Does drying or distillation change the scent of patchouli?

Does patchouli, familiar from perfumery, already smell like its essential oil the moment its leaves are picked? Drying the leaves and then distilling them may do more than simply extract their constituents.

Toshio Hasegawa and colleagues, in a study published in 2017, compared extracts of fresh and dried leaves with essential oil obtained from dried leaves. Their aim was to explore why processing changes the impression of the scent by examining differences in the presence and relative proportions of compounds.

Fresh leaves, two weeks of shade drying, and essential oil

The study used patchouli leaves obtained in Indonesia. Dried leaves were prepared by drying fresh leaves at room temperature for two weeks out of direct sunlight.

Compounds were extracted separately from fresh and dried leaves using the solvent hexane. Essential oil was also obtained by steam distillation of dried leaves. The comparison therefore involved three materials: fresh-leaf extract, dried-leaf extract, and essential oil from dried leaves. It was not a direct comparison of compounds naturally released into the air by the leaves.

The authors reported that each extract smelled similar to the original leaves, and that fresh and dried leaves differed in scent, as did dried leaves and essential oil. Sensory evaluations were conducted by untrained participants in two laboratories, in Japan and Indonesia.

The abundance of a major compound does not fully explain the difference

Patchouli alcohol was the first prominent finding in the analysis. It is one of patchouli’s characteristic constituents and accounts for a substantial proportion of the samples. Analysis of the whole mixtures, however, did not adequately reveal the finer differences in composition corresponding to the perceived differences in scent.

The researchers therefore distilled small amounts of the extracts under reduced pressure, separating portions that emerged over different temperature ranges. These separated portions are called fractions. Dividing the mixture made it easier to compare minor constituents that could otherwise be obscured by abundant ones.

They selected one fraction resembling the scent of fresh leaves and two resembling that of dried leaves for detailed comparison. This approach narrowed the search to fractions with similar smells before examining their compositional differences.

Using the nose while comparing differences and ratios

The study used gas chromatography-olfactometry, or GC-O: compounds are separated by gas chromatography, and a person smells them at the outlet. Gas chromatography-mass spectrometry, or GC-MS, was also used to identify constituents from their characteristic mass spectra and their positions as they emerged from the instrument.

In selecting candidates, the researchers considered compounds found only in particular samples, as well as changes in the relative proportions of shared constituents. Here, a proportion means a peak’s share of the instrument’s total peak area. It is not a share of the scent contribution or of the intensity perceived by a person.

The researchers examined both differences and ratios between these proportions. For an abundant compound, a large difference in percentage points may still represent only a small fold change. Conversely, a trace constituent may change several-fold despite a small percentage-point difference. Looking at both was intended to capture changes in major and minor constituents.

Fourteen candidates associated with the scent of dried leaves

Comparing fresh- and dried-leaf fractions yielded 14 candidate constituents potentially important to the dried-leaf scent. Seven were structurally identified. The chemical structures of the remainder were not established.

The identified compounds included not only patchouli alcohol but also α-patchoulene, seychellene, α-bulnesene, humulene, isoaromadendrene epoxide, and caryophyllene oxide. This suggests that differences in dried-leaf scent cannot all be assigned to a single characteristic major constituent.

For example, patchouli alcohol’s relative peak area in the whole extracts was approximately 46% in fresh leaves and 38% in dried leaves. By contrast, α-bulnesene changed from approximately 6% to 21%. These are relative distributions within mixtures; they do not directly indicate changes in the absolute amount present in an individual leaf.

The candidate compounds change again in the essential oil

Comparing dried-leaf extract with the essential oil distilled from it yielded 12 candidates potentially important to the oil’s scent. Nine were identified. The 14 dried-leaf candidates and 12 essential-oil candidates are sets obtained from two comparisons, not numbers that should simply be added together.

Five constituents were shared by both sets: patchouli alcohol, α-patchoulene, seychellene, α-bulnesene, and isoaromadendrene epoxide. The essential-oil candidates additionally included cis-thujopsene and corymbolone, among others.

Patchouli alcohol accounted for approximately 60% of the essential oil’s relative peak area, compared with approximately 38% in the dried-leaf extract. The authors suggest that even when compounds are shared, differences in their proportions and combinations with other constituents contribute to the differences in scent.

Does heat alone cause the change, or does it happen within the plant?

The researchers also heated an extract at 100°C for one day. Under those conditions, no clear change in the odor constituents was observed.

They therefore considered that heating the extract alone could not explain the differences and that changes within the leaf tissue might be important. They proposed that plant enzymes might be involved during drying, and that enzymes or a combination of enzymes and boiling water might play a role during distillation.

However, the experiment did not measure which enzymes produced which compounds, or test whether inhibiting enzymes prevented the changes. The observed differences before and after processing are one level of evidence; the mechanisms of the reactions that produce them are another.

The candidate compounds still need testing by reconstruction

The method compared similar-smelling fractions and compositional changes to narrow down potentially relevant constituents. It did not test whether blending the candidates could reproduce the original scent or whether removing a particular compound would eliminate a characteristic. Thus, not every candidate was established as indispensable to the scent.

The main text also does not give the number of sensory participants or detailed statistical comparisons of their evaluations. Some candidates remain unidentified, and the findings cannot be generalized without qualification to other drying conditions, growing regions, or distillation methods.

Conclusion

In patchouli, both drying fresh leaves and producing essential oil from dried leaves changed the impression of the scent and the distribution of constituents. The study narrowed potentially relevant compounds to 14 candidates for dried leaves and 12 for essential oil.

Its significance lies in considering minor constituents and manufacturing steps rather than explaining patchouli’s character through one major compound alone. Even within the same plant species, drying, extraction, and distillation help shape the scent that reaches us. Identifying the remaining candidates and testing the reaction mechanisms are the next steps toward a more specific understanding of the relationship between processing and scent.

References

Hasegawa, T., Yoshitome, K., Fujihara, T., Santoso, M., & Aziz, M. A.(2017). Characteristic Changes in the Aroma Profile of Patchouli Depending on Manufacturing Process. Journal of Oleo Science, 66(8), 863–869. DOI: 10.5650/jos.ess17002. Full text (J-STAGE, PDF). © 2017 Japan Oil Chemists’ Society.