Ascorbic acid degradation

Figure 3. Possible ascorbic acid degradation pathways...

Several modifications of the OPDA procedure have been reported for automated analysis. Kirk and Ting (16) described a continuous flow analysis in which DCIP was substituted for Norit in the oxidation step. TAA and DHA can be determined directly RAA is calculated as the difference between TAA and DHA. Good agreement between the manual and automated procedures was achieved, with a considerable decrease in analytical time for the automated assay. Kirk and coworkers (17 19) have used the continuous flow analysis for studies of the kinetics of ascorbic acid degradation in model systems. 

Ascorbic acid loss in frozen foods is highly temperature dependent. For example, when peaches, boysenberries, or strawberries are stored at — 7°C instead of — 18°C, the rate of ascorbic acid degradation increases by a factor of 30-70 (105,106),... 

Anaerobic degradation of ascorbic acid in grapefruit juice is a zero-order reaction (20), but degradation of fish is either first or zero order depending on the type of fish (21). Ascorbic acid degradation in peas is... 

Figures 5 and 6 illustrate the linear relationship between ascorbic acid oxidation and oxygen concentration, and the correlation between browning and ascorbic acid degradation.

Figure 9. The effect of untreated and oxidized polyethylene on ascorbic acid degradation in a model solution, stored at 35°C.

Blasco, R., Esteve, M.J., Frigola, A., and Rodrigo, M. 2004. Ascorbic acid degradation kinetics in mushrooms in a high-temperature short-time process controlled by thermoresistometer. LWT-Food Sci. Technol VI, 171-175. 

FIGURE 11.16 L-Ascorbic acid degradation products (a-dicarbonyl compounds). 

Schulz et al. [234] used dehydro-Shinoda-ascorbic acid, which is the first characteristic intermediate of L-ascorbic acid degradation via the oxidative route, as a starting material of the reaction to distinguish it from the nonoxidative pathway. They found that dehydro-L-ascorbic acid yielded five a-dicarbonyl compounds, namely glyoxal, methylglyoxal, diacetyl, L-threosone, and 3-deoxy-L-threosone (Figure 11.16). They concluded that these a-dicarbonyl compounds are formed from L-ascorbic acid on the oxidative pathway. But, they pointed out that these products can also be produced via the nonoxidative route. They also found that 3-deoxy-L-pentosone is exclusively formed via the nonoxidative route. 

The nse of a high ionic strength acidic extraction solvent is required to suppress metabolic activity upon disrnption of the cell and to precipitate proteins. A metal chelator snch as ethylenediaminetetraacetic acid (EDTA) is also usually required. The use of metaphosphoric acid is the best way to extract and stabilize L-ascorbic acid, as suggested by various authors [72,78,80,81]. For this reason, the samples were extracted with 3% metaphosphoric acid-8% acetic acid-1 mM EDTA solution, which is known to limit L-ascorbic acid degradation to less than 5% [82]. 

In line with recent data on browning inhibition by thiol compounds (77), the present results indicated that the use of a variety of concentrations of L-cysteine inhibited browning (Figure 3) and reduced ascorbic acid degradation in a dose-... 

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. 

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