QUALITATIVE AND QUANTITATIVE STUDY OF VOLATILE CONSTITUENTS OF RHUM.
P. DUBOIS and J. RIGAUD
Station de Technologie des Produits végétaux,
7, rue Sully,
21034 Dijon Cedex
[This is a block buster smoking gun paper that ties the legendary rose ketone, damanscenone, and its various isomers to rum oil. They even say specifically at one point, rum oil. This studies relies on olfactometry which is quite expensive and thus not common in the industry. The birectifier is essentially a cruder, very affordable form of olfactometry and I’ve recently describe how it can assist chromatography in that way. Because they use olfactometry, there are a bunch of incredible sensory descriptors in there.]
Secondary alcohols and volatile phenols were studied in rum by gas-liquid chromatography and mass spectrometry. The results are similar to what is indicated in the literature for rum and other spirits.
The use of a splitter at the end of the gas chromatographic columm has given the possibility to sniff the eluted components of several rum extracts, and to detect characteristic compounds which were identified by GC-MS to trans β-damascenone and to one of its isomers.
A simple method is proposed for preparation of characteristic and concentrated samples for GC analysis.
The main volatile constituents of spirits are aliphatic derivatives: alcohols, aldehydes, acids, esters and acetals, which come from the microbial metabolism of α-ketonic acids and lipids. Distillation and aging modify the relative concentrations of these various constituents, and provide other compounds such as furan derivatives and volatile phenols. In this respect, the rum has a composition very similar to that of other spirits, especially since cane juice and molasses do not seem to possess any particular compounds (YOKOTA and FAGERSON, 1971). We therefore investigated whether it was possible to identify characteristic constituents within other categories of substances, and in particular among secondary alcohols and phenols. In addition, our work focused on the identification of a very characteristic constituent of rums, inspired by the work done at the beginning of this century by MICKO (1908). Finally, we conducted selective extraction trials for the development of quantitative analysis techniques.
[The birectifier was an elaboration of Karl Micko’s techniques and he created the first version of the 8 fraction method. All the extra high value stuff they are looking for that may be particular to rum is basically rum oil. They did not appear to have Micko’s original work but only glimpsed it through Kervegant. In 1975, they do not appear to be aware of Arroyo either. This study makes use of GC-Olfactometry]
I. – SECONDARY ALCOHOLS
Secondary butanol is always more abundant in rums than in whiskeys (SUOMALAINEN and NYKANEN, 1970); it seems to be the same for n-pentanol-2 (LIEBICH et al., 1970) and for n-heptanol-2 (ALLAN, 1973).
We found these compounds in large quantities in some rums, along with another secondary alcohol, n-hepten-1 ol-3. The origin of these compounds is poorly known, but it is conceivable that methyl carbinols with an odd number of carbon atoms are likely to form in ways similar to those leading to the formation, by the molds, of methyl ketones in certain cheeses (DAY, 1967).
Be that as it may, these compounds do not seem to present particular odorous properties.
2. – VOLATILE PHENOLS
The simple phenols identified in the spirit are shown in Table I where they are classified according to their structures (Figure I).
In whiskey fusel oils, BRAUS and MILLER (1958) identified traces of phenol and ortho-cresol as well as more appreciable amounts of guaiacol, methyl-4 guaiacol and ethyl-4 guaiacol. In white rums, DUBOIS et al. (1972) identified guaiacol, 4-ethylphenol, 4-ethyl guaiacol, 4-vinyl guaiacol and 4-propyl guaiacol. More recently, we have found in grand arôme rums, apparently large amounts of ethyl-4 guaiacol, propyl-4 guaiacol and methyl salicylate. These compounds may be thought to be formed during alcoholic fermentation as shown by STEINKE and PAULSON (1964).
All the other phenols seem to come solely from the slow degradation of the lignin of the barrel woods.
These volatile phenols certainly contribute very significantly to the aroma of rum, but it seems unlikely that they can bring the characteristic note that distinguishes a rum from another alcohol during tastings.
Volatile Phenols of Rum
3. — CHARACTERISTIC COMPONENT OF THE RUM
When analyzing rum extracts by gas chromatography, we have olfactorically spotted a compound that seems to be very characteristic of the aroma of rum. This detection was carried out thanks to the use of a flow divider placed at the outlet of a gas chromatography column (FIG. 2). Part of the carrier gas goes to a physical detector, which makes it possible to obtain a chromatogram; the other part of the gas is directed out of the chromatograph, which allows the operator to sense [smell] the eluted compounds.
On the neutral fraction of a rum extract, and on silicone liquid phase (OV 225), we obtained a chromatogram, the part of which we are interested in is the subject of Figure 3. The main peaks correspond, in order, n-octanol-1, ethyl octanoate, menthol, ethyl decanoate, 2-phenyl ethanol-1 and its acetate, ethyl dodecanoate. At the beginning of the elution of peak A, the perceived odor is very characteristic of rum, then it changes and evolves towards that of laurel sauce or juniper. Mass spectrometry shows that peak A is essentially due to trans-β-damascenone (Figure 4), as shown by the comparison with an authentic sample of this compound, a sample for which we thank the laboratories of the Society Firmenich.
