Local distribution of limonene in phospholipid vesicles

The efficacy and quality of food products is affected by the distribution of hydrophobic solutes such as flavours and bioactive compounds. In order to improve food design, it is important to determine the local distribution of these solutes and the factors that affect their stability, incorporation and release. Colloidal assemblies of phospholipids are of particular interest, as they comprise safe, widespread natural amphiphiles that can solubilize hydrophobic compounds. However, there is a lack of accurate and non-destructive methods to study the local distribution of solutes between the sample matrix elements, the aqueous phase, and the vapor phase, making it difficult to assess the effect of structure on stability and release. Short time headspace microextraction allows us to determine the local distribution of hydrophobic solutes and the effect of colloidal structure while keeping the system intact. Using thermodynamic relationships, the detected concentrations of compounds in the vapour phase are used to determine local properties within the sample matrix. The colloids of focus in this study were phosphatidylcholine vesicles which were used to extend our previous work on micellar solutions by developing a quantification method for the solubilization and retention of volatile nonpolar compounds in vesicles. The local partitioning of the aroma molecule, limonene, was investigated in vesicles of various structures, lipid compositions, and at different temperatures. Vesicles were found to be much more effective at solubilizing limonene than short-chain phosphatidylcholine micelles. They yielded vesicle-water partition coefficients of ~104M–1 while the micelles had micelle-water partition coefficients of ~103M-1. Lipid composition and vesicle size did not have a significant effect on the partitioning properties, however, reducing the limonene concentration in the vesicles lowered the partition coefficient, suggesting some interaction effect at higher limonene concentrations. In addition, with saturated lipids, limonene fluidizes the gel membrane and lowers the phase transition temperature.