The Aroma Of Rum II (Adequate Yeast Selection), Rafael Arroyo, 1939

Arroyo R. El Aroma del Ron II (Seleccion de Levadura Adecuada). Revista de agricultura de Puerto Rico, Volume 31 (1939) page 578-580

The Aroma Of Rum
II
(Adequate Yeast Selection)

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.

In the first article on this subject, published in last September’s edition of this same journal, we offered to publicize some of the means used to obtain a pleasant natural and distinctive aroma in commercial rum without having to resort to adulteration of the product with the aforementioned strange agents imparting the bouquet.

With several factors to control for the best realization of our objective, we have decided to present them one by one, in the order of the respective importance that we grant them. We start, then, discussing first the factor that in our opinion and criterion is the most important: The Selection of Adequate Yeast.

In the first place, the selected yeast must be a rum yeast, and secondly, a rum yeast appropriate to the development of the special bouquet that we have in mind to achieve. We will never go very far in quality trying to produce genuine rum with yeast for bread, wine, beer; or with the yeasts generally developed and cultivated specifically for the production of industrial alcohol.

First of all, it will not surprise the reader that we eliminate yeast from bread, wine or beer in the production of rum; but it will be strange, that we include in our eliminative process the yeasts for the production of industrial alcohol; since these and rum yeasts work (especially in tropical countries) on the same raw materials: Molasses or sugar cane juice. If we make a parallel, however, of the objectives sought and desired in the manufacture of rum and industrial alcohol, we will find marked differences.

The ideal yeast for the alcohol distiller would be one capable of using the sugars from the batición to produce ethyl alcohol and carbon dioxide only; omitting the formation of the other congener bodies of the alcoholic fermentation. Not having this ideal yeast, the distiller tries to cultivate and develop those strains that are closest to this type. The problem is the production at a lower cost of the purest ethyl alcohol possible.

The case of the high quality rum distiller presents us with a very different problem. The latter, needs a yeast able to produce reasonable yields of ethyl alcohol of course, but also generate during the fermentation period the special aromatic bodies in adequate quantities. In this case, the yield is of less importance than the quality of the distillate. Quoting the price of their merchandise, especially based on taste and aroma, the rum distiller can not only, they must, sacrifice time and tend to the benefit of genuineness and final excellence of their product.

The following comparative scheme on the desirable requirements in the yeast to be used in each case, will better highlight the ideas expressed above:

FOR INDUSTRIAL ALCOHOL PRODUCTION

A yeast of high fermentative efficiency, able to use practically all the sugars of the batición:
1. In the shortest time possible.
2. With the maximum production of alcohol.
3. With the lowest possible production of:
(a) Organic acids.
(b) Esters.
(c) Aldehydes.
(d) Essential oils.
(e) Higher alcohols.

FOR GENUINE RUM PRODUCTION

A yeast of reasonable fermentative efficiency, able to use the great majority of the sugars in the batición:
1. In a convenient amount of time.
2. With comparatively high yields of ethyl alcohol.
3. With adequate production of convenient classes of:
(a) Organic acids.
(b) Esters.
(c) Aldehydes.
(d) Essential oils.
(e) Higher alcohols.

Accepting, according to the above, that the alcoholic fermentations destined to produce rum must be carried out by special yeasts, we still find that even within this group there are different varieties. As regards the rum bouquet, the differences that concern us are those related to the quantity and class of its metabolic products that are not ethyl alcohol; since to these aromatic bodies the final aroma of commercial rum will be largely due. The so-called “Rum Oil” stands out in the first place among these aromatic bodies. This is an essential oil (or perhaps a mixture of essential oils) of chemical constitution, unknown to date. We could not define its characteristic aroma better than saying it smells like rum. The character of this oil varies somewhat according to the yeast it comes from, and also not all the yeasts produce it; which is why the distiller is greatly concerned with following a line producing this valuable component of a genuine rum. The other congeners of the alcoholic fermentation also vary according to the yeast that we use during the fermentation: for example there are yeasts that produce especially the esters of the acetic radical, while others, although they are also producers of esters based on the acetic radical, do not for that reason produce those much more appreciated and fine in aroma of higher radicals in the series of saturated fatty acids. We also find yeasts that produce mostly ethyl acetate, when others predominate in amyl acetate. This in terms of class of aromatic compound, as in terms of quantities of these compounds generated by different yeasts, the differences are even more marked.

We have to select our yeasts according to the aromatic characteristics of their metabolic products, taking into account not only the quantity of these but also, and especially the class. The important point is the application that we can make of these aromatic substances in the development of the special and distinctive aroma that we wish to impart to our rum. For example, it would be a very big mistake to use a strain of rum yeast whose metabolic products tended to form the bouquet of Jamaica rums, in the fermentation of Puerto Rican or Cuban rum.

We also have that within a same type of rum, as for example, the one of Jamaica, differences of bouquet in the finished product can be established directly attributable to the fermentative agent used.

In our experimental work we have used three different varieties of yeast which produce the characteristic bouquet of the Jamaican rum; but with variations of tone and essence large enough to distinguish, by smelling the finished rum, with which of the three yeasts was fermented the batición mother of the rum in question. We have named these rums as heavy, semi-heavy and light.

In short, we find that for rums of different types we need different varieties of yeasts; but even within the same type of rum we can provide a product with a very distinctive aroma, very characteristic and very distinctive if we take attention with enough care and concern to the Selection Of The Fermentative Agent.

Our next article will deal with the influence exerted on the rum bouquet by the process of pre-treatment of the raw material, using sugarcane juice as an example of raw material.