Louis Pasteur: His Life and Labours

by René Vallery-Radot

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.'