The Aroma Of Rum, Rafael Arroyo 1939

This is possibly part of a seven part series. Later the articles are counted with roman numerals.

The Aroma Of Rum

Por Rafael Arroyo, Especialista en Fermentaciones Industriales; Jefe de la División de Química de la
Estación Experimental Agrícola de la Universidad de Puerto Rico.

The observations made by the author of this work in relation to the aroma of rum are of great importance for all those who work in Puerto Rico in this industry of incalculable perspectives. If someday the rum of Puerto Rico must possess the decided preference and solid reputation that are so covetable, there is no doubt that the men of science, as is the author of this work, should be taken care of, because they will always come from the orientation necessary to go further and sooner, on the frank road to success.

When discussing the aroma of rum, we want to warn our readers that we have in mind the natural aroma of a genuine rum. The aroma imparted to this drink by the incorporation of synthetic essences, extracts of fruits, leaves, flowers, barks, herbs and aromatic roots, will not be considered in this article.

The aroma of a genuine rum is the result of many factors that come into play in its formation; and it can be said that there are no two rums manufactured by different methods that have the same aroma. It is equally difficult to preserve this aromatic purity in the rums produced even in the same factory. This is one of the reasons why many rum manufacturers prefer to impart a peculiar aroma by artificial methods, because it is much more comfortable and simple to add a certain and specific aromatic base made with ingredients that are always the same and in strictly measured or heavy quantities than achieve an aroma invariable natural “bouquet” using special fermentative, distillatory and curative methods.

The great advantages, however, of achieving this characteristic and distinctive aroma of rum by natural development is that in this case imitation is almost impossible, while artificially imparted aromas are always duplicable. In addition, in the case of natural scent there is no need to incur extraordinary expenses to get it, and the resulting drink will be truly rum and not just any liquor we choose to call rum.

The aroma of a genuine rum once matured is influenced by two groups of factors, namely:

1. The set of aromatic bodies originally existing in the crude distillate.

2. Aromatic bodies formed during the duration of the maturing in the oak barrel. Among the latter enter modifications, condensations and oxidations of some of the substances of the first group.

The first group is the most important and the one that has the greatest influence on the complex and compound aroma of commercial rum; but the second is important also for its softening and refining effects, as well as for the body that some of its components impart to the finished rum.

The constituent aromatic bodies of the first group are much better known than those of the second with the exception of rum oil. This is because they are all volatile chemical substances, which pass in the crude distillate, while many of the aromatic bodies formed during barrel maturation are easily decomposed with the application of heat; and since existing methods for the analysis of mature rum require prior distillation of the sample, only those aromatic bodies that do not undergo alteration and decomposition during the distillation period are found in the distillate. The result is that by these methods of analysis accepted in practice we only determine the same classes of aromatic compounds as in the case of crude rum analysis. To verify the above, there is need to distill a sample of genuine aged rum and observe the aroma of this distillate: what great differences will be noticed between the original sample and its distillate! The bouquet has been completely unbalanced by decomposition, degradation and oxidation of some of its components during the distillation.

For a more complete elucidation of the reactions carried out during the maturation of the rum in the barrel, it would be necessary to use more adequate analytical methods, methods capable of letting us know the full aromatic range constituting the bouquet of a mature rum.

The group of aromatic bodies of the first group, which, as we said before, are in the crude distillate, is formed during the fermentation and distillation of the batición. Some of the aromatic principles included in this group were present as such in the raw material used to make the batición; but these are the least.

This first group includes especially esters, aldehydes, higher alcohols, and in some cases furfural and rum oil. Now, it is necessary to keep in mind that not all esters, higher alcohols and aldehydes have a pleasant aroma. Some esters are of little aromatic odor, and there are also acrid and repellent odors; this same condition is noticed in the higher alcohols and aldehydes. Therefore, it is not enough that a rum has a high content of these bodies to possess a pleasant aroma; the degree of fineness of the bouquet will depend more on the kind of aromatic bodies that constitute it than on the quantity of the same.

Raw rums produced without fermentative precautions and through the use of unselected yeasts, are generally not very aromatic although they contain a high percentage of esters and other bodies already mentioned, since the esters and aldehydes common in these rums are the derivatives of the acetic radical, especially ethyl acetate and amyl acetate and acetaldehyde; instead, in the higher alcohols, the amyl alcohol predominates. None of these mentioned bodies has great value as an aroma imparting agent; and much less the furfural.

[I constantly differentiate between the ordinary and extraordinary as well as categorize congeners as high value / low value. Amyl alcohol is also known to have more perceived roughness than other higher alcohols like butyl alcohol.]

