Modelling ascorbic acid thermal degradation and browning in orange juice under aerobic conditions

The thermal degradation of ascorbic acid (AA) in orange juice was analysed over in a 20-45 degreesC temperature range. Dehydroascorbic acid (DA), pH a nd browning were also monitored. Small amounts of AA degradation could be d escribed by first order kinetics, but when only low amounts of AA were reta ined sigmoidal kinetics were clearly appropriate. The Weibull model was use d to describe this pattern (R-adj(2) > 0.995). The rate constant increased with temperature according to an Arrhenius-type relationship. The activatio n energy was 38.6 kJ/mol and at the average temperature of the range tested , 32.5 degreesC, the rate constant was 64.4 x 10(-3) h(-1). The shape const ant decreased linearly with temperature, from 2.17 to 1.13. Before the time when the maximum degradation rate occurred, pH, DA concentration and brown ing remained fairly constant, and then increased. It was found that this be haviour, as well as the dependence of the shape constant on temperature, mi ght be explained by (i) the reconversion of DA into AA, following first ord er kinetics in relation to DA and second order kinetics in relation to AA, and by (ii) different sensitivities of the reaction rate constants to tempe rature. Browning was also well described by the Weibull model with a temper ature independent shape constant.