Part 7

Thus in the destruction of that which has lived, all reduces itself to the simultaneous action of these three great natural phenomena--fermentation, putrefaction, and slow combustion. A living organism dies--animal, or plant, or the remains of one or the other. It is exposed to the contact of the air. To the life which has quitted it succeeds life under other forms. In the superficial parts, which the air can reach, the germs of the infinitely small aérobies hatch and multiply themselves. The carbon, the hydrogen, and the nitrogen of the organic matters are transformed by the oxygen of the air, and under the influence of the life of these aérobies, into carbonic acid, vapour of water, and ammonia gas. As long as organic matter and air are present, these combustions will continue. While these superficial combustions are going on, fermentation and putrefaction are doing their work in the interior of the mass by the developed germs of the anaérobies, which not only do not require oxygen for their life, but which oxygen actually kills. Little by little, at length, by this work of fermentation and slow combustion, the phenomenon is accomplished. Whether in the free atmosphere, or under the earth, which is always more or less impregnated with air, all animal and vegetable matters end by disappearing. To arrest these phenomena an extremely low temperature is required. It is thus that in the ice of the Polar regions antediluvian elephants have been found perfectly intact. The microscopic organisms could not live in so cold a temperature. These facts still further strengthen all the new ideas as to the important part performed by these infinitely small organisms, which are, in fact, the masters of the world. If we could suppress their work, which is always going on, the surface of the globe, encumbered with organic matters, would soon become uninhabitable.

FOOTNOTE:

[8] [A millimeter is 1/25th of an inch.]

_ACETIC FERMENTATION._

THE MANUFACTURE OF VINEGAR.

Soon afterwards Pasteur came upon a most curious illustration of the 'fixation' of atmospheric oxygen by a microscopic organism--the transformation of wine into vinegar. As its name indicates, vinegar is nothing else than wine turned sour. Everybody has remarked that wine, left to itself, in circumstances which occur daily, is frequently transformed into vinegar. This is noticed more particularly when bottles, having been uncorked, are left in a half-empty condition. Sometimes, however, wine turns sour even in corked bottles. In this case we may be sure that the bottles have been standing upright, and that corks more or less defective have permitted the air to penetrate into the wine. The presence of air, in fact, is indispensable to the chemical act of transforming wine into vinegar. How does this air intervene? And what is the little microscopic creature which, in conjunction with the air, becomes the agent of this fermentation?

In a celebrated lecture given at Orleans at the request of the manufacturers of vinegar in that town, Pasteur, after having stated the two foregoing scientific questions, proceeded to examine the difference between wine and vinegar. What takes place in the fermentation of the juice of the grape which yields the wine? The sugar of this juice disappears, giving place to carbonic acid gas, which is exhaled during fermentation, and to alcohol, which remains in the fermented liquid. Formerly, chemists gave the name of 'spirit' to all volatile matters which could be collected from distillation. Now, when we distil wine and condense the vapour in a worm surrounded by cold water, we collect the spirit of wine at the extremity of the worm--this, when the water with which it is mixed during distillation is withdrawn from it, we designate by the name of alcohol. Vinegar contains no alcohol. When distilled it yields water and a spirit. But this spirit is acid, with a very pungent odour, and not inflammable like spirit of wine. Separated from the water which had accompanied it during the distillation, this spirit takes the name of acetic acid. This is the form in which it is used in smelling bottles--in those bottles of English salts the vapour of which is so penetrating.

In the formation of vinegar in contact with air the alcohol disappears, and is replaced by acetic acid. The air has thus given up something to the wine. Atmospheric air every one knows to be a mixture of nitrogen and oxygen, the nitrogen in the proportion of four-fifths of the total volume, and the oxygen of one-fifth. Well, in the transformation of wine into vinegar the nitrogen remains inactive. It is the oxygen alone which enters into combination with the alcohol. You ask for the proof of this? Take a bottle of wine turned sour, a bottle which at the same time is stopped hermetically; if the oxygen of the air contained in the bottle has combined with the alcohol, then, instead of air, there will be nothing in the bottle but nitrogen gas. Turn the bottle upside down and open it in a basin of water. The water of the basin will rush into the bottle to fill the partial vacuum created by the disappearance of the oxygen. The volume of water which enters the bottle is precisely equal to a fifth part of the total original volume of the air which the bottle contained at the time when it was closed. Moreover, it is easy to show that the gas which remains in the bottle has the properties of nitrogen gas. A lighted match is extinguished in it as if plunged into water, and a bird dies immediately in it of asphyxia.

