Different techniques have been successfully used for pre- paring essential oil-loaded microcapsules. Two of the most successful methods apart from spray drying are freeze- drying and coacervation. Other techniques such as annular- jet or spinning-jet drying can be used to produce particles loaded with liquids such as essential oils. Table 3 shows some applications of these techniques for essential oil formu- lation [34-38, 61-75].
In the freeze-drying process solvent is removed from a frozen solution by vacuum sublimation, maintaining the dry- ing chamber pressure and temperature bellow the triple point of solvent. Freeze drying appears as one of the most suitable methods for dehydration of almost all heat-sensitive materi- als and aromas, due to the lower operating temperatures, slow drying rate and to the use of vacuum. This technique has also been used to encapsulate essentials oils [37, 61].
Coacervation consists in the separation of a polymeric solution into two liquid phases, a polymer-rich phase called coacervate and diluted phase called equilibrium solution, induced by media modifications (pH, ionic strength and polyion concentrations), followed by the coating of the coac- ervate phase around suspended core particles or suspended droplets. Complex coacervation is based on electrostatic in- teractions between two oppositely charge polyions in aque- ous media, in which the core material in dispersed form is added to the polymer solution. This mixture is then sus- pended in an aqueous solution phase containing a surface- active agent. Finally, solidification of the coating is achieved by thermal, cross-linking or desolventization techniques. Another closely related technique is the organic phase sepa- ration, which can be considered as a reversed simple coacer- vation: a polymer phase separates and deposits on a core that is suspended in an organic solvent rather than in water Sev- eral essentials oils were encapsulated by coacervation em- ploying gelatine [37, 62], proteins [63] and other polymers [64].