Impact of storage on the physico-chemical properties of microparticles comprising a hydrogenated vegetable oil matrix and different essential oil concentrations.

Microparticles made from hydrogenated sunflower oil without essential oil and with different essential oil concentrations (75-300 g/kg; g of essential oil per kg of microparticles) were stored for 1 or 2 months at 25 or 37 °C. Before and after storage the essential oil concentration, flowability, optical appearance, melting behaviour and crystalline structure of the microparticles were investigated. Essential oil recovery, melting behaviour and crystalline structure were identical for the essential oil containing microparticles and were not affected during storage. The surface structure of the microparticles varied with their essential oil concentration. While the particles containing 75 g/kg essential oil were covered by erect fat crystals, those with 225 g/kg and higher were mostly smooth with some round shaped dumps. However, the surface of all essential oil containing microparticle batches had reached their final stage after production already and did not change during storage. Microparticles without essential oil presented two melting peaks; all microparticle batches with essential oil had one peak. Peaks in the X-ray scattering powder diffraction signal of the essential oil-free microparticles after production can be associated with the α-form of the hydrogenated vegetable oil. During storage, a conversion of the α-form to the stable ß-form was observed. Microscopy showed that these microparticles also developed strong fat crystals throughout storage. The triglycerides in microparticles with essential oil seem to directly take on the stable ß-form. The formation of robust microparticle agglomerates during storage was prevalently observed for the fat crystal forming product batches, meaning the products without or with low essential oil concentration.

Formation of essential oil containing microparticles comprising a hydrogenated vegetable oil matrix and characterisation thereof.

Microparticles with different essential oil concentrations 0, 75, 150, 225 and 300 gkg-1, (g of essential oil per kg of microparticles), were produced by dispersing the essential oils within a hydrogenated vegetable fat matrix and forming spherical solid particles by spray-chilling. Size distribution, flowability, surface structure, essential oil recovery, melting properties and crystallinity of the microparticles were determined. With over 225 gkg-1 essential oil the microparticle surface became stickier, their flowability was reduced and the size distribution broadened. Gas chromatography showed that the essential oil recovery was always above 85% v/v. The surface structure of the microparticles was strongly affected by the essential oil concentration being smooth (225 gkg-1), comprising round-shaped dumps (300 gkg-1) or showing fat blooming (0, 75, 150 gkg-1). With essential oil, the formation of the ß-polymorphic form of the triglycerides was supported leading to changes in the melting behaviour and the crystalline structure.