CHAPTER V

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The Alcoholic Ferments.

§ I.—On the Origin of Ferment.

Amongst the productions that appear spontaneously, or, we should rather say, without direct impregnation, in organic liquids exposed to contact with the air, there is one that more particularly claims our study. It is that one which, by reason of its active energy as an agent of decomposition, has been distinguished and utilized from the earliest times, and is considered as the type of ferments in general; we mean the ferment of wine, beer, and more generally, of all fermented beverages.

Yeast is that viscous sort of deposit which takes place in the vats or barrels of must or wort that is undergoing fermentation. This kind of ferment presents for consideration a physical fact of the most extraordinary character. Take a morsel of the substance and put it in sweetened water, in must, or in dough, which always contains a little sugar; after a time, the length of which varies, a few minutes often sufficing, we see these liquids or the dough rise, so to speak. This inflation of the mass, which is due to a liberation of carbonic acid gas, may cause it to overflow the vessels containing it, if their capacity is not considerably greater than the volume of the matters fermenting. It is equally remarkable that these phenomena are natural and spontaneous; that is to say that the must, the wort, and the dough are able to rise, as we have termed it, when left to themselves, without the least addition of foreign substances. The only difference that may occur in these phenomena is a certain amount of retardation, in cases where the yeast does not reach the saccharine matters in a perfectly natural form, inasmuch as it then requires a certain time to get itself together before it can begin to act.

It is necessary, indeed, that sugar should be present; for if we abstracted by some means or other from the must or dough all the sugar contained in it, without touching the other constituents, the addition of yeast would produce no gas. Everything would remain quiet until the moment when signs of a more or less advanced putrefaction showed themselves. Yeast is one of the most putrescible of substances, and it is worthy of notice that its alteration is also the consequence of the formation of one or more ferments, very different, however, from that of which we are speaking. As for the nature of yeast, the microscope has taught us what it is. That marvellous instrument, although still in its infancy, enabled Leuwenhoeck, towards the close of the 17th century, to discover that yeast is composed of a mass of cells. In 1835 Cagnard-Latour and Schwann took up Leuwenhoeck’s observations, and by employing a more perfect microscope, discovered that these same cells vegetate and multiply by a process of gemmation. Since then the physical and chemical phenomena already mentioned, such as the raising of the mass, the liberation of carbonic acid gas, and the formation of alcohol, have been announced as acts probably connected with the living processes of a little cellular plant, and subsequent researches have confirmed these views.

In introducing a quantity of yeast into a saccharine wort, it must be borne in mind that we are sowing a multitude of minute living cells, representing so many centres of life, capable of vegetating with extraordinary rapidity in a medium adapted to their nutrition. This phenomenon can occur at any temperature between zero and 55° C. (131° F.), although a temperature between 15° C. and 30° C. (59° F. and 86° F.) is the most favourable to its occurrence.

As regards the rapidity of the budding, the following observations will give some idea of what it is in the case of one of the ferments of natural must. The temperature was between 12° C. and 13° C. (55° F.).

“On October 12, 1861, at ten o’clock in the morning, we crushed some grapes, without filtering the juice that ran from them; afterwards, at different times during the day, we examined the juice under the microscope, until at last, although not before seven o’clock in the evening, we detected a couple of cells, as represented in Fig. 25, _a_.

[Illustration: Fig. 25.]

From that time we kept these contiguous cells constantly in view. At 7.10 we saw them separate and remove to some little distance from each other (Fig. 25, _b_). Between 7 and 7.30 we saw, on each of these cells, a very minute bud originate and grow little by little. These buds developed very near the point of contact, where the disjunction had just taken place. By 7.45 the buds had increased greatly in size (Fig. 25, _c_). By 8 they had attained the size of the mother-cells. By 9 each cell of each couple had put forth a new bud (Fig. 25, _d_). We did not follow the multiplication of the cells any farther, having seen that in the course of two hours two cellules had furnished eight, including the two mother-cells.”[82]

An increase like this, which would have been more rapid at a temperature between 15° and 25° (59° and 77° F.), and still more so between 25° and 30° (77° and 86° F.), may indeed seem surprising. It is really, however, nothing to what sometimes occurs. In choosing proper conditions of temperature and medium, of state and nature of yeast, it has sometimes happened that the bottom of a vessel has become covered with a white deposit of yeast cells, in the course of not more than five or six hours after we had sown a quantity of yeast so small as to effect no change at all in the transparency of the liquid contained in the vessel after it had been shaken up. Such a rapidity of vegetation reminded us of those exotic plants which are said to grow several feet in height in the course of twenty-four hours.

