Analysis of Hydrosol of Quaking Aspen
Alexis St-Gelais – Research Notes
Two batches of young male flowers of quaking aspen (Populus tremuloides) were submitted to hydrodiffusion. The resulting hydrosol was analysed by gaz chromatography. The main constituents of sample #1 were 1,2-cyclohexanedione, characteristic of poplars, as well as benzyl alcohol and benzoic acid, while sample #2 was dominated by benzyl benzoate. The identified metabolites are in compliance with those previously reported for poplars. The chemical composition of the aroma of quaking aspen flowers was never reported previously.
When we have the opportunity, we appreciate prospecting the boreal forest for original fragrances. If the aroma of balsam poplar is rather familiar, it is not the case of quaking aspen (P. tremuloides). We have noticed that the male flowers, just after bud burst (figure 1), have a subtle peppery and vanilla scent. We thus collected some in April 2013 and tried to extract an essential oil from it by hydrodiffusion. However, the process only yielded an hydrosol, which a less pleasant aroma. We analysed it out of curiosity. Two samples were analysed by GC-FID, and the identification of the compounds was validated by GC-MS except for vanillin which was found only in GC-FID from its retention indices. Sample #2 was only analysed for a quick comparison.
The results explain well why not oil was obtained (Table 1). Indeed, the volatile compounds from quaking aspen bear oxygen atoms (except for α-farnesene), which makes them relatively soluble in water. Therefore, by mere hydrodiffusion, no oil can be collected. The test was done twice: batch #1 was collected in Chicoutimi on April 8, 2013, while lot #2 was collected in Alma on April 20, 2013.
Table 1. Volatile compounds of the hydrosol of male flower buds of P. tremuloides. Contents in mg/L, expressed in equivalent of tetradecane per litre.
The difference between the two samples is clear, and can easily be seen by comparing the two chromatograms (Figure 2). In sample # 1, there are several abundant compounds. Benzyl alcohol and benzoic acid appear to have been esterified in sample # 2, dominated by benzyl benzoate. This could be caused by the different sampling times, but also by genetic or environmental variations between the two populations of aspens. However, the seasonal hypothesis seems plausible, since the flowers of this species change rapidly through April.
Two compounds (A and B), whose structure is shown in Figure 2, are of interest. They are in fact characteristic of poplars. Compound A was for instance observed in buds of P. nigra (1) and P. simonii leaves (2). Compound B was found in the branches of P. balsamifera, where it contributes to repel herbivores (3). These compounds result from the degradation of glycosides of salicylic and gentisic acids (4), encountered in many poplars, including quaking aspen (5). The presence of salicylic acid (the active form of aspirin) and analogues is also evident from the presence, in the hydrosol, of salicylaldehyde and benzyl salicylate. This is not surprising, since a priori all poplars and willows contain similar molecules, which protect them from herbivores (6). This is also what explains their frequent use worldwide to treat fever and pain.
Moreover, the identified constituents are essentially phenolic compounds, with few exceptions. The most notable ones are linalool, frequently encountered in essential oils, and α-farnesene, whose presence seems surprising, but is validated by GC-MS and retention indices. The reason explaining its appearance in sample #2 is not clear. The initial aroma could quite well be explained by eugenol (clove, spicy), p-vinyl guaiacol (spicy, smoked), guaiacol (spicy, smoky, vanilla) phenol (chemical, pungent), carvacrol (oregano, spicy) and vanillin (vanilla). It is difficult to be entirely sure without organoleptic tests – note that the most concentrated compound is not necessarily the most contributive to the whole aroma.
As of the total volatile content of the hydrosol, its variation from sample #1 to #2 can be explained by the fact that batch #1 was obtained with less plant material, which produced a more diluted hydrosol.
In short, floral volatile constituents of P. tremuloides are characteristic of poplars. They agree well with the known presence of salicylic derivatives in this species and are mainly of phenolic nature, which may explain the notes of spice and vanilla of the floral aroma. There is a marked difference between the two samples tested, possibly caused by a gradient in metabolism through the season.
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