Oxygen availability in model solutions and purées during heat treatment and the impact on vitamin C degradation

•Oxygen is a precondition but not alone triggering degradation of vitamin C at 80 °C.•Ascorbic acid degrades in buffer but not in ultrapure water.•Fe3+ ions do not oxidize ascorbic acid without oxygen.•Oxygen is depleted in food matrices in contrast to aqueous solutions.•Mechanisms differ, which is also indicated by medium dependent temperature sensitivities.

Oxygen availability in different media during heat treatment (8 h at 80 °C) and the related vitamin C loss was assessed. Dissolved oxygen in water containing 3 mmol kg−1 of ascorbic acid decreased initially and seemed to be replaced by oxygen from the headspace in the course of time, as oxygen values increased again. In apple purée and carrot purée in contrast, oxygen was depleted within 60 min. Vitamin C in ultrapure water was stable even in the presence of oxygen. A trigger seemed to be crucial to launch vitamin C degradation. Fe3+ ions added to water, but also the media Mc Ilvaine citrate-phosphate buffer (pH 3.5) or apple purée, initiated degradation. Adding Fe3+ ions to apple purée did not accelerate vitamin C degradation but shifted the equilibrium between ascorbic acid and dehydroascorbic acid to the latter. Oxygen deprivation stabilized completely vitamin C, independently of the medium tested. A temperature decrease to 70 or 60 °C, in contrast, had no effect on the degradation extent of vitamin C in water containing 20 μmol kg−1 Fe3+ ions but led to complete stability in apple purée.