[Illustration: Fig. 26.]

Budding commences in the form of a simple protuberance on the cell—a kind of little boss, as represented in Fig. 26, No. 1. This protuberance goes on increasing, and assumes a spherical or oval form. At the same time, there is a tendency in the points of attachment in the young cell to meet—a kind of strangulation occurs (Fig. 26, No. 2). The junction takes place a little sooner or later, according to the species (Fig. 26, No. 3); the two individuals then separate (Fig. 26, No. 4). In certain cases a single cell may give rise to several protuberances, and, consequently, to several daughter-cells. Where there is only one protuberance or bud we generally see it originate at the thick end and a little on one side of the apex of the oval outline, which, in a greater or less degree, characterizes the cells of the majority of ferments.

Certain authors have maintained that the method of budding which we have just described, and which we think was first promulgated by Mitscherlich, is merely an illusion, and that the cells of yeast break up and scatter their granular contents, and that these scattered granules eventually attach themselves to the cells, growing there, and so giving the appearance of buds or daughter-cells. This error has been revived quite recently.[83] Nothing can be less admissible. We could count the number of yeast cells which we have seen undergo this process of rupture in the course of some ten years of observation, every day of which, we may say, thousands of these cells have passed under our eyes. This breaking up of the cells is really of the most rare occurrence, and may always be explained by some abnormal circumstance affecting the yeast; being indeed a mechanical accident, not a physiological fact. We may easily convince ourselves of this by growing some yeast in a saccharine wort, filtered perfectly clear, and, consequently, deprived of all granular amorphous deposit that might deceive the observer. The cells will be observed to bud and multiply without exhibiting the most minute appearance of granulation, or disruption; moreover, there will always be cells of all sizes, ranging from the smallest visible up to the largest. This very simple piece of observation may be made in all the alcoholic ferments, and with any wort capable of fermenting, and in its presence the hypothesis, which we have been repudiating, cannot hold its own.

In Plate VII. (left side) there is represented a field of yeast, magnified 400 times. We see a mass of disjointed cells, such as appear after fermentations without sufficient aliment; of the kind represented some are nearly spherical, others oval or cylindrical, more or less elongated. If we mix a little of this yeast, of about the size of a pin’s head, with wort, and put the wort into a small, shallow, flat-bottomed basin, having a surface of about 1 square decimetre (10 sq. ins.) exposing it to the surrounding temperature, we shall find next day the bottom of the basin covered with a fine white deposit, of the forms of which we give a sketch in the right half of the plate. In this it will at once be observed that the cells sown have lost their interior granulations, having become more transparent and filled with a gelatinous protoplasm. The principal difference between the two halves of the plate consists in this, that whereas the cells in the left half are isolated and granular, those in the right half are more inflated, more transparent, and provided with buds, which may be seen in every stage of development, from their first appearance till they become as large as the parent cells. They continue to grow until they detach themselves; then they bud in their turn, so that the same figure may furnish examples of cells of the first, second, and third generations. In the right half, the protoplasm contained in the cells exhibits circular spots or vacuoles, which may be made to appear lighter or darker than the rest of the cell by slight movements of the object-glass of the microscope. These spots are due to a migration of the protoplasm towards the sides; they commonly occur in yeast cells the vitality of which, from deficient nourishment, has become suppressed—the shrivelled appearance which they then assume being due to their being forced to live upon themselves, so to speak. However, by introducing such cells into a nutritive and aerated liquid the vacuoles quickly disappear.

[Illustration: Plate 7. Yeast-cells—Worn out and Dissociated (left), after Revival in a Sweet Wort (right).]

In the ordinary yeast, as met with in breweries, the majority of cells show one or more of these vacuoles; if, however, we place a little of this yeast in an aerated wort, and watch under the microscope the changes that occur in the cells, we shall witness, often in the course of a few seconds, a kind of turgescence, a greater tension of the cell-walls, which seem to grow thinner, and a complete disappearance of the vacuoles. At the same time the interior gelatinous matter will become filled with fine granulations that are scarcely visible, but which at a certain distance appear brilliant. At the same time protuberances begin to show themselves, and next day the budding will have already become very active. The newly-formed cells will have such a delicacy of aspect and contour as to be scarcely discernible in the field of the microscope. There will also be a tendency to ramification in the budding, which appearance will be more or less marked according to the kind of alcoholic ferments present, as we shall see presently, attaining its maximum in each case when the cells have been revived after exhaustion by rest and want of food. In the latter case, the process of rejuvenescence may be protracted; but this is not the case with cells of commercial yeast, which is always used within a few days of its formation. And thus, as I said a little ago, speaking of these cells, they often manifest the first signs of their budding in a few seconds.

