Louis Pasteur (1822-1895)

Experiments and new views on the nature of fermentations

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It results from this that beer yeast has two ways of living, essentially distinct. Free oxygen gas can be totally absent, or it can be present in any volume whatever. In the second case it is used by the plant, the life of which is singularly activated. The little plant thus lives then in the manner of the lower plants; and as I have previously recognized with regard to the assimilation of carbon, of phosphates and of nitrogen, beer yeast does not offer essential differences from the moulds. It is well established that the yeast, placed in circumstances where it respires free oxygen gas, has a mode of life comparable in every respect with that of plants and lower animalcules. Now experience proves that the analogy goes much further, and that it extends to the disposition to ferment. In fact, if one determines the fermenting power of the yeast when it is assimilating free oxygen gas, one finds that this fermenting power of the yeast has almost completely disappeared.

I have little doubt that I could succeed in suppressing it completely; but what is certain is that I have already made it nearly twenty times less than it is in ordinary conditions, that is to say that for a development of yeast equal to 1 part, only 6 to 8 parts of sugar are transformed. Let us notice besides that the beer yeast which was just developing in contact with air absorbing oxygen gas, and which, under this influence and by this mode of special life, loses its disposition to ferment, has not however changed its nature. On the contrary: for if one transfers it to sugared water, protected from air, it immediately brings about there the most energetic fermentation. I have never known alcoholic yeast more active, no doubt because all the globules are budded and turgid. It is impossible to see a yeast more homogeneous and more remarkable in shapes, and in health, if I may express myself thus.

To sum up, the little cellular plant, commonly called beer yeast, can develop without free oxygen gas and it is a ferment; a dual property which separates it from all the lower beings. Or else it can develop while assimilating free oxygen gas and with such activity that one can say it is its normal life, and it loses its ferment character; a dual property which brings it near to all the low beings. But let us not forget to notice that if the yeast loses its disposition to ferment while it is multiplying under the influence of the oxygen of the air, it nevertheless puts itself back into the very same state for acting as a ferment, if one suppresses the free oxygen gas.

These are the facts in all their simplicity. Now what is their immediate consequence? Is it necessary to admit that the yeast so avid of oxygen that [it] removes it from atmospheric air with great activity, has no more need of it and does without it when denied this gas in the free state, while provided with it in profusion in the combined form in the fermentable matter? There is all the mystery of fermentation. For if one replies to the question that I have just posed by saying: since beer yeast assimilates oxygen gas with energy when it is free, that proves that it has need of it for living, and consequently it must take it from the fermentable matter if one refuses it this gas in the free state; immediately the plant appears to us as an agent of decomposition of sugar. When with each respiration movement of its cells, there will be molecules of sugar the equilibrium of which will be destroyed by the removal of a part of their oxygen. A phenomenon of decomposition will follow, and from this the ferment character, which on the contrary will be absent when the plant assimilates free oxygen gas.

To sum up, besides all the beings known until the present day, and which, without exception (at least so one believes) can respire and nourish themselves only in assimilating free oxygen gas, there would be a class of beings whose respiration would be active enough for them to be able to live outside the influence of air in taking up oxygen from certain compounds, whence there would result for the latter a slow and progressive decomposition. This second class of organized beings would be made up of the ferments, at all points similar to the beings of the first class, living like them having need of oxygen. But differing from them in that they would be able, in absence of free oxygen gas, to respire with the oxygen gas removed from little stable compounds.