Studies on drying characteristic, nutritional composition, and antioxidant properties of Stevia rebaudiana (Bertoni) leaves

Drying characteristics of Stevia rebaudiana (Bertoni) leaves using radiation drying, convection drying, sun drying, and shade drying treatments were investigated in a convective type. Results for all treatments indicated that drying took place in the falling rate period. The chroma of the dried samples ranged between 29.66 and 23.85. Convection and shade drying samples were found better in colour and had the highest content of total pigments. The proximate and fibre composition were affected by the drying treatments. Extracts obtained from convection drying and shade drying leaves exhibited higher values of Trolox equivalent antioxidant capacities 416.89-423.64 mM mg^-1 and ferric reducing power 85.19-87.97%. These extracts also showed ion chelating capacity 59.05-60.21% for Cu⁺² and 55.05-59.11% for Fe⁺². The results indicate that it is possible to select the drying treatment to obtain the best nutritional and biological properties. Convection drying and shade drying were the least aggressive treatments and leaf powders obtained by these drying methods could be used as sources of nutrients, fibre, and antioxidant compounds.