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

Foods containing polyunsaturated lipids are prone to oxidation. Designing food-grade hydrocolloidal encapsu-lation 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 alpha-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 alpha-tocopherol were characterised by differential scanning calorimetry (DSC), their structural properties were examined by X-ray diffraction (XRD). The miscibility between milk-SM and alpha-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 alpha-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 alpha-tocopherol, the bilayers were no longer in a gel phase below the phase transition temperature Tm, but in the liquid ordered Lo phase. Furthermore, the sphingosomes formed elongated structures in presence of alpha-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.

Automated summary

This study aimed to design food-grade hydrocolloidal encapsulation systems capable of loading lipophilic antioxidant molecules to prevent oxidation in food. The interactions between milk sphingomyelin (milk-SM) bilayers and alpha-tocopherol molecules were investigated using various biophysical techniques, and it was found that alpha-tocopherol molecules can insert into milk-SM bilayers and induce changes in their structure and properties. These findings have important implications for the development of novel encapsulation materials for preventing food oxidation.