Rafael Arroyo: The Alcohol—Temperature— PH Value Complex in Rum Fermentation

This article was found in: Revista de Agricultura de Puerto Rico, Vol 32, 1940 pages 301-304

El Complejo Alcohol—Temperatura—Valor pH en Fermentación de Ron.

The Alcohol—Temperature— PH Value Complex in Rum Fermentation.

Por Rafael Arroyo, Ch. E. & S. E., Especialista en Fermentaciones lndustriales; Jefe Division de Quimica Industrial de la Estacion Experimental Agricola de la Universidad de Puerto Rico.

The intelligent distiller, always alert to improve their product, must keep in mind that the production of rum is primarily a function of one or more fermentation agents among which the yeast is the predominant agent. Any factor that affects the cause, that is, the yeast, must necessarily influence the effect, that is, the rum.

Among the factors that most affect the biological development of the yeast and its fermentative capacity are what we have called the Alcohol—Temperature—pH Value Complex which title heads this article. Let’s see now how yeast is affected by each one of these factors, separately then in joint action.

The yeast, during ethanol fermentation, destroys the sugars of the batición to form ethyl alcohol, carbonic gas and other products of its metabolism. Therefore we can say that the concentration of total sugars in the batición (within certain limits) determines the alcohol concentration that at the end of the fermentation we will find in the liquid. If the yeast were not adversely affected by this same alcoholic concentration of the liquid, we could then say that the amount of alcohol to be expected from a given fermentation would be a direct function of the sugar content of the batición. However, not only are all the yeasts adversely affected by the alcohol that they themselves produce, but they are so in very different degrees, according to the race or variety in question. In addition, it is necessary to count on the conditions existing during the fermentation period in terms of pH value and temperature of the liquid under fermentation.

Among all the industrial yeasts, some from wine are the ones with the highest alcohol tolerance, following by those dedicated to the manufacture of industrial alcohol. On the other hand, the special yeasts for the manufacture of rum do not resist such a high concentration of alcohol as the previously mentioned, although much more than those dedicated to the manufacture of beer. The main cause of this greater lack of resistance to the alcoholic concentration of the medium in the case of rum yeast, lies in the character and quantity of its metabolic products other than ethyl alcohol. Among these we have some that produce more toxic effects to the yeast than those of ethyl alcohol, or that increase the toxicity of the latter. And since the special yeasts for rum usually produce a greater quantity of these congeneric bodies of the ethyl fermentation, it is, by logical consequence, an earlier inhibition of their fermentative power. It has been observed and verified by different researchers on the matter that the higher concentration of these congeneric bodies of alcoholic fermentation, such as organic acids, esters, higher alcohols and aldehydes, the lower the concentration of ethyl alcohol that the yeast can support in the medium. Our own work during four years of research on the manufacture of rum has also proven this.

We see, then, that as the alcohol concentration in the batición increases, the yeast loses its fermentative vigor, and that this inhibitory effect is increased by the presence of the other congeners of the ethyl fermentation. Upon reaching a certain degree of alcohol concentration, (the degree of which varies according to the yeast used) the yeast suspends its activities even when considerable quantities of fermentable sugars remain in the liquid. The distiller must adjust the concentration of sugars from its batición to the fermentative power of its yeast and to the pH and temperature conditions in which their fermentation takes place. Any increase in sugars on this limit will not be taken advantage of by the yeast, and will be lost in the musts of the still. When this happens, you will get a loss in performance; and what is even worse, a distillate with characteristic smell and taste of sugar will be obtained in the case of sugar being the raw material used in the confection of the batición.

Now consider how the other two factors of temperature and pH come into play:

The yeasts will cause the fermentation of the sugars contained in a given batición, within a fairly wide temperature range; zero degree centigrade being considered as the lowest, and fifty above zero as the highest to which fermentation is possible. But there is a much narrower margin, considered in practice as the optimum for the effects of rum fermentation. This varies somewhat according to different breeds and varieties of yeasts; but in general terms, these optimum values are between 25 and 35 degrees Celsius. Within this range, the higher the temperature, the stronger and more violent the fermentation, which is why we can say that an increase in temperature accelerates the fermentation. Temperature also influences the formation of fat and glycogen in the cell; both processes taking place more quickly as the temperature increases. [Fat being derived from grasa which may imply glycerin and fatty acids.]

We have already spoken at the beginning of this article about the inhibitory power exerted on the life of yeast by ethyl alcohol and its congeners. Well, this adverse influence is not the same at all temperatures, but varies directly with increases in the temperature of the liquid in fermentation. For example, a given yeast may begin to feel the inhibitory action of its metabolic products at a concentration of 6.0 percent alcohol when the temperature of the batición is about 36° Celsius; while this same yeast will only begin to feel the same effects when the alcohol concentration reaches 9.0 percent as long as the temperature stays around 25° C. Although as we said before, high temperatures of the medium accelerate the fermentation, it should not be inferred that we will obtain better rum yields. On the contrary, the performance will always be lower at high temperatures due to the joint action of the Alcohol—Temperature complex. Due to the high temperature the alcohol becomes more toxic in its effects and the yeast will be paralyzed in its work when comparatively small amounts of alcohol have been generated in the liquid under fermentation. In other words, the fermentation will have been arrested by the Alcohol—Temperature complex long before the total sugar content in the batición has been depleted. We can therefore state as an axiom that high temperatures of fermentation are adverse or opposed to obtaining high yields.

However, the bad effect of high fermentation temperature does not cease here, but it will also adversely affect the quality of the rum produced. High temperatures operate very effectively in carrying endoproteolysis and autolysis of the yeast cell. When this happens, a large number of decomposition products are formed, many of which then pass with the distillate to increase the bad taste and odor of the raw rum. Apart from these effects, high fermentation temperatures bring about the premature depletion of the yeast by vital over-excitation.

