Differences in Free Volume Elements of the Carrier Matrix Affect the Stability of Microencapsulated Lipophilic Food Ingredients

Fish oil was encapsulated by spray-drying into different matrices based on n-octenylsuccinate-derivatised starch (nOSA starch) and carbohydrate blends varying in dextrose equivalent and molecular weight profile. Based on the development of the hydroperoxide and propanal content upon storage significant differences in the stability of the microencapsulated oil were observed. With 40 mmol/kg oil the hydroperoxide content after eight weeks of storage at 20 °C and 33% relative humidity was lowest in fish oil encapsulated in a matrix containing nOSA starch and maltose. In contrast, the lowest stability was observed in fish oil encapsulated in a matrix based on nOSA starch and maltodextrin with a dextrose equivalent of 18. Physical characteristics like viscosity of the feed emulsion and oil droplet size, which influence drying behaviour as well as particle characteristics like particle size, density or surface area did not differ and thus cannot explain the differences in the rate of autoxidation. Positron annihilation lifetime spectroscopy clearly showed distinct differences in the ortho-positronium lifetime, and thus in the size of free volume elements between the carrier matrices. It is suggested that as a consequence, the matrices differed in oxygen diffusivity, which must be considered to be a key determinant in autoxidation of fish oil encapsulated in glassy carbohydrate matrices.