Influence of Molecular Mobility and Free Volume on the Oxidative Stability of Spray Dried Orange Oil

Abstract

The objective of this research was to determine whether molecular mobility and/or free volume influences the oxidative stability of spray dried orange oil. Encapsulated orange oil was prepared by spray drying using maltodextrin, gum acacia and HiCAP® 100 as carrier materials. The physical properties of the spray dried orange oil were characterized by: viscosity, particle size, moisture content, volatile retention, and absolute density. The molecular mobility was determined by glass transition, free volume was measured positron annihilation lifetime spectroscopy (PALS) and orange oil oxidation was determined by gas chromatography. The infeed viscosity and resultant particle size of the unloaded powders decreased as the molecular weight of the carrier material decreased. Differences between the viscosity of the unloaded and loaded emulsions was due to the addition of orange oil as it is less viscous than water and does not hydrate carrier materials yet contributes to the total percent spray drier infeed solids. The reconstituted emulsion particle size was much smaller than the infeed emulsion particle size due to the high shear of the atomizer in the spray drier. The moisture content and absolute density were independent of carrier material type. The moisture content increased with increasing water activity however, the absolute density was unaffected by a change in water activity. The volatile retention of the spray dried powders improved greatly upon the addition of emulsifier to the carrier wall material. The size of the molecular voids and orange oil oxidation decreased while molecular mobility increased with decreasing molecular weight of the carrier material. As the relative humidity increased, the average size of the molecular voids and the molecular mobility increased. The effect of water activity on the oxidation of spray dried carrier systems appeared to increase to a certain point and then decrease with increasing water activity. Spray dried carrier systems that were formulated as a blend of carbohydrate and emulsifier provided a balance between molecular mobility and molecular free volume that provided the greatest protection against the oxidation of spray dried orange oil.