Ascorbic acid, destruction oxidation

Folate is a relatively unstable nutrient processing and storage conditions that promote oxidation are of particular concern since some of the forms of folate found in foods are easily oxidized. The reduced forms of folate (dihydro- and tetrahydrofolate) are oxidized to p-aminobenzoylglutamic acid and pterin-6-carboxylic acid, with a concomitant loss in vitamin activity. 5-Methyl-H4 folate can also be oxidized. Antioxidants (particularly ascorbic acid in the context of milk) can protect folate against destruction. The rate of the oxidative degradation of folate in foods depends on the derivative present and the food itself, particularly its pH, buffering capacity and concentration of catalytic trace elements and antioxidants. 

The aerobic oxidation of RAA (Figure 1) occurs rapidly when metal catalysts, particularly copper or iron, or enzymes such as ascorbic acid oxidase, polyphenol oxidase, peroxidase, and cytochrome oxidase are present. The anaerobic destruction of ascorbic acid may proceed by a variety of mechanisms that have been postulated (2,3) but not verified. 

It is frequently stated that steaming vegetables is most beneficial in retaining ascorbic acid in the food. An examination of the sparse data (56) in the literature does not support this theory, however. In many cases, the lengthened cooking time necessary to soften the vegetables can result in destruction of vitamin C by heat and oxidation. If vegetables... 

Figure 5.2. Dependence of the relative rates of various processes on water activity at constant temperature. Curve 1 relative destruction rate of chlorophyll in spinach at 37°C. Curve 2 relative decay rate of ascorbic acid. Curve 3 relative oxidation rate of potato chips stored at 37°C. Curve 4 relative rate of inactivation of phosphatase in skim milk. Curve 5 relative rate of inactivation of Clostridium botulinum. Curve 6 drying rate of a slab of glucose at 30°C slab temperature. (From Thijssen, 1979.)...

Destruction of vitamins A, C, D and E induced by riboflavin-photosensitized oxidation has been reported. Vitamin A and its esters, along with carotenoids with pro-vitamin A activity, undergo ring opening upon sunlight exposure in the presence of riboflavin. Although an excellent antioxidant, ascorbic acid is rapidly photooxidized in the presence of riboflavin. Even a small decrease in riboflavin content in milk due to photosensitization can lead to virtual complete destruction of ascorbic acid. Fortunately, milk is not an important source of vitamin C in most diets. 

Ascorbic-acid reductase is assayed (19, 47, 50, 51, 52) by incubating dehydroascorbic acid and GSH in the presence of enzyme and determining the amount of ascorbic acid formed or GSH oxidized (or both). The incubation is carried out anaerobically or in the presence of cyanide, in order to inhibit or prevent the action of ascorbic-acid oxidase. Although the pH optimum of the enzyme reaction is at about 6.8, the assay is often run at pH 6.0 to 6.3 in order to minimize the nonenz3nnatic reaction and the irreversible destruction of dehydroascorbic acid which occurs more rapidly at the higher pH s. A blank determination containing all components of the assay system except the enzyme is run simultaneously. A correction is then applied by subtracting the value of the blank from the... 

Since ascorbic acid is water-soluble and very easily oxidized, loss of this factor in the storage and preparation of food may be great. Destruction is more rapid in neutral and alkaline solutions than in acid solutions. In patients who have achlorhydria and in those who have received alkaline medication, ascorbic acid may be destroyed in the upper intestinal tract. The vitamin is absorbed readily, and the levels of ascorbic acid in plasma are related to recent dietary intake. Tissue saturation is achieved easily with large doses of ascorbic acid, and amounts in excess of this are excreted in the urine. It has been estimated that when the tissues of an adult human being are saturated the body contains about 5 g. of ascorbic acid. 

Folic acid is quite stable. There is no destruction during blanching of vegetables, while cooking of meat gives only small losses. Losses in milk are apparently due to an oxidative process and parallel those of ascorbic acid. Ascorbate added to food preserves folic acid. 

The oxidation of ascorbic acid to dehydroascorbic acid and its further degradation products depends on a number of parameters. Oxygen partial pressure, pH, temperature and the presence of heavy metal ions are of great importance. Metal-catalyzed destruction proceeds at a higher rate than noncatalyzed spontaneous autoxidation. Traces of heavy metal ions, particularly Cu and Fe +, result in high losses. 

The effects of pH in the oxidative destruction of folates has been reviewed by several authors (6,10,28). Each vitamer has its own unique pH stability, and it is therefore sometimes difficult to optimize extraction conditions for all folate forms in a single extraction. The pH of the extractant, presence of oxygen, the temperature used, and in addition, buffer type, can all affect the efficiency of folate extraction (8). Several folate derivatives can be altered during extraction. The presence of phosphate accelerates conversion of 10-HCO-H4-folate to (1) 5,10-CH+-H4-folate and (2) 5-HCO-H4-folate at pH values below pH 7 during extraction. Also, 5,10-CH2-H4 folates can be converted to H4 folate during extraction (29). As the use of ascorbic acid reduces 5-CH3-H2-folate to 5-CH3-H4-folate, the former can be quantified only in the 5-CH3-H4-folate pool (30). Thus, extraction conditions can affect the vitamer distribution and therefore need always to be carefully considered and fully reported. The chosen extraction conditions are dependent on the purpose of the analysis to be carried out. 

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