Louis Pasteur: His Life and Labours

by René Vallery-Radot

I.

All that has lived must die, and all that is dead must be disintegrated, dissolved or gasified; the elements which are the substratum of life must enter into new cycles of life. If things were otherwise, the matter of organised beings would encumber the surface of the earth, and the law of the perpetuity of life would be compromised by the gradual exhaustion of its materials. One grand phenomenon presides over this vast work, the phenomenon of fermentation. But this is only a word, and it suggests to the mind simply the internal movements which all organised matter manifests spontaneously after death, without the intervention of the hand of man. What is, then, the cause of the processes of fermentation, of putrefaction, and of slow combustion? How is the disappearance of the dead body or of the fallen plant to be accounted for? What is the explanation of the foaming of the must in the vintage cask? of dough, which, abandoned to itself, rises and becomes sour? of milk, which curdles? of blood, which putrefies? of the heap of straw, which becomes manure? of dead leaves and plants embedded in the earth, which transform themselves into soil?

Many different attempts were made to account for this mystery before science was in a condition to approach it. In our age, and at the time when Pasteur was led to the study of the question, one theory held almost undisputed sway. It was a very ancient theory, to which Liebig, in reviving it, had given the weight of his name. 'The ferments,' said Liebig, 'are all nitrogenous substances—albumen, fibrine, caseine; or the liquids which embrace them, milk, blood, urine—in a state of alteration which they undergo in contact with the air.'

The oxygen of the air was, according to this system, the first cause of the molecular breaking up of the nitrogenous substances. The molecular motions are gradually communicated from particle to particle in the interior of the fermentable matter, which is thus resolved into new products.

These theoretic ideas regarding the part played in fermentation by the oxygen of the air were based upon experiments made in the beginning of the century by Gay-Lussac. In examining the process of Appert for the preservation of animal and vegetable substances—a process which consisted in inclosing these substances in hermetically sealed vessels and heating them afterwards to a sufficiently high temperature—Gay-Lussac had seen, for example, the must of the grape, which had been preserved without alteration during a whole year, caused to enter into a state of fermentation by the simple fact of its transference to another vessel—that is to say, by having been brought for an instant into contact with the oxygen of the air. The oxygen of the air appeared, then, to be the primum movens of fermentation.

The illustrious chemists Berzelius and Mitscherlich explained the phenomena of fermentation otherwise. They placed these phenomena in the obscure class known as phenomena of contact. The ferment, in their view, took nothing from, and added nothing to, the fermentable matter. It was an albuminoid substance, endowed with a force to which the name catalytic was given. The ferment in fact acted by its mere presence.

A very curious observation, however, had been made in France by Cagniard-Latour and in Germany by Schwann. Cagniard-Latour, however, was the first to publish this observation, which was destined to become so fruitful. One of the ferments most in use, and known as early as the leavening of dough or the turning of milk, is the deposit formed in beer barrels, which is commonly called yeast. Repeating an observation of the naturalist Leuwenhoeck, Cagniard-Latour saw this yeast, which was composed of cells, multiplying itself by budding, and he proposed to himself the question whether the fermentation of sugar was not connected with this act of cellular vegetation. But as in other fermentations the existence of an organism had not been observed even by the most careful search, the hypothesis of Cagniard-Latour of a possible relation between the organisation of the ferment and the property of being a ferment was abandoned, though not without regret by some physiologists. M. Dumas, for example, recognised that in the budding of the yeast globules there must be some clue to the phenomenon of fermentation. I, however, repeat that as nothing of the kind had been found elsewhere, and as all other fermentations presented the common character of requiring, to put them in train, organic matter in a state of decomposition, the hypothesis of Cagniard-Latour remained a simple incident, instead of having the value of a scientific principle.

Liebig, moreover, carrying general opinion along with him, contended that it is not because of its being organised that yeast is active, but because of its being in contact with air. It is the dead portion of the yeast—that which has lived and is in the course of alteration—which acts upon the sugar.

The new memoirs published on the subject agreed in rejecting the hypothesis of any influence whatever of organisation or of life in the process of fermentation. Books, memoirs, dogmatic teaching, all were favourable to the theoretic ideas of Liebig. If a few rare observers indicated the presence in certain fermentations of living organisms, this presence was, in their opinion, a purely accidental fact, which, instead of favouring the phenomenon of fermentation, was injurious to it.

From his first investigation on lactic fermentation Pasteur was led to take an entirely different view of the matter. In this fermentation he recognised the presence and the action of a living organism, which was the ferment, just as yeast was the ferment of alcoholic fermentation. The lactic ferment was formed of cells, or rather of little rods nipped at their centres, extremely small, being hardly the thousandth part of a millimeter in diameter.[8] It reproduced itself by fission—that is to say, the little rod divided itself at its middle and formed two shorter rods, which became elongated, nipped, in their turn, at their centres, each giving rise, as before, to two rods. Each of these, again, soon divided itself into two, and so on. Why had not this been observed prior to Pasteur? For the simple reason that chemists had never observed the production of lactic fermentation except in complex substances. They mixed chalk with their milk for the purpose of preserving the neutrality of the fermenting medium. They employed substances such as caseine, gluten, animal membranes, all of which, when examined by the microscope, exhibited a multitude of mineral or organic granules, with which the lactic ferment was confounded. Thus the first care of Pasteur, with the view of proving the presence of the ferment and its life, was to replace the cheesy matter and all its congeners by a soluble, nitrogenous body, which would permit of the microscopic examination of all the living cellular products.

