Gellan hydrogels: preparation, rheological characterization and application in encapsulation of curcumin

Abstract

Hydrogels can be used to protect some labile active principles, as polyphenol-rich substances, that can be added to foods to prepare functional ones. Rheological properties of gels formed through the addition of calcium chloride to gellan solutions were studied. It can be concluded that preparation variables and not only formulation ones are determinant in rheological properties of the resulting gels, as they are not in an equilibrium state but they are continuously evolving during hours to stronger gels corresponding to a denser network. It could be related to the fact that local non-gelled domains are formed surrounded by a shell of gel where Ca2+ ions take some time to arrive. A minimum Ca2+/gellan ratio (CG) is required to reach the gel point (GP), determined as the CG where the ratio loss modulus/elastic modulus (G^/G') collapse for all frequencies. Calcium-induced external gelation of oil-in-water (O/W) emulsions where a curcumin-in-oil solution is the disperse phase and a watery solution of gellan is the continuous phase was used to prepare beads were curcumin is entrapped in order to prevent its degradation. Smaller droplet-sized emulsions were obtained with higher gellan concentrations, since a higher viscosity of the continuous phase allowed to reach the critical Capillary number CaC at lower radius of droplets. An encapsulation yield around 90% was reached for gellan concentrations of 1% w/v, and the resulting encapsulated curcumin presented around 6 times slower light degradation than free curcumin-in-oil solutions.