Effect of metal ions on stability of ascorbic acid determined by high-performance liquid chromatography (HPLC) and novel modelling of its degradation kinetics

Stability of ascorbic acid (AA) as catalyzed by different metal ions (i.e., Fe3+/Fe2+, Cu+/Cu2+, Zn2+, and Na+/K+/Ca2+/Mg2+) in model solutions at 25 °C was determined by HPLC. A novel first order reversible consecutive reaction model which takes into account uncatalyzed and catalyzed degradation of AA in parallel with metal ion as catalysts for both AA oxidation (with apparent rate constant k1app) and dehydroascorbic acid (DHAA) reduction (with apparent rate constant k2app) has been developed. Experimental data fit the reversible model well, but not the irreversible model except for AA and AA with high concentrations of Cu+/Cu2+. At 0.263 mmol/L, Na+/K+/Ca2+/Mg2+ accelerated AA oxidation by about (ca.) 0.36-fold, while Zn2+, Fe3+/Fe2+, and Cu+/Cu2+ by ca. 2-, 14-, and 88-fold, respectively. Metal ions can induce catalyzed DHAA reduction to AA in presence of oxygen, and the reversible nature of AA degradation became less remarkable by nitrogen purging, as revealed by a decrease in k2app/k1app. Significance of the finding that Cu+/Cu2+ and Fe3+/Fe2+ ions accelerated the AA oxidation and Zn2+ ion moderately enhanced it was discussed in relation to the concentrations of those metal ions in 307 foods. Our simulation for half-life (t1/2,rev) of AA in foods at 25 °C by using reversible model and considering the existence of Cu2+, Fe3+, and Zn2+ ions only revealed that for green peas (with 1.17 mg/kg Cu2+), spinach (with 0.81 mg/kg Cu2+), and green beans (with 0.63 mg/kg Cu2+), t1/2,rev were 0.98, 1.26, and 1.65 h, exhibiting a prediction error of −86.2%, 3.3%, and −68.6%, respectively.