Decomposition of ascorbic acid

Moore et al. (56) reported that the decomposition of ascorbic acid in orange juice was directly associated with darkening. According to Curl (57), the development of off flavors in orange juices at 13-71% soluble solids was closely paralleled by the loss of ascorbic acid and by darkening. In mandarin juice, Aiba et al. (58) found that the rate of browning was related to the decomposition of ascorbic acid. Studies with the juice of natsu-... 

When several temperature-dependent rate constants have been determined or at least estimated, the adherence of the decay in the system to Arrhenius behavior can be easily determined. If a plot of these rate constants vs. reciprocal temperature (1/7) produces a linear correlation, the system is adhering to the well-studied Arrhenius kinetic model and some prediction of the rate of decay at any temperature can be made. As detailed in Figure 17, Carstensen s adaptation of data, originally described by Tardif (99), demonstrates the pseudo-first-order decay behavior of the decomposition of ascorbic acid in solid dosage forms at temperatures of 50° C, 60°C, and 70°C (100). Further analysis of the data confirmed that the system adhered closely to Arrhenius behavior as the plot of the rate constants with respect to reciprocal temperature (1/7) showed linearity (Fig. 18). Carsten-sen suggests that it is not always necessary to determine the mechanism of decay if some relevant property of the degradation can be explained as a function of time, and therefore logically quantified and rationally predicted. 

It has been found that glucose, sucrose, casein, albumin, sodium chloride, flour groats, and starch stabilize ascorbic acid (vitamin C).771 The addition of 5% of starch inhibits the decomposition of ascorbic acid.772 Studies on the stability of ascorbic acid, sodium ascorbate, erythorbic acid, and sodium erythorbate showed that some stabilization is observed after the addition of 8% starch.773 It was reported that the interaction is purely physical in nature.774... 

The decomposition of ascorbic acid is catalyzed by trace metal ions in solution, hence the addition of sequestering agents such as EDTA and its salts has been shown to enhance the stability of ascorbic acid (291,292,293),... 

Huelin, RE. Studies on the anaerobic decomposition of ascorbic acid, Food Res., 15, 78, 1953. 

Vernin G, Chakib S, Rogacheva SM, Obretenov TD, Parkanyi C. Thermal decomposition of ascorbic acid. Carbohyd Res. 1998 305 1-15. 

The reaction of vitamin C and 6-0-stearoyl ascorbic acid with various stable vitamin E radicals has been monitored spectrophotometrically using a stopped-flow technique. The reaction was dramatically slowed by steric hindrance about the aryloxy radical.87 Sodium nitrite has been shown to accelerate the decomposition of ascorbic acid under aerobic conditions, and an ascorbate-nitrile complex was observed by nmr.88... 

Foods derived from plant materials contain pentose as well as hexose sugars, which are usually formed under weakly acidic conditions. It is well known that pentose contributes more to browning by the Maillard reaction than hexose does, because the oxo-or reducible form of sugars is higher in pentose than in hexose. Hydroxymethylfurfural (HMF) is one of the major decomposed products of such hexoses as glucose and fructose under acidic conditions, while furfural is the corresponding one of pentose and is also formed by the decomposition of ascorbic acid. 

Note Dehydroascorbic acid, the decomposition product of ascorbic acid, does not react. But it can be detected as a yellow-orange chromatogram zone (hRf 65-70) by further treatment of the chromatogram with 2,4-dinitrophenylhydrazine. This sequence... 

Stablizers. Stabilizers are ingredients added to a formula to decrease the rate of decomposition of the active ingredients. Antioxidants are the principal stabilizers added to some ophthalmic solutions, primarily those containing epinephrine and other oxidizable drugs. Sodium bisulfite or metabisulfite are used in concentration up to 0.3% in epinephrine hydrochloride and bitartrate solutions. Epinephrine borate solutions have a pH range of 5.5 7.5 and offer a more difficult challenge to formulators who seek to prevent oxidation. Several patented antioxidant systems have been developed specifically for this compound. These consist of ascorbic acid and acetylcysteine, and sodium bisulfite and 8-hydroxyquinoline. Isoascorbic acid is also an effective antioxidant for this drug. Sodium thiosulfate is used with sodium sulfacetamide solutions. 

