The Aroma Of Rum III (Influence of Fermentation Technique), Rafael Arroyo, 1940

Arroyo R. El Aroma Del Ron III (Influencia de la Técnica Fermentativa). Revista de agricultura de Puerto Rico, Volume 32, (1940) page 112-115

The Aroma Of Rum
(Influence of Fermentation Technique)

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

At the beginning of the fermentative process, and during its course, the rum distiller is presented an appropriate occasion to develop the quality and aromatic intensity of their product in the degree that they desire. This sometimes involves a somewhat complicated technique, capable of being carried out in large distilleries that have adequate technical personnel, as well as the necessary equipment; but that would be difficult and even impracticable for the small distiller. However, certain of the procedures to be exercised are available to all distillers, large and small.

The native rum industry is today one of the most important; but its possibilities have no limits if we know how to tear it away from the traditional and wasteful empiricism and channel it by the efficient and economical methods of rational technique. It is necessary and essential to think a little more about improving the product. They are many, and of very varied aspects, the complaints that are made against the poor quality of a large part of our rums. It is necessary to think a little less about the competition based on price, to focus our attention on competition based on the goodness of the product, its purity and quality. Only with this ideal in mind will our rums acquire, inside and outside the country, the just fame and unconditional acceptance so desirable for the expansion and flourishing of an industry of this nature.

The practices within the fermentation technique for the development and conditioning of the aroma that we are going to present here, are intimately related to the improvement work that in the previous paragraph we have mentioned as necessary.

Let us now enumerate, and then examine, some of these methods for modifying and intensifying, the nature of the aroma. The factors on which it is necessary to operate mainly during this stage of the formation of the bouquet to achieve the desired effects are:

1. pH value of the batición at the start of the fermentation process.
2. Variability or constancy of this pH value during the course of fermentation.
3. Amount of yeast seed used when inoculating the batición.
4. Fermentation temperature.
5. Use of other fermentative agents in pure culture before; in conjunction with, or after the completion of the alcoholic fermentation.

A scientific control over these factors, together with the discreet and constructive manipulation of them can produce truly surprising results in strength, tonality and cleanliness of the aroma in the finished rum. Let us examine, then, in its order, the different factors mentioned above, commenting on the brevity that an article of this nature imposes on us, the influence exerted by each one on the development, modification and tonality of the aroma of rum:

1. pH Value of the Batición at the Beginning of the Fermentation Process

The pH value, that is the ionic concentration of hydrogen existing in the batición at the beginning of the fermentation lapse, acts favorably, or unfavorably in the formation of the aroma, according to the existing concentration: values between 4.5 and 5.0 pH are unfavorable; between 5.0 and 5.5 neutrals; and between 5.5 and 6.0 favorable.

When the fermentation starts within the optimum values (between 5.5 and 6.0 pH), not only a greater quantity of esters tend to be formed, but these are also the ones that produce the best and finest aromas. At the same time the medium is in more favorable conditions for the formation of valuable essential oils, more important than the esters in the formation of the natural and genuine rum bouquet.

2. Variability or Constancy of the Initial pH Value During the Course of Fermentation

When we start the alcoholic fermentation within the optimum pH values for the formation of aromatic products, that is, between 5.5 and 6.0, we find that if we do not take precautions, the organic acids generated during the course of the fermentation will soon begin to change the initial pH value, tilting it towards the acidic side. Therefore it has only been possible to obtain the benefits of this ionic concentration of hydrogen that we call optimal during part of the fermentation period, depending on the greater or lesser capacity of the yeast in use to produce these acids.

The distiller can, however, sustain the optimum pH value for the production of fine aroma, and in a very simple way. It is enough to incorporate to each fermentor at the moment of inoculation a certain amount of calcium carbonate; whose quantity varies with the yeast lineage and the nature of the raw material used. This addition of calcium carbonate maintains the initial pH value within narrow or almost constant variations. For example, a fermentation of cane juice that starts at pH 5.8 will produce a final pH between 4.0 and 4.5 when the excess calcium carbonate is not added; but if we control the pH with this substance, then we will find that the final pH of the fermented batición will not go from 5.5 and 5.6.

This action of calcium carbonate is very favorable in terms of constructively modifying the aroma, since by its use, a large part of the esters that are formed are those of the acid radicals of the saturated fatty series with high molecular weights, such as the butyric, caprilic, capric, lauric, etc., etc. This is because calcium carbonate acts first as a neutralizer of acetic acid, which is much stronger and is also produced in greater quantities than the others mentioned above. The success of this operation, therefore, is to neutralize, by means of calcium carbonate, most of the low molecular weight fatty acids, particularly acetic acid, thus giving greater opportunity for esterification of the most valuable fatty acids in the formation of fine aroma. We must warn here two very important points.

a. In no way can we substitute calcium carbonate with hydroxide, because in that case it would be very easy to go beyond the limits of optimum pH; and in extreme cases we could even prevent the success of the fermentative course.

b. In this practice we will greatly improve the quality of our rum; but at the expense of efficiency. The loss suffered by the yield is much more noticeable in the case of using molasses as raw material, than when we use cane juice. Our experiments indicate the following values: in the case of using final molasses as raw material, it would result in yield loss between 5 and 10 percent; and in the case of using cane juice, this reduction is reduced to 2 or 3 percent.

