## 3 Results
### Rheological Characterisation of Gels Stabilised with Birch Bark Extracts  
Jojoba oil stabilised with OS-E, SCF-E, R-E and by the modified RESS (MR) procedure extract showed gel characteristics (examples in Figure 5), as indicated by the storage modulus (G') being larger than the loss modulus (G') in the linear viscoelastic region of the flow curve. A comparison between gels, stabilised by the different types of extract, confirmed a higher gel strength for those gels stabilised with extracts obtained by supercritical fluid technology (Figure 6) compared to gels formed with OS-E. The OS-E gel, SCF-E gel and RESS-E gel were stabilised with 6% of dry extract, whereas the extract content of the gels produced by the modified RESS-M process remains undefined. All three supercritical fluid-based particle deposition methods gave extracts that lead to gels with a gel strength of more than 8500 Pa. The OS-E was only able to form gels with a gel strength  
Figure 4: Triterpene composition of fractional RESS extracts; \(n=2\)–\(3\), mean \(\pm\) SD or span width. Figure 3: Triterpene composition of birch bark extract obtained by different extraction methods; mean \(\pm\) SD. of less than 1000 Pa (Figure 6). Different extract surface properties resulting from the fast particle deposition from the supercritical solution could be the reason for the ability to form stronger gels. To investigate the gelling speed of the gels an amplitude sweep was performed every 30 min, starting immediately after the gel production. For the SCF-E gel, the G' values started and remained at a higher level compared to the OS-E gel. As a reference for almost complete gelling, an amplitude sweep after 24 h was performed. The SCF-E gel, however, reached this reference point already within six hours, whereas the OS-E gel only reached 28% of the reference value (Figure 7). This experiment could not be performed with the two RESS-extracts due to the design of the RESS-system. Figure 5: Amplitude sweep of gels stabilised by different types of birch bark extract. Figure 6: Gels stabilised by different types of birch bark extract (gel strength: amplitude sweep (AS) after 24 h); mean \(\pm\) SD. For a gel, a dissipation factor less than one is typical. Studying the dissipation factor during the first six hours of gel formation (AS; LVE) confirmed differences between a SCF-E gel and an OS-E gel. Directly after gel production, the SCF-E gel showed a dissipation factor of 0.12 \(\pm\) 0.02, which slightly increased during six hours. The OS-E gel started at 0.38 \(\pm\) 0.04, the value decreased over time (Figure 8). The SCF-E particles obviously were able to form a stable particulate network direct after homogenising and therefore faster, compared to the OS-E particles. Again, this might hint at differences in extract particle size and surface properties between the different types of extract, which enables a faster formation of a network structure in the oil. After three hours, both types of gels were on the same dissipation level. To study the minimum extract content required for gel formation, gels with different amounts of birch bark extract were produced. A decrease of extract content caused a decrease in gel strength, especially for the OS-E gel. With two percent OS-E it was hardly possible to produce a gel, and gel strength dropped under 3 Pa, whereas one percent SCF-E was enough to obtain a gel with over 250 Pa (Figure 9). Figure 8: Dissipation factor (amplitude sweep) during the first six hours of gel formation for organic solvent and supercritical fluid extract gel (six percent extract in oil); mean \(\pm\) SD. Figure 7: Gel strength (amplitude sweep) during the first six hours of gel formation for organic solvent and supercritical fluid extract gel (six percent extract in oil); mean \(\pm\) SD. The dissipation factor for the SCF-E gel remained constant over an extract content range from one to five percent. For the OS-E gel a smaller amount of extract had a negative influence on the gel character.