Evaluation of the fluidized bed drying process on the quality attributes of stevia leaves (stevia rebaudiana)

Stevia leaves (Stevia rebaudiana) are a source of bioactive compounds beneficial to human health; among them, steviol glycosides standout. The commercial value of dehydrated stevia leaves is related to their physical appearance and the concentration of glycosides. The hypothesis of the study was that the quality of stevia leaves, based on the content of bioactive compounds present in the leaves, can be visible and modified by drying. To test it, the objective was to evaluate the effect of temperature, air velocity, and load density of stevia leaves in a fluidized bed dryer unpacked and packed on the characteristics of colour, steviol glycoside concentration (stevioside and rebaudioside A), total phenolic compound concentration, flavonoid concentration, and antioxidant capacity. The physicochemical variables were evaluated by CIEL*a*b* coordinates, reverse phase liquid chromatography, Folin-Cicolteau colourimetric methods, aluminum chloride, and DPPH* free radical inhibition assay. The effect of process variables was evaluated with a full factorial arrangement with 24 combinations of factors and three sampling replicates. The fluidized bed configuration affected (p£0.05) the concentration of steviol glycosides in the dry leaf material. In both configurations the steviol glycoside content increased between 230-290% compared to fresh leaf material. For the unpacked bed configuration, the bitter aftertaste (rebaudioside/stevioside ratio) was higher than the original evaluated in the fresh material. All process variables had a significant influence on polyphenol concentration. The packed bed configuration increased the concentration of phenolic compounds (p£0.05) up to 248%. Whereas, with unpacked bed it decreased up to 17% (p£0.05) in relation to fresh material. The decrease in drying times and the increase of bioactive compounds (steviol glycosides and antioxidant compounds) in stevia leaf make of this drying method a useful tool for the management and preservation of plant materials containing secondary metabolites of economic importance.