Arroyo R. The Aroma of Rum : Influencia del Proceso del Pretratamiento de la Materia Prima. Revista agricultura industria commercio; Puerto Rico Vol. 32 (1940), no. 2, p. 284-286;
The Aroma Of Rum
(Influence of the Raw Material Pre-treatment Process)
By RAFAEL ARROYO, Ch. E. & S. E.,
Especialista en Fermentaciones Industriales; Jefe, División de Química Industrial de la Estación
Experimental de la Universidad de Puerto Rico.
For the purposes of this fourth article we will take the sugarcane juice as the raw material to be considered, as it is the most representative for what we wish to expose and illustrate; and by pre-treatment process, the one to which the raw material is submitted must be understood before proceeding with the preparation of the batición and its subsequent fermentation.
We have been able to observe during the course of our research work that we can achieve variations in the aroma of rums produced by the same raw material and using the same yeast, according to the pre-treatment we give to this raw material. These differences in aroma will not be as pronounced as in the case of a change of yeast (a matter that we have already discussed in article No. II of this series) but they are sufficiently so that we can immediately distinguish their effects.
If we follow the prevailing practice on our island of grinding the cane, diluting the juice to the required density and then inoculating it with the yeast that we have in use, we will obtain a certain type of aroma in the rum produced. We will name this case as No. I.
If, in addition to conveniently extracting and diluting the juice, we subject it to a sterilization process by means of heat, we cool rapidly, and then inoculate with our yeast; We will obtain another type of aroma in the rum produced, which we will call No. II.
If, in addition to following the operations indicated in case No. II, we treat the juice with the amount of calcium hydroxide necessary to obtain a hydrogen ion concentration equivalent to pH 7.0 or 7.2, and then acidify with an appropriate acid until obtaining a pH value between 5.6 and 5.8 before inoculating with our yeast; we will still obtain another aromatic type different from the two preceding cases. We’ll call this type No. III.
When performing the organoleptic tests to determine the quality of the aroma in each of the rums produced in the three cases under consideration, the expert taster will find that:
1. Rum No. 1 will have a more or less pleasant or unpleasant aroma; but in all cases very variable. That is, the tones of the aroma will vary in such a way during the tests, which will give the impression of smelling a mixture of different rums instead of just one. In a word, the aroma would lack stability and purity even if it was pleasant.
2. Rum No. 2 would have a more fixed, softer, more delicate and pure aroma. It would have greater aromatic harmony and a single persistent note would be received during the entire time of the test. There may be complexity in this aroma; but never instability; Its aromatic range is harmonically encompassed to produce a single note, a single effect.
3. Rum No. 3 will have a bouquet very similar to that of No. 2; but in this case we will find greater breadth and penetration in the aromatic note. You will also notice some characteristic smell of old rum, which was not highlighted in the other two rums. The aroma of this third rum will be of greater penetration and persistence; enduring undoubtedly greater dilution with water or neutral alcohol, before one ceases to perceive its bouquet relative to either of the other two rums. It would be a very good rum to mix in punches and “cocktails”.
So far what we have been able to observe in our experimental work regarding the bouquet differences produced by different methods of pre-treatment of the raw material, before proceeding to its fermentation.
How to explain the causes of these results? Cases Nos. 1 and 2 are comparatively easy to explain; but case No. 3 is much more complex and difficult. To find a reasonable explanation we would have to enter the speculative field, provide theories, etc. Let’s see, however, what can be elucidated in each case: The cane juice freshly extracted by the mill represents a highly contaminated material with all kinds of microorganisms capable of fermenting its sugars, among these adventitious yeasts, fungi and bacteria are especially represented. These fermentative agents are found on the bark of the cane, in its roots and in the waste material that usually comes attached to the cane. During the milling, these organisms are largely dragged by the juice, being then in an ideal environment for the development of their lives, growth and reproduction. The same mill is generally a new source of infection for the juice extracted by it.
Now, in case No. 1 we have inoculated this juice with our yeast; but without eliminating the fermentation agents already existing in it. Will our lineage prevail? Everything will depend on its vigor and cell concentration of the seed at the time of inoculation. In any case, there will always be a greater or lesser opportunity for the growth and development of at least some of the microorganisms originally found in the juice; and therefore we will be running all kinds of risks in terms of the final quality of the aroma of the rum thus produced. We cannot expect this aroma to be that specifically produced by our particular strain of yeast, since the other fermentative organisms existing in the raw material will also contribute odors peculiar to their metabolic products in the formation of the final aroma of our rum. Therefore operating with raw juice in these conditions we will entrust the final aroma of our rum to the luck factor, but we will never have uniformity or stability of bouquet; The final aroma will always be variable, insecure, and accidental.
In case No. 2 we were careful to sterilize the cane juice before its inoculation with our line of pure and selected yeast; freeing it in this way from existing contamination. This fact explains the stability and purity obtained in the aroma of rum, since in this case only the aromatic products generated by our particular line of yeast entered into its formation during the fermentation period.
As for case No. 3, we have already said that the explanation is more difficult. The difference between the pre-treatment of No. 2 and that of No. 3 lies in the use of calcium hydroxide in the latter, with subsequent acidulation until the desired pH value is obtained. Therefore to this chemical treatment we have to attribute the differences in the respective aromas. Having no exact verification of the role of this edition of calcium hydroxide in cane juice in terms of the aroma of rum, we dare to advance the following theories:
1. We all know that “calcium grout” is used in sugar mills as a juice clarifying agent. That same effect occurs in juices in our case, which means the elimination of a large amount of harmful substances to the formation of pleasant aroma in rum. In addition, at the time of distillation of the fermented mixture, we will feed our still with a purer liquid, almost completely stripped of objectionable suspended matter. These materials, when decomposed by the action of heat, give rise to stinky products that pass largely with alcoholic distillate.
2. It may well be that there are certain compounds in the cane juice, which, when activated by calcium hydroxide, are prepared for the generation of fine and aromatic odors later during the fermentation period. Of one thing we are, however, confident, and this is that we have noticed greater development of essential oils when the raw material has been pre-treated according to the directions given in case No. 3.
We see, then, that it is possible to develop bouquet variants according to the pre-treatment to which we subject the raw material before fermentation; since this pre-treatment is prepared and conditioned for the generation of a superior and distinctive aroma during fermentation and distillation of the finished product.