In a memoir presented to the Academy of Sciences in 1857 Pasteur stated that there were 'cases where it is possible to recognise in lactic fermentation, as practised by chemists and manufacturers, above the deposit of chalk and the nitrogenous matter, a grey substance which forms a zone on the surface of the deposit. Its examination by the microscope hardly permits of its being distinguished from the disintegrated caseum or gluten which has served to start the fermentation. So that nothing indicates that it is a special kind of matter which had its birth during the fermentation. It is this, nevertheless, which plays the principal part.'

To isolate this substance and to prepare it in a state of purity, Pasteur boiled a little yeast with from fifteen to twenty times its weight of water. He then carefully filtered the liquid, dissolved in it about fifty grammes of sugar to the litre, and added to it some chalk. Taking then, by means of a drawn-out tube, from a good ordinary lactic fermentation a trace of the grey matter of which we have just spoken, he placed it as the seed of the ferment in the limpid saccharine solution. By the next day a lively and regular fermentation had set in, the liquid becoming turbid and the chalk disappearing, and one could distinguish a deposit which progressed continually as the chalk dissolved. This deposit was the lactic ferment.

Pasteur reproduced this experiment by substituting for the water of the yeast a clear decoction of nitrogenous plastic substances. The ferment invariably presented the same aspect and the same multiplication. These results, however, did not yet satisfy Pasteur. He desired more rigour in a subject of such theoretic importance. Might not the partisans of Liebig's theory argue, if not without subtlety yet with a semblance of justice, that the fermentation was not due to the formation and progressive growth of this feeble nitrogenous globular deposit, but rather to the nitrogenous matter dissolved during the decoction of the yeast used in the composition of the liquor? Up to a certain point it might be maintained that the dissolved matters which had been in contact with the oxygen of the air had been thrown into molecular motion, that this motion had been communicated to the fermentable matter, and that the deposit of the pretended organised ferment was but an accident—one of the physical changes or one of the precipitates so frequently observed in the modifications of albuminoid matters. In the observation of Cagniard-Latour and of Schwann as to the life of the yeast, Liebig saw nothing more. 'One cannot deny,' said he, 'the organisation of the yeast or its multiplication by budding, but these living cells are always associated with other dead cells in process of molecular alteration. It is these molecular motions which communicate themselves to the molecules of the sugar, break them up, and cause them to ferment.'

The arguments of Liebig derived great strength from the belief which was shared by all chemists that the cells of yeast perish during fermentation and form lactate of ammonia. On examining this assertion, Pasteur found that not only was there no ammonia formed during alcoholic fermentation, but that even if ammonia were added it disappeared, entering into the formation of new yeast cells. Was not this a proof of the potency of the organised ferment?

Tormented, however, by the idea that, notwithstanding all these facts, the reasonings of Liebig might still find some credit, Pasteur worked earnestly to discover new facts capable of demonstrating that Liebig's theory was absolutely false. He made two crucial experiments, the one relating to the yeast of beer, or of alcohol, and the other relating to the lactic ferment. He introduced into a pure solution of sugar a small quantity of crystallisable salt of ammonia, then some phosphates of potash and magnesia, and he sowed in this medium an imponderable quantity, if we may so express it, of fresh cells of yeast. The cells thus sown multiplied, and the sugar fermented. In other words, the phosphorus, the potassium, the magnesium of the mineral salts, united to form the substances which compose the ferment. By this experiment, so simple and yet so demonstrative, the power of the organisation of the ferment was once for all established. The contact theory of Berzelius had no longer any meaning, since it was evident that the fermentable matter here furnished to the ferment one of its essential elements, namely, carbon. Liebig's theory of communicated molecular motion, originating in a nitrogenous albuminoid substance, had no better claim, since such substances had been discarded. The whole process took place between the sugar and a ferment germ which owed its life and development to nutritive matters, the most important of which was the fermentable substance. Fermentation, in short, was simply a phenomenon of nutrition. The ferment augmented in weight, feeding upon the sugar, and its vitality was such that it contrived to build up the complex materials of its own organisation by means of sugar and purely mineral elements.

In a second experiment, Pasteur demonstrated that, notwithstanding their smallness and the possibility of confounding them with the amorphous granules of caseine and gluten, the little particles of lactic ferment were indeed alive, and that they, and they only, were the cause of lactic fermentation. He mixed with some water, sweetened with sugar, a small quantity of a salt of ammonia, some alkaline and earthy phosphates, and some pure carbonate of lime obtained by precipitation. At the end of twenty-four hours the liquid began to get turbid and to give off gas. The fermentation continued for some days. The ammonia disappeared, leaving a deposit of phosphates and calcareous salt. Some lactate of lime was formed, and at the same time one could notice the deposition of the little lactic ferment. The germs of the lactic ferment had, in this case, been derived from particles of dust adhering to the substances themselves, of which the mixtures were made, or to the vessels used, or from the surrounding air. The chapter on spontaneous generation will render this clear.

It suffices here to state that the results of this second experiment were absolutely conclusive, and that the theories of contact force or of communicated motion, which up to that time had reigned in science, were completely overthrown.