Mechanistic model to couple oxygen transfer with ascorbic acid oxidation kinetics in model solid food

A mechanistic model was developed taking into account both oxygen and ascorbic acid transfer phenomena and oxidation reaction of ascorbic acid. Studied system was cylindrical agar gel initially spiked with ascorbic acid and exposed to air on one face. This model describes oxygen and ascorbic acid diffusion (using Fick's law) and ascorbic acid oxidation rate. A sensitivity study showed that partial order of reaction with respect to oxygen was the critical factor in this model coupling transfer and reaction phenomena. This model has been validated at 20 °C using local measurements of ascorbic acid and oxygen contents in agar gels, and also using average ascorbic acid content, both type of data having been measured at different sampling times. It was shown that the reaction occurred at a significant extent only near from the interface air/gel, i.e. the part of the gel near from the interface air/gel contained high level of oxygen and low level of ascorbic acid, whilst the part near the other extremity of the gel contained high level of ascorbic acid and low level of oxygen. This model could be a useful tool to precisely locate the zone in a solid food submitted to oxidation, to determine which reactant and which phenomenon (diffusion and/or reaction) is rate limiting and consequently to assess better designs for strategies of protection against oxidation.