Coating of oil-loaded alginate core with chitosan by polyelectrolyte complexation

Abstract

In food industries, many active compounds are lipophilic in nature. These compounds can be encapsulated to protect them from evaporation or oxidation, to control the release of the compounds and to facilitate the incorporation of the compounds in aqueous medium. Alginate, a hydrophilic polyanionic biopolymer derived from plant, can used as an encapsulant to entrap the oil under mild conditions. For certain food and beverages applications, the beads containing oil are stored in liquid medium before use or they are incorporated into a liquid product. This may cause oil leakage from the beads to the storage medium and thus, reducing the oil content or contaminating the quality of the liquid medium. This research aimed to encapsulate high oil-loaded alginate beads, coat them with chitosan and invesigate how well oil is retained within the chitosan-coated beads during storage in liquid medium. Preliminary research includes the preparation of a stable alginate-based O/W emulsion with monodisperse oil droplet size. Non-ionic surfactants (i.e., Tween 80 and Span 20) were incorporated in the alginate based O/W emulsion. The synergistic effects of alginate and surfactants showed improvement on the O/W emulsion properties which were evaluated in terms of oil droplet size and emulsion stability. The size distribution of oil droplets was narrow and monomodal at 2 %w/v of alginate and 1 %w/v of surfactants, even at an oil loading of 70 %v/v. The emulsions formed were also stable. Subsequently, the alginate based O/W emulsion was extruded into a gelation bath containing calcium ions forming encapsulated alginate beads containing oil and was coated with chitosan. The O/W emulsion formulation containing alginate and surfactants blend yielded beads with high encapsulation efficiencies (i.e., > 98%), with or without coating. The SEM images showed rough and porous surface on 50 %v/v beads suggesting less alginate-chitosan interaction compared to 10 %v/v beads which had smooth surface. From the confocal images, more chitosan was bound on the surface of alginate core containing lower oil loading and when 2-step coating process was used, thus resulting in a thicker and denser coating. This result was consistent with the mechanical strength data where thicker coating ruptured at higher applied force. Surprisingly, thicker coating resulted in higher oil leakage during storage. This result indicates that the increased chitosan-alginate binding generated higher internal pressure within the beads. Consequently, more oil were squeezed out from the beads during storage. Under the experimental conditions, the 1-step coating process gave the lowest oil leakage (< 0.1% at 22 days of storage) at both 10% v/v and 50 % v/v oil loading, compared to the beads without coating or beads coated using a 2-step process. In conclusion, an O/W emulsion formulation that allows high encapsulation efficiency has successfully been developed and the oil leakage during storage has effectively been minimized by chitosan coating.