In our preceding remarks we have expressly assumed that there are many kinds of alcoholic ferment. This is, beyond doubt, the case, as we have given incontestable proofs, first in 1862, in the _Bulletin de la Société chimique_ of Paris, and later on, in 1864 and 1866, in a Note in the _Comptes rendus_, on the diseases of wines, as well as in our “Studies on Wine.” Moreover, we know that brewers have long recognized two distinct methods of fermentation—“high” fermentation and “low” fermentation—and two corresponding yeasts. It is true that the differences presented by these fermentations were believed to be caused by the different conditions under which they took place, and that it was supposed that we might change “high” yeast into “low” yeast, or inversely, by subjecting the first to a low temperature, or the second to a high one. In our observations of 1862, which we have just mentioned, we discovered that must gives rise to several yeasts; that the ferment of “high” beer cannot develop except with great difficulty in must, whilst one of the ferments of the grape grows rapidly and luxuriantly in wort; that it is easy to isolate the smallest of the ferments of the grape from its congeners, by subjecting filtered must to fermentation; and finally, that the secondary fermentations of wines which remain sweet furnish a remarkable ferment, very different in aspect to the ferment of beer.

We have not given specific names to these different ferments, any more than we have to the other microscopic organisms which we have had occasion to study. This was not from any disregard for names, but from a constant fear that, since the physiological functions of these minute forms was the exclusive object of our study, we might be led to attach too much importance to exterior characters. We have often found that forms, having nothing apparently in common, belong to one and the same species, whilst similarity of form may associate species far apart. We shall give some fresh examples of this fact in the present paragraph. A German naturalist, Dr. Rees, who has discovered new proofs of the diversity of alcoholic ferments, putting aside, perhaps rightly, such scruples, has attached specific names to the different kinds of ferments, in his _brochure_ published in 1870, which we have already cited (p. 71). Indeed, we have often ourselves, for brevity’s sake, made use of the names proposed by Dr. Rees.[84]

[Illustration: Fig. 27.]

In a Note inserted in the _Bulletin de la Société chimique de Paris_, in 1862, we figured a ferment of small dimensions, which develops spontaneously in must, filtered or unfiltered, and which is very different from the ordinary ferment of wine. It is the first to make its appearance in the fermentation of the grape, and may even appear alone if the must has been previously well filtered, doubtless because its germs, being smaller than those of other ferments, pass through the filter more easily and in greater number. Fig. 27, extracted from our Note of 1862, represents this ferment, together with some spherical cells of _high_ yeast, with the object of giving a more exact idea of the relative dimensions of these two ferments and their dissimilarities. Dr. Rees has named it _saccharomyces apiculatus_.

The same savant has given the name of _saccharomyces pastorianus_ to the yeast of the secondary fermentations of sweet liquids, such as wine that has remained sweet after its principal fermentation. We have described this yeast in a Note published in 1864, on the diseases of wine, from which we give the following extract:—[85]

[Illustration: Fig. 28.]

“Fig. 6 (Fig. 28 in this work) represents a very interesting variety of alcoholic ferment. It happens pretty often, especially in the Jura, where the vintage takes place about October 15th, when the season is already cold and little favourable to fermentation, that the wine is still sweet at the moment when it is put into casks. This is especially the case in good years, when the sugar is abundant and the proportion of alcohol high, a circumstance which prevents the completion of fermentation when effected at a low temperature. The wine remains sweet in cask sometimes for several years, undergoing a continuous but feeble alcoholic fermentation. In such wines we have always observed the presence of this peculiar ferment. In form it consists of a principal stem, forming nodes at various points, from which short branches arise, ending in spherical or ovoid cells. These cells readily detach themselves, and act as spores of the plant. It is rarely, however, that we see so perfect a vegetation as we have represented, because the different parts fall to pieces, as we have shown in the left half of the figure.”

What is the origin of cellular plants of this remarkable type? Where and how are the ferments of the grape generated?

In