Chapter VII (3)

But this new theory raised a chorus of controversy. Pasteur held to his proofs; he recalled what he had published concerning the typical ferment, the yeast of beer, an article inserted in the reports of the Académie des Sciences for 1861, and entitled, _The Influence of Oxygen on the Development of Yeast and on Alcoholic Fermentation_. In this article Pasteur, à propos of the chemical action connected with vegetable life, explained in the most interesting manner the two modes of life of the yeast of beer.

1. The yeast, placed in some sweet liquid in contact with air, assimilates oxygen gas and develops abundantly; under those conditions, it practically works for itself only, the production of alcohol is insignificant, and the proportion between the weight of sugar absorbed and that of the yeast is infinitesimal. 2. But, in its second mode of life, if yeast is made to act upon sugar without the action of atmospheric air, it can no longer freely assimilate oxygen gas, and is reduced to abstracting oxygen from the fermentescible matter.

“It seems therefore natural,” wrote Pasteur, “to admit that when yeast is a ferment, acting out of the reach of atmospheric air, it takes oxygen from sugar, that being the origin of its fermentative character.” It is possible to put the fermentative power of yeast through divers degrees of intensity by introducing free oxygen in variable quantities.

After comparing the yeast of beer to an ordinary plant, Pasteur added that “the analogy would be complete if ordinary plants had an affinity for oxygen so strong as to breathe, by withdrawing that element from unstable components, in which case they would act as ferments on those substances.” He suggested that it might be possible to meet with conditions which would allow certain inferior plants to live away from atmospheric air in the presence of sugar, and to provoke fermentation of that substance after the manner of beer yeast.

He was already at that time scattering germs of ideas, with the intention of taking them up later on and experimenting on them, or, if time should fail him, willingly offering them to any attentive scientist. These studies on beer had brought him back to his former studies, to his great delight.

“What a sacrifice I made for you,” he could not help saying to Dumas, with a mixture of affection and deference, and some modesty, for he apparently forgot the immense service rendered to sericiculture, “when I gave up my studies on ferments for five whole years in order to study silkworms!!!”

No doubt a great deal of time was also wasted by the endless discussions entered into by his scientific adversaries; but those discussions certainly brought out and evidenced many guiding facts which are now undisputed, as for instance the following--1. Ferments are living beings. 2. There is a special ferment corresponding to each kind of fermentation. 3. Ferments are not born spontaneously.

Liebig and his partisans had looked upon fermentation as a phenomenon of death; they had thought that beer yeast, and in general all animal and vegetable matter in a state of putrefaction, extended to other bodies its own state of decomposition.

Pasteur, on the contrary, had seen in fermentation a phenomenon correlative with life; he had provoked the complete fermentation of a sweet liquid which contained mineral substances only, by introducing into it a trace of yeast, which, instead of dying, lived, flourished and developed.

To those who, believing in spontaneous generation, saw in fermentations but a question of chance, Pasteur by a series of experimental proofs had shown the origin of their delusion by indicating the door open to germs coming from outside. He had moreover taught the method of pure cultures. Finally, in those recent renewals of old quarrels on the transformations into each other of microscopic species, Pasteur, obliged by the mycoderma vini to study closely its alleged transformation, which he had himself believed possible, had thrown ample light on the only dark spot of his luminous domain.

“It is enough to think,” writes M. Duclaux concerning that long discussion, “we have but to remember that those who denied the specific nature of the germ would now deny the specific nature of disease, in order to understand the darkness in which such opinions would have confined microbian pathology; it was therefore important that they should be uprooted from every mind.”