## 4 Discussion  
Birch bark extraction with supercritical carbon dioxide, as an alternative to conventional organic solvent extraction, turned out to be a promising method. Employing supercritical fluid extraction and subsequent separation of the extract in a separation vessel provided acceptable extraction yields, being comparable to other SCF-methods [51]. RESS-extraction by spraying the supercritical solution through a nozzle was successful, but showed a poor extraction yield due to the limited carbon dioxide mass flow. This low yield might, however, also be affected by the entrainment of submicron particles by the CO\({}_{2}\) stream. A direct production of a gel by spraying the supercritical solution through a nozzle directly into jojoba oil (modified RESS procedure) would here be preferential and turned out to be possible, giving stable gels. The control of this process has, however, to be improved by adaption of the pilot plant with regard to the risk of ice formation during expansion. Repetitive extraction of the same material with supercritical carbon dioxide provided extracts with a different triterpene composition. Early fractions contained less betulin and more lupeol, compared to an organic solvent extract. The betulin content increased with further extraction steps whereas the lupeol content and the content of unidentified substances decreased. By this method, the supercritical fluid extraction allows the production of extracts with different triterpene compositions. This might give extracts with different pharmacological and stabilizing properties. The more important finding is, however, that obviously lupeol and the unidentified compounds dissolve faster in supercritical carbon dioxide than betulin, giving an extract composition which is, at least in part, different from the organic solvent extract. Extracts from supercritical fluid extraction showed better and faster gelling abilities, compared to the organic solvent extract. Gels challenged by an amplitude sweep for rheological characterization showed much higher modulus values when the stabilisation was performed with supercritical fluid extracts from all tested particle deposition methods compared to the gel stabilised by the organic solvent extract. For these gels, the gel character, indicated by a dissipation factor less than one, was detected instantly after the gel production, whereas the gel stabilised by organic solvent extract needed several hours to reach the same level. The minimum extract content, necessary for gelling, was lower for the supercritical fluid extracts. This indicates a more effective formation of the network structure in these gels. The supercritical fluid extracts differ from the organic solvent extract in several ways. First of all, the specific area of the particles is higher, compared to the organic solvent extract. This could be a reason for a better particle to particle interaction and an improved interaction of the liquid phase (jojoba oil) with the particle surface and thereby the formation of the gel network. The dispersive surface energy, however, was less when compared to the organic solvent extract. That might be due to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Extract** & \begin{tabular}{c} **Specific Surface Area** \\ **(BET) (m2/g)** \\ \end{tabular} & \
\begin{tabular}{c} **Dispersive Surface** \\ **Energy (m/m2)** \\ \end{tabular} &
\begin{tabular}{c} **Gel Strength (Pa)** \\ \end{tabular} \\ \hline OS-E & 29.6 \(\pm\) 0.3 & 62.42 \(\pm\) 0.33 & 716 \(\pm\) 278 \\ SCF-E1 & 27.9 \(\pm\) 1.3 & 51.59 \(\pm\) 0.47 & 4486 \(\pm\) 888 \\ SCF-E2 & 37.3 \(\pm\) 0.1 & 51.67 \(\pm\) 0.59 & 2211 \(\pm\) 245 \\ SCF-E3 & 36.2 \(\pm\) 2.0 & 51.27 \(\pm\) 0.65 & 11,081 \(\pm\) 1540 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Surface characterisation of birch bark extract obtained by different extraction methods. the micromorphology of the supercritical carbon dioxide-treated particles, having an obviously more polar surface structure.