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 degrees 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 alpha-form of the hydrogenated vegetable oil. During storage, a conversion of the alpha-form to the stable beta-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 beta-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.