating for 30 min at 100 C (X.L. Cao and A. Becalski, unpublished, 2006). These discrepancies could be attributed to differences in the light source, the use of UV stabilizers in the packaging materials, and the composition of beverages and test solutions, temperature, and the exposure time. The effectiveness of EDTA as a chelating agent was not always obvious for some beverages formulated with EDTA and calcium. The International
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