Local distribution of limonene in phospholipid vesicle dispersions

Abstract: The efficacy and quality of food products is affected by the distribution of hydrophobic solutes such as flavours and bioactive compounds. 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 are safe and widespread natural amphiphiles that can solubilize hydrophobic compounds. Short-time headspace microextraction is used to determine the local distribution of hydrophobic solutes and the effects of structure, while keeping the system intact. Through thermodynamic relationships, the detected concentrations of compounds at equilibrium in the vapour phase are used to determine local properties within the sample matrix. We have extended our previous work on micellar solutions by developing a quantification method for the solubilization and retention of volatile nonpolar compounds in phosphatidylcholine 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 that were 3–8 times larger than micelle-water partition coefficients. Experiments done with fixed phospholipid concentrations show that vesicles can solubilize limonene up to solute-lipid mole ratios of ~3 before the system becomes saturated, compared to <0.9 for micelles. With saturated lipids, a higher concentration of limonene fluidizes the gel membrane and lowers the phase transition temperature. Therefore, when this effect was removed by using sufficiently low concentrations of limonene, a decrease in the partition coefficient for vesicles in the gel phase was observed at lower temperatures. When above the gel transition temperature, however, lipid composition and vesicle size did not have a significant effect on the partitioning properties. Increasing the limonene concentration in the vesicles increased the partition coefficient, suggesting some interaction effects at higher limonene concentrations.