Effect of different drying techniques on the volatile compounds, morphological characteristics and thermal stability of Stevia rebaudiana   Bertoni leaf

Purpose: To examine the volatile compounds, thermal stability and morphological characteristics of stevia (Stevia rebaudiana Bertoni) leaves after sun, oven and microwave drying.
Methods: Gas chromatography-mass spectrometry with a spectral analysis manager was used to separate the volatile compounds. Dried stevia leaf powder was characterized morphologically by scanning electron microscopy while thermal properties were determined by differential scanning calorimetry (DSC).
Results: The plant material contained large amounts of spathulenol and caryophyllene oxide. The main compounds were 1-docosanol and hexanoic acid; trans-β-ionone, 5-methylundecane, 2,5,6-trimethyldecane, (+) spathulenol, propanoic acid and 1-chlorononadecane. The surface of the dried leaf powder varied with the treatment applied. Following microwave drying, the particles were even, regular, and compact while the sun- and oven dried particles resembled angular bricks. All samples exhibited a strong endothermic response, indicating stability up to 150 °C; from 150 to 200 °C, thermal decomposition occurred.
Conclusion: Drying method has a significant effect on the structure, thermal stability and volatile compounds contents of stevia leaves. All drying methods examined in this study have potential applications in the preparation of stevia as a functional ingredient for the food and pharmaceutical industries.

Keywords
Differential scanning calorimetry; Drying techniques; Stevia rebaudiana; Pharmaceuticals; Scanning electron microscopy; Volatile compounds