Use of Weibull distribution to quantify the antioxidant effect of Stevia rebaudiana on oxidative enzymes

The effect of Stevia rebaudiana Bertoni (Stevia) at different concentrations (0.5-2.5 g dried Stevia leaves/100 mL H2O) on polyphenoloxidase (PPO) and peroxidase (POD) activities of orange, mango and papaya during incubation at 10 and 37 degrees C was studied. A decrease in both enzyme activities was observed when Stevia was added, showing an inhibitory effect that could correspond to an antioxidant capacity. To quantify this effect the experimental data values of the logarithm of enzyme activity (Log (A/A(0))) versus time as a function of Stevia concentration and incubation temperature were fitted to the Weibull distribution function. For both enzymes, independently of incubation temperature, the scale parameter, a, increased with Stevia concentration and can therefore be considered an index to quantify the antioxidant capacity of Stevia. The results clearly showed that Stevia had significant potential to be used not only as a sweetener but also as a natural antioxidant and antibrowning agent to prevent fruit quality degradation. (C) 2014 Elsevier Ltd. All rights reserved.