Even when both are called rosemary oil, distilled essential oil and oil made by soaking leaves in vegetable oil do not necessarily have the same contents. What the constituents dissolve in and the conditions under which they are extracted change the resulting material.
A study published by Maha Mohamed Soltan and colleagues in 2024 compared hydrodistillation, maceration in olive oil, and extraction with supercritical carbon dioxide. It examined the combinations of constituents obtained and their effects on cultured microorganisms.
Three methods produce different forms of extract
The researchers used dried rosemary sold in Egyptian markets. The reference method was hydrodistillation: heating the plant material with water and cooling the rising vapor to collect essential oil.
The second method involved soaking ground plant material in olive oil for 24 days at room temperature. The product was olive oil containing dissolved rosemary constituents, not isolated essential oil.
The third method used pressurized carbon dioxide as a solvent. This study used 40°C and pressures of 100–300 bar, producing a paste-like material. It is important that the comparison involved three products of different character: hydrodistilled essential oil, macerated olive oil, and a paste-like extract.
Shared major constituents, but different proportions
GC–MS analysis of volatile constituents showed that all three extracts contained a large proportion of 1,8-cineole, with camphor also a major constituent. However, their proportions of measured signal area differed.
The proportion of 1,8-cineole was approximately 51% with hydrodistillation, 38% with supercritical CO₂ extraction, and 63% with oil maceration. α-Pinene was prominent in the hydrodistilled and macerated oils but less abundant in the supercritical CO₂ extract, whereas borneol was more abundant in the supercritical CO₂ extract.
These are relative proportions among the analyzed constituents. The 63% figure for oil maceration does not mean that 63% of the bottle’s contents was cineole. Nor does a higher proportion of a constituent mean that its odor is perceived as proportionally stronger. The study did not conduct sensory comparisons with human participants.
Some minor constituents detected in the hydrodistilled oil, such as p-cymene, were not detected with the other methods. The authors suggest that differences in solubility in oil and extraction conditions affect which constituents can be recovered.
“70% extracted” was an estimate based on what remained
Hydrodistillation yielded 1.4 mL of essential oil per 100 g of dried material. Hydrodistilling the plant residue after oil maceration yielded the equivalent of 0.42 mL remaining. The difference, 0.98 mL, was therefore estimated to have transferred into the oil, corresponding to approximately 70% of the reference amount.
This was not a direct measurement made by separating all the essential oil from the olive oil. The paste-like supercritical CO₂ extract was reported by mass, so its value cannot simply be ranked against essential oil volumes to determine which method yielded the most.
The authors propose that recovery through oil maceration might be improved by examining mixing, particle size, temperature, and other factors. The study did not compare and optimize all the conditions proposed.
Read antimicrobial effects in the context of the test conditions
For microbial testing, paper discs containing samples were placed on culture plates with four bacterial species, two yeasts, and two molds. The researchers measured the diameter of any zone around each disc in which microorganisms did not grow.
Under the conditions tested, all three extracts produced inhibition zones against bacteria and yeasts, with the supercritical CO₂ extract tending to produce larger diameters. In contrast, only the hydrodistilled essential oil produced zones against the two molds; none were observed with the macerated oil or supercritical CO₂ extract.
However, the essential oil was diluted in olive oil, the macerated oil was used as obtained, and the supercritical CO₂ extract was dissolved in another solvent. The amounts were not expressed on a uniform basis either. Because solvents and diffusion through the plate influence the result alongside composition, these diameter differences cannot directly establish a ranking of antimicrobial potency or multiples of medicinal effectiveness.
The authors suggest that minor constituents and combinations of compounds may contribute to differences among microorganisms. However, they did not establish the causes through experiments such as removing individual constituents.
Summary
Despite sharing the plant name rosemary, changing the extraction method changed both constituent proportions and the form of the product. The authors conclude that oil maceration merits consideration as a way to incorporate fat-soluble constituents, while supercritical CO₂ extraction offers another route to a composition different from that obtained by distillation.
The paper also discusses potential applications in cosmetics and infectious disease, but the measurements concerned composition and effects on culture plates. It did not establish therapeutic effects in humans, skin safety, or the safety of home preparation and long-term storage. Advancing these applications requires selecting extraction conditions suited to the purpose and testing effects with product form and concentration standardized.
References
Soltan, M. M., Mahfouz, S., Motawe, F. H., Karam, E. A., & El-Hagrassi, A. (2024). In sight on olive oil maceration and supercritical CO₂ in extracting rosemary essential oil. Scientific Reports, 14, 29730. DOI: 10.1038/s41598-024-79832-y / Full original article.
Chemical analysis was supported by the National Research Centre in Egypt, and publication costs by STDF and the Egyptian Knowledge Bank. The authors declared no competing interests. Original article: © 2024 The Author(s), CC BY 4.0.