If we confine our knowledge to what has gone before, it would seem that alcohol diluted with water and exposed to the air ought to furnish acetic acid. It is not so, however. Pure water alcoholised to the degree of ordinary wines may remain for whole years in contact with the air, without the least acetification. In this difference between natural wine and pure water alcoholised, and exposed to contact with air, we touch upon a vital point in the phenomena of fermentation. The celebrated theory of Liebig, which Pasteur was destined to overthrow, might be thus summed up:--If pure alcoholised water cannot become sour in contact with air, as is the case with wine, it is because the pure alcoholised water lacks the albuminoid substance which exists in the wine in a state of chemical alteration, and which is a ferment capable of causing the oxygen of the air to combine with the alcohol. And the proof, according to Liebig, that things act rigorously thus is, that if you add to the mixture of water and alcohol a little flour, or a little meat-juice, or even a minute quantity of any vegetable juice, the acetic fermentation arises, as if by compulsion. In other words, by the addition of a small quantity of any nitrogenised substance in process of alteration, you cause the union of the oxygen of the air with the alcohol.

There is doubtless always in the wine, when it turns sour, a necessary intermediary, producing the fixation of the oxygen of the air; since in no circumstances can pure alcohol, diluted to any degree whatever with pure water, transform itself into vinegar. But this necessary intermediary is not, as the German theory would have it, a dead albuminoid substance; it is a plant, and of all plants one of the simplest and most minute, which has been known from time immemorial under the name of flower of vinegar. This little fungus is invariably present on the surface of a wine which is being transformed into vinegar. Liebig was not ignorant of this, but he regarded it as a simple coincidence. Do we not know, said he, that whenever an infusion of organic matter is exposed to the air it becomes covered with a cryptogamic vegetation, or is invaded by a crowd of animalculæ? Is not vinegar a vegetable infusion? Vinegar affords a refuge to the flower of vinegar, just as it gives refuge to what are called the little eels of vinegar.

We can appreciate here the uncertainties of pure observation. The great art--and no one practised it better than Pasteur--consists in instituting decisive experiments which leave no room for an inexact interpretation of facts. These decisive proofs of the true part played by the little microscopic fungus, by this flower of vinegar, this mycoderma aceti, are thus formulated by Pasteur. It is but another example of the method which he used in alcoholic, lactic, and tartaric fermentations. The theories of Berzelius, of Mitscherlich, and of Liebig were destined again to receive the rudest shocks by the demonstration of these rigorous facts.

Let us place a little wine in a bottle, then hermetically seal it, and leave it to itself. In these conditions the wine becomes sour. But if we take the precaution of putting the bottle into hot water, so that the wine and the air in the bottle may be heated for some instants to a temperature of 60° Centigrade, and if, after cooling, we leave the bottle to itself, the wine in these conditions will never become transformed into vinegar. The heating, however, must have left intact the albuminoid or nitrogenous substances contained in the wine. These, then, cannot constitute the ferment of the vinegar. Can it be maintained that by heating the wine to 60° we have altered the albuminoid matter, which is, on this account, no longer able to act as a ferment, or, in other words, no longer able to determine the union of the oxygen of the air with the alcohol? This hypothesis falls to pieces before the following experiment. Open the bottle, blow into it with bellows, so that the once heated wine shall come into contact with ordinary air, and the acetification of the wine will take place.

But the master experiment is the following. We have seen that pure alcoholised water never turns sour unless some albuminoid matter is introduced into it. Pasteur saw that this albuminoid matter might be completely suppressed and replaced by saline crystallisable substances, alkaline and earthy phosphates, to which has been added a little phosphate of ammonia. In these conditions, especially if the alcoholised water be acidulated by small quantities of pure acetic acid, one actually sees the mycoderm developing, and the alcohol transforming itself into acetic acid. It is not possible to demonstrate in a more convincing manner that the albuminoid matters of the wine are not in this case the acetic ferment. These albuminoid matters, however, contribute to the acetic fermentation, but only as being an aliment to the mycoderma aceti, and notably a nitrogenous aliment. The true and only ferment of vinegar is the little fungus; it is the great agent of the phenomenon; it, indeed, accomplishes all.

