Kinetics of ascorbic acid loss during thermal treatment in different pH buffer solutions and the presence of oxygen

Ascorbic acid stability is greatly influenced by temperature, oxygen content. Both exposures to oxygen and prolonged heating in the presence of oxygen destroy ascorbic acid. Thermal degradation kinetics of ascorbic acid in sodium acetate buffer (0.2 M, pH 5.0) and sodium phosphate buffer (0.1 M, pH 7.0) were studied at the temperature range of 80 to 100°C and the different molar ratio between oxygen and ascorbic acid. The obtained results showed that the decrease in AA concentration as a function of time at constant temperature occurred in two phases. The biphasic model was used to describe the loss of AA in aqueous solution due to thermal treatment. The AA degradation had occurred during thermal processing probably due to oxidation (aerobic degradation). When the oxygen was totally used up, the anaerobic degradation dominated and took place much more slowly than aerobic degradation. At low AA concentration (≈0.28 mM) and an oxygen concentration of 0.25 mM, most of the degradation of AA occurred under aerobic conditions, however, AA was further degraded through an anaerobic pathway at a higher ratio of AA to oxygen concentration [0.57:0.25 (mM:mM) and 1.42:0.25 (mM:mM)]. The estimated activated energy Ea values, the temperature sensitivity of the kvalues was lower at pH 7.0 than at pH 5.0. Studying the degradation of ascorbic in a model system is essential in order to fully understood and applied effectively in food products processing.