Milk sphingosomes as lipid carriers for tocopherols in aqueous foods: Thermotropic phase behaviour and morphology

Abstract

Foods containing polyunsaturated lipids are prone to oxidation. Designing food-grade hydrocolloidal encapsulation systems able to load lipophilic antioxidant molecules, such as tocopherols (vitamin E), is necessary to prevent oxidation and its deleterous consequences. In this study, we hypothesised that α-tocopherol molecules could incorporate in a host membrane composed of milk sphingomyelin (milk-SM) and performed a multi-scale biophysical study. The thermal properties of milk-SM bilayers with various molar proportions of α-tocopherol were characterised by differential scanning calorimetry (DSC), their structural properties were examined by X-ray diffraction (XRD). The miscibility between milk-SM and α-tocopherol was investigated in mixed Langmuir monolayers. The morphology of milk-SM sphingosomes was observed by confocal laser scanning microscopy (CLSM). We found that molecules of α-tocopherol inserted into the milk-SM bilayers and induced a physical desorganisation in the membrane packing, both in the ordered and fluid states. In the presence of α-tocopherol, the bilayers were no longer in a gel phase below the phase transition temperature T_m, but in the liquid ordered L_o phase. Furthermore, the sphingosomes formed elongated structures in presence of α-tocopherol as a result of membrane softening and changes in the bilayer curvature associated to membrane fusion. The findings of this work contribute in a better understanding of the capacity of milk-SM bilayers to incorporate guest molecules. Milk-SM sphingosomes loaded with tocopherols could be used to prevent oxidation in aqueous foods containing polyunsaturated lipids such as oil-in-water emulsions.

Keywords: Encapsulation; Lipid membrane; Lipophilic antioxidant; Liposome; Milk polar lipid; Milk sphingomyelin; Phase transition; Vesicle.