Extraction of Terpenoids from Essential Oils

Scientists from the Zheijiang University of Technology in Zheijiang, China published a review article in the Journal of Chromatography A, pulling from two decades of research, on extracting terpenoids from essential oils and separating enantiomers (1).

essential oils and medical flowers herbs | Image Credit: © Olga Miltsova - stock.adobe.com

Essential oils are valuable metabolites extracted from parts of plants including leaves, seeds, bark, and roots (2). Different methods, such as steam distillation and solvent extraction, can be used to extract substances into oils (3). Essential oils can provide potential health benefits, with some scientists believing they can send chemical messages to parts of the brain that affect mood and emotion. These can help relieve stress, among other functions.

Essential oils consist of terpenoids, aromatic and aliphatic compounds. Terpenoids, which include terpenes and modified terpenes with additional functional groups, known as oxygenated terpenoids, make significant contributions to the physiological activity of essential oils. Oxygenated terpenoids contribute significantly to the aroma profile of essential oils. Considering the chemical properties of terpenoids, isolation of terpenes, and oxygenated terpenoids could improve the quality of the essential oils. Some terpenoids have chiral centers, which are atoms that are bonded with four different groups of elements and have nonsuperimposable mirror images (4). These centers can form enantiomers with distinct physiological activities. Extracting and separating terpenoid enantiomers has become important processes that have attracted extensive attention in recent years.

Further, the specific distribution and enantiomer excess results (the excess of one enantiomer over the other in a mixture of enantiomers) could be used as quality markers for illegitimate adulteration, origin identification, and exploring component variations and functional interrelations across different plant tissues.

The scientists reviewed extraction methods, which were then retrieved by resultant network visualization findings, with the predominant methods being hydrodistillation, solvent-free microwave extraction, headspace solid-phase microextraction (HS-SPME) and supercritical fluid extraction (SFE) methods. These novel techniques are mainly characterized by ecological friendliness and alignment with sustainable development concepts, all while having potential for being synergistically employed to increase target compound yields. Future development of more selective essential oil extraction procedures can help simplify subsequent processes, including separation, purification, and identification. Technologies, such as combining structural characteristics of compounds with conductor-like screening model (COSMO) models for creating specific deep eutectic solvents, as well as utilizing mechanochemical techniques combed with solid-phase reactions for selective extraction, have demonstrated effectiveness, with these practices holding promise for the future.

Increasing numbers of enantiomers are being isolated and identified, with more studies on biological activity and functional applications. Separating and identifying enantiomers from essential oils are generally achieved through gas chromatography–mass spectrometry (GC–MS); when used alongside various cyclodextrin-modified capillary columns, the enantiomers can realize chiral recognition. Furthermore, developing new modulators for optimizing comprehensive two-dimensional GC–MS (2DGC-MS) can further improve its analytical capabilities.

References

(1) Wang, Y.; Huang, J.; Lin, X.; Su, W.; et al. Recent Progress in the Extraction of Terpenoids from Essential Oils and Separation of the Enantiomers by GC–MS. J. Chromatogr. A 2024, 1730, 465118. DOI: 10.1016/j.chroma.2024.465118

(2) Begum, J. Dos and Don’ts of Essential Oils. WebMD 2023. https://www.webmd.com/skin-problems-and-treatments/ss/slideshow-essential-oils (accessed 2024-7-10)

(3) Untapping the Power of Nature: Essential Oil Extraction Methods. New Directions Aromatics Inc. 2024. https://www.newdirectionsaromatics.com/blog/articles/how-essential-oils-are-made.html (accessed 2024-7-11)

(4) Introduction to Chirality and Chiral Centers. Socratic Q&A 2024. https://socratic.org/organic-chemistry-1/r-and-s-configurations/introduction-to-chirality-and-chiral-centers (accessed 2024-7-10)