Extraction of lipophilic compounds

Extraction is one of the key processing steps in isolation and purification of less hydrophilic or lipophilic compounds contained in plant materials.

Such plant constituents are referred to as “extractives”, or sometimes “secondary plant metabolites”. They dissolve in fats, oils, lipids, and non-polar solvents such as hexane, dichloromethane, toluene and others. Several classes of compounds can be extracted from plant materials, such as free fatty and resin acids, sterols, waxes, steryl esters, phenols, polyphenols, triglycerides, flavonoids, and many others.

ALICE core facility offers the most advanced techniques for extraction of lipophilic compounds, namely supercritical fluid extraction (SFE) and accelerated solvent extraction (ASE).

Supercritical fluid extraction, mostly operating with supercritical carbon dioxide (scCO₂), is considered to be the most accurate and gentle technique for isolation of thermo-sensitive extractive compounds. The unique peculiarity of this approach is the application of supercritical fluids, which possesses gas-like properties of better diffusion, lower viscosity, and negligibly small surface tension, and at the same time liquid-like density and solvation power. Due to the very low critical temperature and pressure (31°C, 74 bar), carbon dioxide is considered as one of the ideal solvents for SFE both for laboratory and industrial purposes, allowing to extract most of lipophilic compounds present in natural materials. The technique is advantageously used to extract lipophilic components from plant material in a “green” – solventless and especially mild – way.

Nowadays, extraction with scCO₂ is being used industrially for niche applications, such as coffee decaffeination and hop extraction, but more and more applications are emerging with the intent of replacing common organic solvents for extractions, such as hexane or heptane, of which the polarities are comparable. The increase of pressure leads to higher density and the solvent power is shifted towards more hydrophilic molecules. This way, the selectivity for compound classes to be extracted can be tuned, as shown in the following figure.  

Even larger scCO₂ polarity shifts are possible by means of a cosolvents added, a so-called “entrainer”, usually applied in the range of about 5%-10%. HPLC pumps allow exact dosage of an organic polar co-solvent during SFE (e.g. EtOH, MeOH, acetone), which is completely miscible with the stream of carbon dioxide.

“Cascade fractionations” using two or more separators can also be exploited for fractionation of interesting compounds. scCO₂ extraction does not leave any traces of solvent or other contaminations in the extract or in the starting material. Therefore, food-grade extracts can be obtained, which in turn can be committed to nutraceutical, pharmaceutical and cosmetics application sectors.

As an alternative to extraction with scCO₂, accelerated solvent extraction ASE (Dionex™ ASE™ 350) offers high efficiency and flexibility with regards to extraction conditions (temperature, solvents, extraction time), allowing not just extracting the sum of lipophilic compounds, but additionally fractionating them. ASE has almost no restrictions with regard to the organic solvents or solvent mixtures used as extractants. 

Accelerated solvent extraction offers unique opportunities for the extraction of solubilizable components from insoluble natural matrices, such as bark, pulp, lignin, wood, and lignocellulosic materials in general, with the starting material being efficiently depleted of the extractives but otherwise left chemically unchanged.

Very lipophilic components, such as triglycerides, sterol esters, or waxes, to medium polarity compounds, such as sterols, triterpenoids, fatty acids, resin acids, mono and diglycerides, to hydrophilic molecules, such as lignans, flavonoids, phenolics and other polyphenols, can be extracted according to the choice of solvent or solvent mixture and the setting of other extraction conditions. The pressurized extraction vessel (up to 100 bar) allows the extraction at temperatures higher than the normal-pressure boiling points of the extraction solvents, maximizing solubility and minimizing the extraction time.

References

1) Richter, B. E.; Jones, B. A.; Ezzell, J. L.; Porter, N. L.; Avdalovic, N.; Pohl, C. Accelerated Solvent Extraction:  A Technique for Sample Preparation. Anal. Chem. 1996, 68(6), 1033-1039, DOI: 10.1021/ac9508199.

2) Cho, S. K.; Abd El-Aty, A. M.; Choi, J. H.; Kim, M. R.; Shim, J. H. Optimized conditions for the extraction of secondary volatile metabolites in Angelica roots by accelerated solvent extraction. J. Pharm. Biomed. Anal. 2007, 44(5), 1154-8, DOI: 10.1016/j.jpba.2007.03.011.

3) de Melo, M. M. R.; Silvestre, A. J. D.; Silva, C. M. Supercritical fluid extraction of vegetable matrices: Applications, trends and future perspectives of a convincing green technology. J. Supercrit. Fluids 2014, 92, 115-176, DOI: 10.1016/j.supflu.2014.04.007.

4) Attard, T. M.; Bukhanko, N.; Eriksson, D.; Arshadi, M.; Geladi, P.; Bergsten, U.; Budarin, V. L.; Clark, J. H.; Hunt, A. J. Supercritical extraction of waxes and lipids from biomass: A valuable first step towards an integrated biorefinery. J. Cleaner Prod. 2018, 177, 684-698, DOI: 10.1016/j.jclepro.2017.12.155.

5) Chrastil, J. Solubility of solids and liquids in supercritical gases. J. Phys. Chem. 1982, 86(15), 3016-3021, DOI: 10.1021/j100212a041.

6) Reverchon, E.; De Marco, I. Supercritical fluid extraction and fractionation of natural matter. J. Supercrit. Fluids 2006, 38(2), 146-166, DOI: 10.1016/j.supflu.2006.03.020.

7) Attard, T. M.; Theeuwes, E.; Gomez, L. D.; Johansson, E.; Dimitriou, I.; Wright, P. C.; Clark, J. H.; McQueen-Mason, S. J.; Hunt, A. J. Supercritical extraction as an effective first-step in a maize stover biorefinery. RSC Adv. 2015, 5(54), 43831-43838, DOI: 10.1039/c5ra07485a.