UC Davis

The solubilization and retention of aromas in foods can be improved using phospholipid vesicle dispersions which protect these aromas from evaporation, degradation, and chemical reactions. In this work, a quantification method was developed for the solubilization and retention of volatile aromas in phosphatidylcholine vesicles, with headspace solid phase microextraction (HS–SPME) combined with gas chromatography and mass spectrometry (GC/MS). This method allows us to selectively sample volatile compounds in the vapor phase using a sorbent fiber. HS–SPME is fast, accurate and non-invasive and allows for in situ measurements of partition coefficients for local regions within food products. With this approach, limonene partition coefficients were determined both between vapor and water and between vesicle bilayers and water. The unilamellar, nearly monodisperse vesicles were very effective at solubilizing limonene molecules, with large bilayer–water partition coefficients of ~104 M–1. These values are 3–8 times larger than those observed for short chain phospholipid micelles, which have smaller core volumes. In addition, vesicles can solubilize limonene up to very high mole fractions x_lim≤0.8, where x_lim is the moles of limonene relative to moles of limonene + phospholipid in the bilayers. This maximum solubilization is much greater than that measured for short chain phospholipid micelles (x_lim≤0.5). The bilayer–water partitioning behavior of limonene was investigated as a function of phospholipid composition and vesicle size, using lecithin enriched in phosphatidylcholine and pure dimyristoylphosphatidylcholine (DMPC). There was no significant difference in extent of solubilization whether vesicles were made using DMPC or lecithin, despite the fact that DMPC has saturated 14-carbon fatty acid tails, while lecithin is predominantly unsaturated with 18-carbon fatty acid tails. With an increase in vesicle diameter from ~100 to 200 nm, there was a marginal increase in solubilization for DMPC and a marginal decrease in solubilization for lecithin.The vapor phase concentration of limonene was used to analyze the mixing thermodynamics through calculations of chemical potentials. For a wide range of limonene–lipid mole ratios, constant partition coefficient values could quantitatively capture solubilization in the bilayer, as predicted by ideal–dilute mixing theory for the solute. At high limonene–lipid mole ratios, however, we saw evidence of non-ideal behavior with higher measured lipid–water partition coefficients. Using regular solution theory with an interaction parameter (χ_(PL,L)) of ~1 predicted this partitioning behavior quite well, suggesting that preferential solute–solute interactions may enhance solubilization at higher limonene concentrations. With saturated phospholipids, high mole fractions of limonene in the bilayer lowered the gel–to–fluid lipid phase transition temperature through entropic contributions that could be predicted quite well with freezing point depression theory. Gel, gel/fluid coexistence, and fluid regions were observed over small to large limonene–lipid mole ratios. In the gel/fluid coexistence region, we observed constant vapor phase concentrations of limonene. At low mole fractions of limonene, formation of the gel phase decreased partition coefficients at least two-fold in comparison to the fluid phase. Above the phase transition temperature, decreases in temperature also lowered the partition coefficients, but to a much lower extent. Gel phase partition coefficients were used to modify the freezing point depression model and predict the solubilization behavior of limonene at all compositions at 15 and 20ºC. At these temperatures, experimental data agreed with the theory very well for low total limonene concentrations.