Is there not a great charm in seeing an obscure subject clearly illuminated by facts well understood and well interpreted? If in a bottle containing wine and air and raised to a temperature of 50° or 60° the wine never turns sour, it is because the germs of the mycoderma aceti, which the wine and the air hold in suspension, are deprived of all vitality by the heat. Placed, however, in contact with ordinary air, this once-heated wine can turn sour; because, though the germs of the mycoderma aceti contained at first in the wine are killed, this is not the case with those derived from the surrounding air. Pure alcoholised water never turns sour, even in contact with ordinary air, and with whatever germs this air may carry, or that may be found in the dust of the vessels which receive it. The reason is that these germs cannot become fertile because of the absence of their indispensable food. Wine in bottles well filled and laid flat do not acetify; this is because the mycoderm cannot multiply for lack of oxygen. Without doubt the air constantly penetrates through the pores of the cork, but always in such feeble quantities that the colouring matters of the wine, and other more or less oxydisable constituents, take possession of it without leaving the smallest quantity for the germs of the mycoderm which are generally suspended in the wine. When the bottle is upright the conditions are quite altered. The desiccation of the cork renders it much more permeable to the air, and the germs of the mycoderm on the surface of the liquid, if any exist there, are enveloped by air.

Thus, to recapitulate in a few words the principles which have just been established; it is easy to see that the formation of vinegar is always preceded by the development, on the surface of the wine, of a little plant formed of strangulated particles, of an extreme tenuity, and the accumulation of which sometimes takes the form of a hardly visible veil, sometimes of a wrinkled film of very slight thickness, and greasy to the touch, because of the various fatty matters which the plant contains.

This cryptogam has the singular property of condensing considerable quantities of oxygen and of provoking the _fixation_ of this gas upon the alcohol, which is thereby transformed into acetic acid. The little mycoderm is not less exacting than larger vegetables. It must have its appropriate aliments. Wine offers them in abundance: nitrogenous matters, the phosphates of magnesia and of potash. The mycoderm thrives, moreover, in warm climates. To cultivate it in temperate regions like ours it is well to warm artificially the places where it is cultivated. But if wine contains within itself all the elements necessary to the life of the little mycoderm, this life is further promoted by rendering the wine more acid through the addition of acetic acid.

What, then, can be more simple than to produce vinegar from wine--a manufacture which justly makes the reputation of the town of Orleans? Take some wine, and after having mixed with it one-fourth or one-third of its volume of vinegar already formed, sow on its surface the little plant which does the work of acetification. It is only necessary to skim off, by means of a wooden spatula, a little of the mycodermic film from a liquid covered with it, and to transfer it to the liquid to be acetified. The fatty matters which it contains render the wetting of it difficult. Thus, when we plunge into the liquid the spatula covered with the film, the latter detaches itself and spreads out over the surface instead of falling to the bottom. When we operate in summer, or in a room heated to 15° or 25° Centigrade in winter, in twenty-four or forty-eight hours at most, the mycoderm covers the whole liquid, so easy and rapid is its development. After some days all the wine has become vinegar.

On one occasion, in a discussion which he was holding at the Academy of Sciences, Pasteur, wishing to affirm the prodigious activity of the life and multiplication of this little organism, expressed himself thus:--

'I would undertake in the space of twenty-four hours to cover with mycoderma aceti a surface of vinous liquid as large as the hall in which we are here assembled. I should only have to sow in it the day before almost invisible particles of newly-formed mycoderma aceti.'

Let the reader try to imagine the millions upon millions of little mycoderma particles which would come to life in that one day.

But how is the mycoderm seed to be obtained in the first instance? Nothing more simple. The mycoderma aceti is one of those little so-called 'spontaneous' productions which are sure to appear of themselves on the surface of liquids or infusions suitable to their development. In wine, in vinegar, or suspended in air, everywhere around us, in our towns, in our houses, there exist germs of this little plant. If we wish to procure some fresh mycoderm it is only necessary to put a mixture of wine and vinegar into a warm place. In a few days, generally, if not always, there appear here and there little greyish patches scattering the light instead of regularly reflecting it, as does the surrounding liquid. These specks go on increasing progressively and rapidly. This is the mycoderma aceti raised from the seeds which the wine or the added vinegar contained, or which the air deposited; just as we see a field covered with divers weeds by seeds naturally distributed in the earth, or which have been brought to it by the wind or by animals. Even in this last circumstance the comparison holds good, for after you have put wine or vinegar in a warm place there soon appear, whence we know not, little reddish flies, so commonly seen in vinegar manufactories, and in all places where vegetable matter is turning sour. With their feet, or with their probosces, these flies transport the seed.