Other workers have used the tristimulus parameters to study the kinetics of decomposition reactions. The fading of tablet colorants was shown to follow first-order reaction kinetics, with the source of the illumination energy apparently not affecting the kinetics [49]. The effect of excipients on the discoloration of ascorbic acid in tablet formulations has also been followed through determination of color changes [50]. In this latter work, it was established that lactose and Emdex influenced color changes less than did sorbitol. 

The kinetic models for these reactions postulate fast complex-formation equilibria between the HA- form of ascorbic acid and the catalysts. The noted difference in the rate laws was rationalized by considering that some of the coordination sites remain unoccupied in the [Ru(HA)C12] complex. Thus, 02 can form a p-peroxo bridge between two monomer complexes [C12(HA)Ru-0-0-Ru(HA)C12]. The rate determining step is probably the decomposition of this species in an overall four-electron transfer process into A and H202. Again, this model does not postulate any change in the formal oxidation state of the catalyst during the reaction. 

Although most steroids are stable on the silica gel layers, some steroids (e.g., estrogens and vitamin D) can decomposed readily. To avoid the decomposition, a preliminary treatment of the sorbent with a solution of ascorbic acid (antioxidants) in ethanol is recommended. 

Parenteral formulations often contain excipients considered to be chemically stable and inert however, all excipients in a formulation may influence the photochemical stability of the product. Dextrose and sodium chloride are used to adjust tonicity in the majority of parenteral formulations. Sodium chloride can affect photochemical processes by influencing solvation of the photoreactive molecules (see Section 14.2.3). The ionic strength is reported to affect the photochemical decomposition rate of minoxidil until a saturation level is reached (Chinnian and Asker, 1996). The photostability of L-ascorbic acid (vitamin C) in aqueous solution is enhanced in the presence of dextrose, probably caused by the scavenging effect of the excipient on hydroxyl radicals mediated by the photolysis of ascorbic acid sucrose, sorbitol, and mannitol have the same effect (Ho et al., 1994). Monosaccharides (dextrose, glucose, maltose, and lactose), disaccharides (sucrose and trehalose), and polyhydric alcohols (inositol, mannitol, and sorbitol) are examples of commonly used lyo-additives in parenterals. These excipients may also affect photochemical stability of the products after reconstitution. 

It is known that the redox potential of the Fe+3/Fe+2 pair can vary by complexing ligands (27). EDTA reduces the redox potential of Fe+2 (28) and this increases the rate constant transfer of the electron from Fe+2 to H202, which is formed during autooxidation of ascorbic acid (29), and decomposition of the latter to H0-. However, at low pH 3-4, EDTA was found to inhibit ascorbic acid oxidation by ferric ions (29). Thus, the form the metal chelate takes, as a function of pH, plays a key role in its effectiveness as a catalyst. Cupric ions are known to accelerate ascorbic acid oxidation however, EDTA inhibits its catalytic effect at both neutral and low pH (24). 

Very little information is available about the species which react in the further decomposition of ascorbic add. Apparently, both dehydroascorbic acid and 2,3-diketogulonic acid can be oxidized directly, since both oxalylthreonic acid and the same two free acids have been identified. The former would result from oxidative fission of the chain while the lactone ring was intact. Even less is known of the oxidation mechanism. The oxidation occurs with iodine and with acid permanganate (H12) and also with oxygen or peroxide (R23). Recent studies on the similar decomposition of the enols of aryl pyruvates to aryl aldehydes plus oxalic acid identified these reactions as examples of the direct attack... 

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