Here is how the selection of fermentative organisms and the use of proper control of fermentation has a significant influence on the quality of the aroma, since the combinations of aromatic bodies produced by different strains of yeasts are diverse and varied. For example, there will be a huge difference between the aromas of two crude rums, the first of which contains butyraldehyde as its predominant aldehyde, and ethyl butyrates, propyl, butyl, amyl, etc., etc., as its main esters; while the second one has its aldehydes and esters formed on the basis of the acetic radical; that is, acetaldehyde, and acetates of ethyl, propyl, butyl, amyl, etc., etc.

However, the chemical analysis as it is usually executed and reported will not establish differences between these two rums, except in the quantity of aromatic bodies found in one and the other. This is because in the determination of aldehydes and esters in a rum it is customary to report in terms of acetaldehyde in the case of the determination of aldehydes and in terms of ethyl acetate in the determination of the esters. This practice means that the chemical analysis lacks any value in the evaluation of a rum. Regarding the determination of rum oil content, it does not even appear in the analytical methods, although among all the aromatic bodies that make up the complex bouquet and composition of a genuine rum it is the most valuable ingredient but also the one that is found in least quantities. The influence of the aroma of this essential oil is so important that a rum entirely devoid of esters and aldehydes would still retain its distinctive aroma of a good rum provided that this valuable essential oil is present in sufficiently high quantities.

[They were limited by titration methods hence the convention of counting everything as ethyl acetate or acetaldehyde.]

Unfortunately, it is very complex and difficult to obtain this essential oil in appreciable quantities. Its presence in the raw rum depends first of all on the particular yeast used during the fermentation, and secondly on the conditioning carried out on the raw material in the preparation of the batición.

From what in a general and superficial way we have exposed above about the first group of factors that come into play in the formation of the aroma of a rum, we can take in clear the following summary:

1. The aromatic bodies that together form the complex and composite aroma of a freshly distilled rum, comprise: esters; aldehydes; higher alcohols; furfural, and in some cases the essential oil known as rum oil.

2. Not all of the aforementioned bodies (with the exception of rum oil) have distinctive or intensely aromatic odors. Therefore, the CLASS of these bodies and not the QUANTITY is the predominant factor.

3. These aromatic bodies are formed during the fermentation and distillation of the batición. Therefore both processes significantly influence the formation of the bouquet and can be modified in order to obtain the best possible results in terms of aroma.

4. The yeast strain used in fermentation will have a predominant influence on the quality of the aroma.

5. The essential oil called rum oil is the most important factor in the bouquet of a genuine rum.

6. The methods generally used in the chemical analysis of rum, nothing inform us as to the quality of its aroma. A rum can exhibit identical analysis to another on which it is nevertheless infinitely superior. [Only by now antiquated titration methods.]

7. When re-distilling a aged and mature rum to carry out its analysis, it unbalances and decomposes its original bouquet. The distillate obtained is not representative of the original sample.

[Much of this was solved with vacuum distillation techniques. We can extrapolate for this effect in pragmatic affordable day to day work such as with the birectifier, but for pure research we would need low temp solutions.]

This bouquet of the crude rum, later, during the maturation period, is perfected, developed and modified by the action of the barrel, the oxygen of the air that enters through the pores of the same, the chemical reactions carried out there and by the heat and relative humidity of the environment.

We accept then that the aroma of commercial rum is the result of the agents mentioned above, plus those that originate in the barrel itself during the maturation of the crude rum.

The formation of new aromatic bodies and the modifications of the raw rum in the barrel are due to an extremely complex process that is still to be elucidated. There is no doubt, however, of the influence of the chemical compounds and catalytic agents found in barrel staves that act on the constituents of the crude rum, either by exchanges of acid radicals, or by oxidations and condensations between the bodies brought by the raw and those extracted to the pipe by the powerful dissolving action of alcohol. The oxygen of the air, the temperature and humidity of the environment, and catalytic mineral agents play an important role in this mysterious maturation process between the barrel and the rum contained therein; it is undeniable that if the rum is cured by the barrel, it in turn benefits and improves in the process.

If we analyzed the chemical components of the wood of a barrel before and after being used in the rum maturation, we would find great alterations in its chemical constitution and physical texture, caused by this chemical exchange between constituents of the wood and those contained in the rum.

Due to this intimate relationship and reciprocal reaction between contents and container is that a bad commercial rum will results after curing an inferior raw rum, which at the same time deteriorates the quality of the barrel, to the point of leaving it incapacitated to age and cure another raw rum of high quality.

[Garbage in / garbage out! Never put stuff in a barrel that held junk!]

In our next article on the same subject we will discuss the means employed in the practice of manufacturing rum to achieve a true pleasant and genuine aroma.

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