Trans-β-damascenone does not smell rum, so we trapped the components responsible for peak A in Figure 3, and we reinjected them on a column of different polarity (Carbowax 20 M). We then obtained the chromatogram of Figure 5, in which the peak B corresponds to the elution of the odor-producing compound characteristic of rum. The mass spectrum of this compound is quite similar to that of trans-β-damascenone; so we think it is an isomer of this compound.
Trans β-damascenone has been identified in a large number of products of plant origin and we have just discovered a trace in Cognac.
It appears to be more abundant in the grand arôme rum than in other types of rums, suggesting that it might form during alcoholic fermentation, as reported by MICKO about what he called rum oil. [#smoking gun]
The question we ask ourselves is whether trans-β-damascenone is always accompanied by its characteristic odor isomer. If this isomer is also present in cognac, for example, only quantitative analysis can explain the differences in aroma of these two spirits. [Other differences are going to come from its perceptual radiant properties. This all the very artful stuff understood by perfumers like Arcadi Boix Camps.]
4. — TRIALS OF SELECTIVE EXTRACTIONS
The major constituents of “non-alcohol” spirits are either polar compounds such as propyl, isobutyl and isoamyl alcohols; or low polar compounds such as ethyl esters of higher fatty acids and some acetals. Based on the results obtained by WILLIAMS and TUCKNOTT (1973), we used pentane to avoid the extraction of a large quantity of higher alcohols. With this same solvent, on the other hand, the low polar compounds are very well extracted even when the alcoholic degree of the hydroalcoholic solution is high.
Thus, on a spirit at 75°GL, a single pentane treatment makes it possible to eliminate most of the ethyl esters of the higher fatty acids, and in particular the heavier ones which have little interest in the olfactory plane. Following this first operation, the dilution of the hydroalcoholic solution with 75°GL by four times its volume of water makes it possible to extract, still with pentane, the most interesting constituents of the “non-alcohol”. [So, down to 15%ABV. I’ve seen other people use 13%]
[A lot of things are really interesting here. First is that heavier esters are of little interest to the olfactory plane. Are they talking extremely high molecular weight? And in the first process the high ethanol holds onto the rum oil and the pentane merely washes away the esters. Then they dilute the ethanol and the second rinsing of pentane captures the rum oil. This is quite the trick to know.]
Thus, using the method shown diagrammatically in FIG. 6, the chromatograms A and B of FIG. 7 are obtained. The peaks of the ethyl esters are hatched and the numbers indicated correspond to the carbon number of the acids of these esters.
This very simple methodology makes it possible to obtain extracts that are very concentrated and, it seems, very significant, highly concentrated, because type B extracts are low in higher alcohols, heavy esters and acetals. Very significant, because the extracts of type A have an aroma reminiscent of that of Cognac when the rum studied is rich in esters, and an aroma reminiscent of Calvados when acetals dominate. On the other hand, in all cases, the extracts of type B have an aroma very close to that of the rum of departure, with its qualities and its defects.
[Incredible sensory details here. I don’t really know what they mean by departure, possibly an export rum concentrate? or, could they mean the tails fractions where I’ve found incredible amounts of high value congeners?
Sean, thinks departure could refer the original sample that these two extracted departed from which is sounding really smart to me. Its a fascinating exercise, but it helps you conceive the role of these congeners in the identity of the original spirit.]
In conclusion, it seems important to us to develop a quantitative method for the analysis of trans-β-damascenone and its isomer in order to study its mode of formation and role. This is the subject of ongoing work.
ALLAN D. A., 1973. A new approach to the identification of flavour compounds in rum. Austral. Wine
Brew., Spirit Rev., 42-44.
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BRICOUT J., 1971. Analyse de quelques constituants dérivés du chêne dans les vieilles eaux-de-vie d’Armagnac. Ann. Technol. agric., 20 (3), 217-223.
DAY E. A., 1967. Cheese flavour. In SCHULTZ H. W., DAY E. A., LIBBEY L. M. Chemistry and physiology of flavours, 331-361. Avi Publ. Co. Westport-Connecticut.
DUBOIS P., BRULE G., DEKIMPE J., 1972. Analyse des phénols volatils de deux rhums martiniquais. Industr. alim, agric., 89 (1), 7-9. [this likely has details on Grand Arôme rhums.]
JOSEPH E., MARCHE M., 1972. Contribution à l’étude du vieillissement du Cognac. Connaissance Vigne Vin, 6 (3), 273-330.
KAHN J. H., SHIPLEY P. A., LA ROE E. G., CONNER H. A., 1969. Whisky composition : identification of additional components by gas chromatography-mass spectrometry. J. Food Sci., 34, 587-591.
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