Stevia leaves are usually used in dried state and undergo the inevitable effect of drying process that changes the quality characteristics of the final product. The aim of this study was to assess temperature effect on Stevia leaves through analysis of relevant bioactive components, antioxidant capacity and content of natural sweeteners and minerals. The drying process was performed in a convective dryer at constant temperatures ranging from 30 to 80 °C. Vitamin C was determined in the leaves and as expected showed a decrease during drying proportional to temperature. Phenolics and flavonoids were also determined and were found to increase during drying below 50 °C. Antioxidant activity was determined by DPPH and ORAC assays, and the latter showed the highest value at 40 °C, with a better correlation with the phenolics and flavonoids content. The content of eight natural sweeteners found in Stevia leaves was also determined and an increase in the content of seven of the sweeteners, excluding steviol bioside, was found at drying temperature up to 50 °C. At temperatures between 60 and 80 °C the increase in sweeteners content was not significant. Stevia leaves proved to be an excellent source of antioxidants and natural sweeteners.

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The drying process of Stevia leaves was studied from 30 to 80C. Experimental drying curves showed that the drying process took place only in the falling rate period. Effective moisture diffusivity of Stevia leaves was in the range of 4.67-14.9 × 10 −10 m 2 /s. Several drying models available were evaluated based on statistical tests as a sum squared error (SSE), chi-square and determination coefficient. A value of 38.78 kJ/mol was determined as an activation energy. When comparing the experimental with predicted drying curves, the Midilli-Kucuk model was found to give the best fit quality (SSE < 0.001, χ 2 < 0.001, r 2 > 0.99). In addition, all drying curves were generalized and then modeled by Midilli-Kucuk model (r 2 = 0.99), showing this equation can very accurately predict the drying time of Stevia leaves under the drying conditions studied. At last, energy consumption and efficiency values for Stevia leaves drying process were found to be in the range of 316.59-716.19 kJ/kg and 21.46-54.34%, respectively. PRACTICAL APPLICATIONSA great demand for medicinal plants with extended shelf life has urged the dried food industry to look for raw materials of desirable nutritional and functional properties. Stevia rebaudiana leaves, with its high content of biologically active health-promoting components (natural non-caloric sweetener compounds, antibacterial capacity, antioxidant activity and other), is therefore an excellent raw material for the healthy food industry. Dehydration has the potential to deliver safe food products through decreasing moisture content and water activity. Therefore, knowledge about moisture transfer kinetics, as well as energy consumption and/or data normalization, is needed by the industry to manage and control efficiently drying process. Thus, this could serve to demonstrate the environmental consciousness of the food processing industry, greatly appreciated by consumers.

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