The Aroma Of Rum VII, 1941 (Influence Of The Period of Maturation)

Arroyo R. El Aroma del Ron (VII) Influencia del periodo de curación. Revista de agricultura de Puerto Rico, Volume 33 (1941), 127-132

(Influence of the Maturation Period)
Especialista en Fermentaciones Industriales; Jefe, División de Química Industrial de la
Estación Experimental Agrícola de la Universidad de Puerto Rico.

For those of our readers who have not had the opportunity to read the preceding articles of this series; we want to make an important clarification: when speaking about the aroma of rum we refer only and exclusively to the natural aroma developed during the fermentation and distillation of the product, and to the improvements and modifications that act on this aroma during the curing period; but in no way are those aromas imparted by the addition of: [In the title I put maturation but curación probably best translates to curing.]

1. Aromatic chemicals.
2. Artificial essences of rum.
3. Infusions, or alcoholic extracts of fruits; barks; Leaves or roots of aromatic plants.

Once obtaining the raw distillate, it is necessary to impart the smoothness and roundness of aroma that are so desirable, and that without a doubt, significantly influence the good taste of the product.

If the norms already expressed in the preceding articles have been followed, the raw distillate will be free of all “tufo” and will instead present a very pleasant and distinctive aroma; therefore, the so-called “destufe treatment” of crude rum will be automatically eliminated.

We will simply submit the crude distillate to the curing process that we consider necessary. This process is divided into two main classes: (1) Slow curing, and (2) Accelerated curing. As to date we do not know of any accelerated curing method that without resorting to artificial flavoring additives has ever equaled the method of slow healing in obtaining genuine aroma. We are going to discard it in this exhibition of how the curing process influences the aroma. In other words, we are going to limit ourselves to a consideration of the process of slow curing in barrels.

The factors that influence more powerfully in the modification and improvement of the original aroma of the crude during the period of the cure are:

1. The percentage of ethyl alcohol per volume existing in the crude distillate at the time of its barrel entry.
2. Variety and individual chemical constitution of the components that make up the “Non-Alcohol Coefficient” of the rum.
3 The barrel used to cure the raw product, in relation to
(a) Class.
(b) Size.
(c) Conditioning.
4. Relative humidity and temperature conditions in the room used to store the barrels during the course of the curing process.


Rums are distilled at different degrees of alcohol concentration. In some cases where the distillation is carried out to a high degree, it is customary to dilute the distillate with water before entry in oak barrels for curing. In other cases the distillate is put in barrels at distillation proof, and after some time it is then diluted to the suitable proof degree and subjected to final aging; and still in other cases this dilution with water does not take place except when the rum is already cured and ready to be bottled. Each and every one of the dilution methods to which the distillate is subjected, before or after being in the barrel, will have a great influence on the aroma of the final product.

Our opinion is that the best method in maintaining aroma quality would be to distill to a proof degree low enough to avoid having to use (or use as little as possible) dilution water at any time. This practice would not present great difficulties if the pre-distillation precautions were taken with the fermented batición that we have already dealt with in previous articles, and more extensive use of batch distillation system was made.

Any alteration of the proof of the distillate brings with it the aromatic imbalance thereof; but apart from this disequilibrium of the “bouquet” that results from the effect of the dilution itself, there is another modifying factor of the aroma that depends on the degree of alcoholic concentration of the crude distillate when it is packed in the barrel. The degree of alcoholic concentration of the distillate will determine its solvent power over the different substances found in the oak of the barrel. Some of these extractive substances are more soluble in alcohol than in water, and others more in water than in alcohol. Therefore, the ratio of alcohol to water in the distillate establishes an index of the extent to which they will be extracted from the staves of the barrel and dissolved in the distillate, those substances that impact their aroma during aging. Hence the great importance of a careful selection of the distillate entry proof to barrel.


Among the component bodies of the “Non-Alcohol Coefficient” there are large variations between fermented and distilled rums in different ways. During the curing period this composition of the “Non-Alcoholic Coefficient” will have great influence on the final aroma of the commercial rum. The variety of products, the individual chemical constitution of each one, and their physical properties, constitute factors of great importance in the conditioning of the aroma during the curing period.

If we group the constituent bodies of the “Non-Alcoholic Coefficient” according to their chemical nature, we find that the presence of a great variety of substances has been discovered, although they are never represented in a given sample of rum: We think it convenient to declare here that despite the variety of bodies found in the “Non-Alcoholic Coefficient” the existence of many others is presumed indeterminate up to the present, due especially to the ignorance of sufficiently accurate analytical methods for the separation and determination of components represented in tiny, almost negligible amounts.

Below we will find the classification and individual members in each group of these components:
I. Acetals:
1. Acetal

II. Alcohols:
1. Mono Alcohols
(a) Methyl alcohol
(b) Normal propyl alcohol
(c) Isopropyl alcohol
(d) Normal butyl alcohol
(e) Isobutyl alcohol
(f) Normal amyl alcohol
(g) Secondary amyl alcohol
(h) ) Caproic or hexyl alcohol
(i) Oenanthyl or heptylic alcohol
(j). Caprylic or octyl alcohol
(k) Capric or decyl alcohol.
2. Di-Alcohols.
1. Isobutylglycol.
3. Tri-Alcohols.
1. Glycerin.