Our final opinion that this practice would be advantageous to the quality rum distiller even if it lost 10 or 15 percent of its normal yield, since fermentation and distillation expenses are the least important in the rum manufacturing business.

3. Amount of Yeast Seed Used When Inoculating the Batición

The action of this factor depends on the effect it has on the fermentation time. Using seed that causes low cellular concentration in the batición, we will have a longer fermentation time lapse than if we use a quantity of seed that originates a high cellular concentration. However, slower fermentations result in greater aromatic intensity in the finished product, since the yeast has a greater opportunity to form those higher aromatic bodies that require some time in their confection.

However, this control of seed quantity should not be attempted in distilleries that are not sufficiently equipped for it. The following are basic requirements: (a) the services of a bacteriologist or fermentologist; (b) equipment for sterilization, or at least pasteurization of the baticións; (c) use of pure yeast cultures.

4. Temperature of Fermentation

Every rum distiller knows the effects produced by the more or less elevated temperature in his fermenters in terms of fermentation efficiency and yields; but few have thought about how the degree of temperature influences the fermentation in terms of quality and aromatic intensity of the finished product.

As a first point, the factor that has the most influence on the speed of fermentation is that of temperature. Within the limits acceptable as healthy practice, we will find that the higher the temperature, the greater the corresponding fermentation speed. We have already indicated that for the development of fine bouquet the fermentation must be slow rather than fast; then it is necessary that the distiller try to conserve the optimum temperature for formation of the desirable aroma.

Our experiments in this regard have indicated that relatively high temperatures (between 35°-38° C.) in the fermenters lead to undesirable bouquet in the finished product. And for these two important reasons, among many others: (a) the formation of undesirable metabolic products, of many penetrating odors; but always little aromatic; (b) the tremendous increase recorded in the autolysis of the yeast, with consequent bad odors during the period of the distillation. On the other hand fermentation temperatures between 27° and 30° C. lead to the development of fine aroma in the distillate. This is because other conditions remaining equal and constant, we will obtain better distillates in winter time than during the summer; and at all times, better distillates at considerable height above sea level than near the coast….

In the practice of rum manufacturing, this is the factor that offers the most difficulties for its control, since our climate does not lend itself to getting temperatures between 27° and 30° C. in the fermenters for most of the year. And even in those distilleries where the fermenters are equipped with devices for artificial cooling of the batición by circulation of water in the interior, or application of water on the walls of the fermenters, we often find that the difference in temperature between water and the batición is not large enough to exert any practical benefit. This happens, of course, during the hottest time of the year. As we pointed out above, distilleries located at considerable height above sea level have a great advantage in this regard.

5. Use of Other Fermentation Agents in Pure Culture and Under Strict Control, Before; in Conjunction With; or After the Ethyl Fermentation is Finished.

This process consists of incorporating in the batición, sometimes before the yeast begins working; other times during the ethyl fermentation; and others once the yeast is finished, certain amount of other fermentation agents in pure culture whose metabolic products greatly influence the formation or conditioning of the bouquet. Both the quantity, the purity of the crop, and the precise moment of incorporation to the batición must be under the supervision of a competent person; or else it is better not to try this practice.

These controlled additions of other fermentation organisms produce varied and characteristic effects on the nature of the aroma in the finished rum. At the same time they shorten the period necessary for the aging and curing of the crude distillate. So surprising are sometimes the obtainable results, that we do not doubt that the creation of completely new aromatic types is reached.

Another of the great advantages obtainable from this practice is the production of a rum of molasses with the aroma characteristics inherent to the rums produced from cane juice.

The perspectives that the progressive distiller offers this practice are, in our opinion, inexhaustible. This has been one of the most beautiful practical results of our research on the formation and conditioning of aroma through variations of fermentation technique.

However, we frankly understand and declare that carrying out this fifth production and aroma improvement system to the daily practice of the commercial distillery is extremely difficult. For this special and modern equipment is needed which practically all the facilities now in operation on the Island lack. Then we have to have a very competent and well trained personnel in these practices, since we must work under a special technique where any omission or commission of activities out of order would ruin the ends that are pursued.

But for those who after reading this article are interested in doing small-scale tests and are willing to make the necessary expenses, we offer our most determined and hearty help. Everything would come to bear on the prestige of the rum industry and the good of the community.

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