Let’s analyze now the effect of the ionic concentration of hydrogen, known as pH:

Rum yeasts vary greatly in the production of organic acids during the fermentation period. The amount of acid formed varies between 4.7 and 10.0 cubic centimeters equivalent of a tenth normal solution of sodium hydroxide per 100 cubic centimeters of the fermented liquid. Of this total amount, the fixed organic acids vary between 2.1 and 5.4 c.c. and volatiles between 2.1 and 5.8 c.c. [We can use our new titration skills and translate these numbers into Δ acidity which may help us better monitor aroma creation. We need to know what is normal from our yeast before we add acids from bacteria on top.]

This increase in acidity of the medium during fermentation is measured in practice by readings of the pH values.

As in the case of temperature, the pH of the medium during the fermentation period is of enormous importance in the manufacture of rum, both from the point of view of quality and the quantity of the product.

Maintaining the optimum pH of a certain yeast throughout the fermentation period will have a great influence not only on the final yield, but also on the quality of the aroma. Its action will be specifically felt in the influence it exerts regarding the production of rum oil and the class and aromatic quality of the esters formed during the fermentation and distillation of the product. For each yeast, the inhibitory effect of the alcohol produced on the fermentative power of the yeast will depend directly on the amount of acid that the yeast produced during the fermentation. In this way, a fermentation can be paralyzed when 6.0 to 7.0 percent of alcohol has been produced, if simultaneously the pH value of this batición has dropped to a reading between 2.5 and 3.0. But if this pH value had been maintained all the time at the optimum for the yeast in question, this same fermentation would have continued until reaching a much higher alcohol content, say 10 percent or more. [pH 3.0 for comparison is basically 5% acetic vinegar]

We can understand from the above, that in a rum fermentation the combination HIGH TEMPERATURE—LOW pH VALUE —HIGH ALCOHOL CONTENT is not possible, and that if good yields are desired it is necessary to maintain high pH value and low temperature throughout the period of fermentation.

If the maintenance of a constant pH suitable for the yeast line in use, during the whole fermentation time, results in high alcoholic yield, it also brings a great benefit in the aromatic effect of rum. This note of pleasant aromatic tone is obtained for the following reasons:

By partially neutralizing the organic acids produced by the yeast during fermentation, two results are achieved leading to the formation of pleasant bouquet. On the one hand, only the class of desirable esters is formed in a quality rum, by means of the elimination in the medium of formic and acetic acids in the free state. This elimination of formic and acetic acids in free state, will give an opportunity to the other acids of the saturated fat series that have higher molecular weight for the formation of esters, during the fermentation and distillation of the product. [This is a major difference between Arroyo’s process and Jamaica process. Jamaica doesn’t due this and ends up with grand arôme rums very high in surplus ethyl acetate that requires blending down. However, the Arroyo method is largely unproven. Arroyo concerned with a ratio of ordinary to extraordinary esters.]

The esters formed by these superior acids are those of true and unmistakable value in the formation of the bouquet of a good rum. In addition, the large decrease in esters of the formic and acetic radicals, as well as the absence of these acids in free state greatly contributes to the softness, palatability and sweetness of the distillate. A distillate produced under these conditions will mature rapidly in the barrel during aging.

On the other hand, in the case that the yeast in use was of the good producers of Rum Oil, there is nothing that helps more to this production than the maintenance of the optimum pH in the batición. It is sometimes necessary to lean towards high pH values to receive the full benefit of this valuable oil. In fermentation for rum we mean high pH values between 5.7 and 6.0.

[This may take advantage of rum oil formation via enzymatic process rather than staling under acid conditions over extended time which is another channel of formation. Each channel may exhibit a different character.]

From the above we can expose, for greater understanding of the practical distiller, the predominant ideas and teachings in summary form, as follows:
1. During the fermentation of rum, ethyl alcohol and its congeners tend to inhibit the yeast producing them.
2. There is great variation between different varieties and breeds of yeasts in terms of the alcohol concentration necessary to effect this inhibition of activities, so it is necessary that each distiller determines the maximum concentration of metabolism products that their particular line can support.
3. As soon as the alcoholic concentration reaches the inhibitory level for the yeast, it will cease to act on the sugars that still remain without decomposing in the liquid. The distiller must therefore take care that the concentration of sugars in the batición does not exceed the fermentative limits of its yeast. Otherwise their yields on sugars will be very poor.
4. High fermentation temperatures will result in:
a. Poor rum yields.
b. Lower quality of the product.
5. Inadequate pH values, especially those very low in relation to the optimum pH of the yeast, will give the same results as high temperatures.
6. Combinations of high temperature and low pH value will result in much more adverse consequences than when only one of these factors comes into play.
7. If the conditions of the distillery are such that the fermentations have to be carried out under adverse conditions in terms of temperature and pH value, then use concentrations of sugars as low as possible within the economy of your factory, when making the batición. Only in this way can it be possible to maintain good, or at least moderately good, yields and qualities of the product.
8. baticións of high concentration in total sugars are recommended for many reasons, and can be fermented successfully, if:
a. The yeast resists high concentrations of alcohol in the medium.
b. There is an adequate balance of the necessary nutrients to the liquid.
c. The temperature of the liquid under fermentation can be controlled and maintained between 27 and 30 degrees Celsius.
d. The pH value of the fermented liquid can be maintained at all times within the optimum values.
9. Low temperature and high pH value will give us a raw rum of the following characteristics:
a. With softness and sweetness of aroma.
b. Of pleasant taste.
c. It will be an easy aging, or rather, acquire the requirements of a mature and ripe rum with little time of barrel aging.