III. Aldehydes:
1. Ethyl aldehyde. [acetaldehyde]
2. Propyl aldehyde.
3. Isobutyl aldehyde.
4. Butyl aldehyde.
5. Oenantílico aldehyde.
6. Hexyl aldehyde.
7. Furfural.

IV. Ketones:
1. Acetone.

V. Organic Acids:
1. Formic.
2. Acetic.
3. Propionic.
4. Butirico.
5. Capróico.
6. Heptico.
7. Caprílico.
8. Capric:

VI. Estéres:
1. Acetates of:
(a) Methyl,
(b) Ethyl,
(c) Propyl,
(d) Butyl,
(e) Isobutyl,
(f) Amyl.
2. Propionates of:
(a) Methyl.
(B) Ethyl.
(C) Amyl.
3. Formats of:
(a) Ethyl
(b) Propyl
(c) Butyl
(d) Amyl.
4. Butyrates or Isobutyrates of:
(a) Ethyl.
(b) Amyl.
5. Ethynylate of Ethyl.
Caprilate of Ethyl.
Caproate of Ethyl.

VII. Bases.
1. Amines.
2. Pyridines.

VIII. Essential oils:
1. Rum Oil. [Which we now know are rose ketones]
2. Several Others.

The “presence of methyl alcohol in rum has been affirmed by researchers and denied by others no less trained. The acetal is assumed to be formed during barrel aging by the reaction of acetaldehyde and ethanol, which condense to form the acetal. The presence of aldehydes and acetals in rum predisposes it to the formation of new bodies of condensation of complex molecules, and that undoubtedly affect the final aroma of commercial rum.

[Acetaldehyde and acetal form an equilibrium where each is about 50/50. I have distilled wines full of acetaldehyde and watched them transform dramatically over months as acetal is formed. Excessive acetaldehyde in an unsound ferment may be best recycled to a ferment where it can be oxidized by yeast back to ethanol.]

Some of the components of the “Non-Alcohol Coefficient” have lower boiling points than ethyl alcohol. They also usually have very intense and irritating odors to the nasal mucosa, whose odors usually lack true aroma. Among these are: ethyl aldehyde, methyl formate, propyl aldehyde, methyl acetate, acetone, isobutyl aldehyde, methyl alcohol, ethyl acetate and others. Although aldehydes are an important part of the group, we should not believe that all aldehydes are undesirable due to their pungent odors, as we have aldehydes of great aromatic value such as normal butyl aldehyde, oenantyl aldehyde and hexyl aldehyde, whose presence in rum is very desirable.

[Interesting that Arroyo mentions ethyl acetate because its perception shifts as its concentration increases. Completely understanding ethyl acetate has been a bit of a puzzle.]

Other of these bodies have higher boiling points, than ethyl alcohol. They also possess aromatic qualities superior to the bodies cataloged in the previous group. Among these we will find: ethyl butyrate, amyl formate, butyl acetate, amyl acetate; oenantilate, caprylate and ethyl caproate; butyl aldehyde; caprioic acid and oenantílico [heptanoic?] aldehydes, and the essential oils.

During the curing period, the maturity and aromatic modifications suffered by the crude distillate will largely depend on the class of bodies included in the above classification, which form their “Non-Alcoholic Coefficient”. The relative reactivity or chemical affinity of these bodies will be a factor of great importance, since it will determine their capacity to form condensation and oxidation products. Its chemical affinity to combine with the extractive products of the barrel staves is also a valuable factor in the formation or modification of aroma.

The physical properties of these bodies, especially the molecular size, the volatility relative to that of ethanol, and the susceptibility to the formation of azeotropic mixtures with other products; they are physical factors that also exert their influence on the development and improvement of the aroma during the curing period. The pores of the barrel staves offer escape avenues by evaporation, not only to alcoholic vapors and water vapor, but also to these components of the “Non-Alcoholic Coefficient”. The proportion in which the vapors of these aromatic bodies will pass out through the pores of the wood of the barrel will depend on the individual size of the molecules; of their boiling points; of the existing quantities of each of these bodies, and of the relative tendency of their vapors to form azeotropic mixtures with the vapors of other substances. From this we infer that the greater softness we observe in the rums as the time of their barrel curing passes is largely due to the evaporation loss of some of their more pungent components in odor; or that these components have reacted chemically between themselves or with other bodies to form condensations, with corresponding changes of aromas.

Nowadays it is customary to use white oak barrels in the curing of raw rums. Apart from the fact that there is great variation in the quality of the wood with which the barrel was manufactured, we also found several modifications within the same origin in terms of construction wood. We have natural white oak barrels on the market; slightly charred; intensely charred; charred barrels to which the carbon part has been scraped to expose the reddish underlying zone; Barrels internally paraffined, and barrels covered inside by a film of sodium silicate.