* * * * *

At Orleans the process for the manufacture of vinegar is very simple. Barrels ranged over each other have on each of their vertically-placed bottoms a circular opening some centimeters in diameter, and a smaller hole adjacent, called _fausset_, for the air to pass in and out when the large opening is closed, either by the funnel, through which the wine is introduced, or by the syphon, which is used for drawing off the vinegar. These barrels, of which the capacity is 230 litres, are half filled. The manual labour consists in keeping up a suitable temperature in the vessel, and in drawing from it every eight days about eight or ten litres of vinegar, which are replaced by eight or ten litres of wine.

A barrel in which this give-and-take of wine and vinegar goes on is technically called a 'mother.' The starting of a 'mother' is not a rapid process. We begin by introducing into the barrel 100 litres of very good and very limpid vinegar; then two litres only of wine are added. Eight days after, three litres of wine are added, a week later four or five, until the barrel contains about 180 to 200 litres. Then for the first time vinegar is drawn off in sufficient quantity to bring back the volume of the liquid to about 100 litres. At this moment the labours of the 'mother' begin. Henceforward ten litres of vinegar may be drawn off every eight days, to be replaced by ten litres of wine. This is the maximum that a cask can yield in a week. When the casks work badly, as is often the case, it is necessary to diminish their production.

This Orleans system has many drawbacks. It requires three or four months to prepare what is called a 'mother,' which must be nourished with wine _very_ regularly once a week under penalty of seeing it lose all its power. Then it is necessary to continue the manufacture at all times, whether the vinegar be required or not. To reconstitute a 'mother,' one must begin from the very beginning, a process which involves a loss of three or four months' time. Lastly--a condition which is at times very inconvenient--a 'mother' cannot be transported from one place to another, or even from one part of the same locality to another. The 'mother,' in fact, must rest immovable.

Pasteur advised the suppression of the 'mothers.' He recommended an apparatus, which is simply a vat, placed in a chamber the temperature of which can be raised to 20° or 25° Centigrade. In these vats vinegar already formed is mixed with wine. On the surface is sown the little plant which converts the wine into vinegar. The mode of sowing it has been already explained. The acetification begins with the development of the plant.

A great merchant of Orleans, who had from the first adopted Pasteur's process, and who had won the prize offered by the 'Society for the Encouragement of National Industry' for a manufactory perfected after these principles, has stated that at the end of nine or ten days, sometimes even in eight, all the acetified wine is converted into vinegar. From a hundred litres of wine he drew off ninety-five litres of vinegar. After the great rise of temperature observed at the moment of the formation of the vinegar, and which is caused by the chemical union of the alcohol and the oxygen of the air, the vinegar is allowed to cool. It may then be drawn from the vat, introduced into barrels, refined, and straightway delivered, fit for consumption. When the vat is quite emptied, and well cleaned, a new mixture is made of vinegar and wine, the little plant is sown as before, and the same facts are reproduced in the second as in the first operation.

* * * * *

In the vessels where vinegar is preserved, whether in the manufactories, in private houses, or in grocers' shops, it often happens that the liquid becomes turbid, and impoverished in an extraordinary manner; it even ends in putrefaction, if a remedy be not promptly applied. Pasteur has pointed out the cause of these phenomena. After the alcohol has become acetic acid by the combustive action of the mycoderm, the question remains, what becomes of the mycoderm? Most frequently it falls to the bottom of the vessel, having no more work to accomplish. This is a phase of the manufacture which must be watched with care. It is shown by the experiments of Pasteur that the mycoderma aceti can live on vinegar already formed, maintaining its power of fixing the oxygen on certain constituents of the liquid. In this case the acetic acid itself is the seat of the chemical action--in other words, the oxygen unites with the carbon of the acetic acid, and transforms it into carbonic acid, and as the acetic acid has a composition which can be represented by carbon and water, it follows that if the combustion is allowed to take its course, instead of vinegar we have eventually nothing but water mixed with a small proportion of nitrogenous and mineral matters, and the remains of the mycoderm. We have thus an ordinary organic infusion exempt from all acidity, and one which could not be better fitted to become the prey of the vibrios of putrefaction or of the aérobic mucors. By these mucors, moreover, which form a film on the surface of the liquid after the mycoderm has fallen, the anaérobic vibrios, protected from the action of the air, can come into active existence. Here we find ourselves in presence of one of those double phenomena, of putrefaction in the deeper parts of the liquid, and of combustion at the surface which is in contact with the air. Nothing is more prejudicial to the quality of the vinegar than the setting in of this combustion after the vinegar has been formed, and when it contains no more alcohol. The first materials of the vinegar upon which the oxygen transmitted by the mycoderm fixes are, in fact, the ethereal and aromatic constituents which give to vinegar its chief value.