Louis Pasteur: His Life and Labours

by René Vallery-Radot

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.

II.

The light shed by these experiments quickly extended its sphere; and Pasteur lost no time in discovering a new ferment, that of butyric acid. Having shown the absolute independence which exists between the ferment of butyric acid and the others, he found, contrary to the general belief, that the lactic ferment is incapable of giving rise to butyric acid, and that there exists a butyric fermentation having its own special ferment. This ferment consists of a species of vibrio. Little transparent cylindrical rods, rounded at their extremities, isolated or united in chains of two or three, or sometimes even more, form these vibrios. They move by gliding, the body straight, or bending and undulating. They reproduce themselves by fission, and to this mode of generation their frequent arrangement in the form of a chain is due.

Sometimes one of the little rods, with a train of others behind it, agitates itself in a lively manner as if to detach itself from the rest. Often, also, the little rod, after being broken off, holds on still to its chain by a mucous transparent thread.