Among all these kinds of barrels, the most commonly used are those of natural white oak and those slightly charred. Some manufacturers are aware of the importance of immediate contact of the crude rum with the red zone, they select this type of barrel as fast aging. These barrels, in all classes, can be new or second-hand. When they are of second use they have served first, generally, for the packaging of wine, brandi or “whiskey”.

The search for second-use barrels is due to three different factors: (1) Initial cost economy; (2) to which they are already “cured” according to the buyer’s opinion; (3) to the belief that a new barrel never cures the raw distillate as effectively and satisfactorily as the one previously used.

For lack of time and space to be an article of this nature, we will not stop to discuss what is real and ephemeral in the reasons given above to prefer second-use pipes to new ones; but we will say that there is much to talk about the matter.

Having extended this last article outside the limits to which we thought to restrict it, we will now limit ourselves to exposing succinctly how the barrel affects the aroma of the product that is cured in it.

One of the factors inherent to the barrel that most influences the conditioning of the aroma in the degree of porosity of its timbers. During the first months of aging, the aroma change is effected largely by the loss of those bodies with a low boiling point and which at the same time have an acrid or very penetrating aroma, and this loss is made in direct proportion to the degree of porosity of the barrel. That’s why we first notice the sweetness of the aromatic tone. The high degree of porosity in the wood of the barrel at the same time facilitating the exodus of pungent and penetrating odors of crude rum, also makes more accessible the entrance of the outside air to the liquid contained in it, facilitating in this way the oxidation process that plays such a high role in the conditioning of the aroma.

The degree of porosity of a barrel can be easily increased in the desired proportion, provided that this (the barrel) is natural white oak, not charred or covered inside by artificial films.

Another very important factor in the conditioning of the aroma is the size. This is, of course, related to porosity. The aroma of the crude rum will undergo faster modification the smaller the size of the barrel, due to two factors, one of which is the porosity already discussed above. The other factor depends on the surface ratio of the continent to the content volume. Per unit of volume the small barrel offers more contact surface between wood and liquid. This implies that those aroma imparting substances contained in the barrels’ timbers will be extracted by the dissolving action of the crude rum more quickly in the case of the small barrel.

The third factor that we are going to consider is that of the conditioning or previous preparation given to the barrel before using it in the curing of the crude distillate. This preparation is equally necessary irrespective of the size of the barrel; but it can not be affected in the same way or with the same intensity in all cases. The preparation of the barrel is governed first by the aromatic type we want to develop, secondly, by the time we have for the curing of the crude rum, and thirdly by the size and type of barrel.

For example, if we want to produce a heavy type of rum, like the Jamaican type, the treatment will be different than if we want to produce a very light type of delicate and subtle aroma. In the first case, the treatment would be less complicated and intense than in the second. Essentially, the treatment consists of extracting certain bodies from the barrels, leaving, however, others that are necessary for our purposes. Another result of the treatment is to increase the degree of porosity of the barrel.

In this way we will eliminate from the final product certain undesirable characteristics (for a certain type) of the aroma, and we will impart those which in our opinion have to modify the aroma of the crude rum beneficially.


The conditions of temperature and relative humidity existing in the room destined to store the barrel during the course of the curing process constitute another factor that exerts its influence on the conditioning of the aroma of the crude rum although in an indirect way. At higher temperatures, greater chemical activity between the components of the crude rum; as well as between the components extracted from the barrel and those originally brought by the crude. Also greater intensity in the oxidation process caused by the oxygen of the air that has access to the interior of the barrel. We will also have greater activity in the evaporation through the pores of the barrel of those bodies already mentioned above that have low boiling point. Therefore, aroma modification is taking place more quickly as a result of all these operations. Of course, there are limits on the increase in temperature, indicated by other factors in the production of rum, which have no direct relation to the subject on which we are dealing. And even when dealing exclusively with the aromatic tone, we could reach an increase in temperature that would produce negative results in terms of improving the aroma.

The relative humidity of the environment also takes its part in the development of the aroma in the liquid contained in the barrel. During the first stage of the curing process it is convenient to have a high relative humidity around the aging barrels. This is because it is necessary to facilitate the exodus of those bodies of low boiling point that provide roughness to the aroma of the crude rum. Rum being a mixture of these bodies, ethyl alcohol, other bodies with higher boiling points; and water; it is necessary to retard or prevent as much as possible the passage of water vapor molecules through the walls of the barrel so that the molecules of the vapors of the other bodies that we wish to eliminate have free access to the external environment. And this is achieved by increasing the amount of water vapor in the atmosphere around the barrel, which constitutes high relative humidity.

[This idea of humidity may have greater bearing on very heavy rums.]

Once these undesirable bodies have been eliminated from the crude rum, it is convenient to reduce the percentage of relative humidity to avoid immoderate alcohol losses, as well as possible losses due to evaporation of aromatic bodies that we wish to preserve in our rum.

We have finished the series of articles that we have been publishing for some time in this magazine under the title of El Aroma del Ron. It only remains for us to express our deep gratitude to Mr. Rafael Sacarello for the determined and sincere cooperation he always lent us, and especially for the words of encouragement and praise with which he honored us on several occasions when commenting, in his brilliant style, on the content of our articles.

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