Encapsulation of aqueous leaf extract of Stevia rebaudiana Bertoni with sodium alginate and its impact on phenolic content

The aim of this study was to encapsulate aqueous leaf extract of Stevia rebaudiana Bertoni with sodium alginate and evaluate the effect on the total phenolic content (TPC) and antioxidant stability. An experimental design was initially performed in order determine the best extraction conditions on yield of TPC from stevia leaves. The encapsulation of the optimised extract was conducted through extrusion technology. Wet and lyophilised calcium alginate beads were examined and compared in relation to their TPC and entrapment efficiency based on the effects caused by storage conditions. High encapsulation efficiency values were obtained for the wet (69.8%) and for the lyophilised (97.7%) beads stored in the optimised extract. A high correlation was observed between the TPC of the optimised extract and its antioxidant activity values, determined in terms of the ability to scavenge free radicals and its ferric reducing ability (r=0.99 and 0.97, p<0.001, respectively). Lyophilisation significantly affected bead size and morphology and proved to be an appropriate method for preservation of encapsulated polyphenols. Confocal microscopy images showed that both the wet and the lyophilised beads exhibited an intense red fluorescence emission attributed to the phenolic constituents. Release of encapsulated polyphenols in water was measured to analyse the diffusion and kinetic behaviour of the system. Both the wet and the lyophilised beads showed stability in TPC and preserved antioxidant potential throughout 30 days of storage at 4 °C. Thus, encapsulation of stevia extracts in alginate beads is a promising technique for food supplementation with natural antioxidants.