High pressure extraction of plant materials with near critical liquid carbon dioxide

Different plant materials including thyme (Thymus vulgaris L.), Kekik (Thymus longicaulis C Preisl.) and Savory (Satureja montana L.), were extracted by liquid CO₂ under liquid-vapour conditions near its critical conditions. The apparatus used is shown in figure 1. The liquid CO₂ extracts were compared with hydrodistillates using Thin Layer Chromatography as described in British Pharmacopoeia [1]. Some compounds were identified based on the R_f values given in [1]. For each plant was observed, that every compound present in hydrodistillate, was also present in the CO₂ extracts, and the chromatograms of CO₂ extracts showed the presence of other unidentified compounds as well. The quantitatively determined yields of the CO₂ extractions were 2.5 to 4.9 times larger than the yields of hydrodistillations. For the extraction of the plant materials approximately 4 hours were needed and for the hydrodistillation 6 hours. The yield of the extractions as function of time could be described with a simple equation.

Figure 1: Apparatus for the high pressure extraction with liquid CO₂ under liquid – vapour equilibrium conditions.
1. Cooling finger; 2. Pressure gauge; 3. Sapphire Window; 4. Valve; 5. Upper cover; 6. Steel Cylinder; 7. Bottom cover; 8. Extracting thimble; 9. Syphone; 10. Product droping glass; A. Product; B. Extract solution; C. Plant material

Reference: [1] Bernan, British Pharmacopoeia 2003 & British Pharmacopoeia Veterinary, Bernan Association, Monographs: Medicinal and Pharmaceutical substances Thyme Oil, f.1534.