Concentrating the chlorophylls in extract by pretreatment of stinging nettle leaves with nonpolar organic solvents and supercritical carbon dioxide

It is well known that stinging nettle leaves contain volatile compounds such as essential oils and nonvolatile compounds such as cuticular waxes, pigments (chlorophylls and carotenoids), vegetable oils and other solutes. Petroleum ether (PE) and hexane (Hex) have good solvent properties for the extraction of nonpolar components such as vegetable oils, essential oils, oleoresins and also carotenoids. Therefore, these nonpolar solvents have been used to pretreat the material in order to remove lipophilic substances, undesired oils and carotenoids, and to concentrate the chlorophylls in extracts obtained by re-extracting the residual material with ethanol. By pretreating the material with PE and Hex, the concentration of chlorophylls in ethanol extracts increased from 11.7 ± 0.52 gCHL/100 gextract to 17.3 ± 0.61 and 14.3 ± 0.95 gCHL/100 gextract, respectively. For comparison, extractions using supercritical carbon dioxide (SC-CO₂) at three temperatures and pressures were performed in order to remove lipophilic compounds and, consequently, to concentrate the content of chlorophylls in extracts obtained by re-extraction of the residual by conventional extraction. The results show much higher concentration of chlorophylls in extracts obtained from residual material when pretreating the material with SC-CO₂ (from 9.6 ± 0.41 gCHL/100 gextract to 19.9 ± 0.65 gCHL/100 gextract). There is a considerable demand for replacing the synthetic colorants in the food, drug and cosmetic industry. In this work, stinging nettle was selected as raw material because it is known as a rich source of chlorophylls and is a plant abundantly available worldwide. Chlorophylls are used as a green coloring agent (E140) extensively as a dye in coloring inks, resins, soaps, waxes, edible fats and cosmetics. The present work deals with the different techniques in order to concentrate the content of chlorophylls in extract and to increase the efficiency of isolation. It was shown that the best results were obtained by using supercritical CO₂ extraction for pretreating the material.