Close upon this pleasantry followed a serious and important communication from Liebig [[1839]], in which the nature of fermentation, putrefaction, and decay was exhaustively discussed. Liebig did not admit that these phenomena were caused by living organisms, nor did he attribute them like Berzelius to the catalytic action of a substance which itself survived the reaction unchanged. As regards alcoholic fermentation, Liebig's chief arguments may be briefly summarised. As the result of alcoholic fermentation, the whole of the carbon of the sugar reappears in the alcohol and carbon dioxide formed. This change is brought about by a body termed the ferment, which is formed as the result of a change set up by the access of air to plant juices containing sugar, and which contains all the nitrogen of the nitrogenous constituents of the juice. This ferment is a substance remarkably susceptible of change, which undergoes an uninterrupted and progressive metamorphosis, of [p009] the nature of putrefaction or decay, and produces the fermentation of the sugar as a consequence of the transformation which it is itself undergoing.
The decomposition of the sugar is therefore due to a condition of instability transferred to it from the unstable and changing ferment, and only continues so long as the decomposition of the ferment proceeds. This communication of instability from one substance undergoing chemical change to another is the basis of Liebig's conception, and is illustrated by a number of chemical analogies, one of which will suffice to explain his meaning. Platinum is itself incapable of decomposing nitric acid and dissolving in it; silver, on the other hand, possesses this power. When platinum is alloyed with silver, the whole mass dissolves in nitric acid, the power possessed by the silver being transferred to the platinum. In like manner the condition of active decomposition of the ferment is transferred to the sugar, which by itself is quite stable. The central idea is that of Stahl (p. [2]) which was thus reintroduced into scientific thought.
In a pure sugar solution the decomposition of the ferment soon comes to an end and fermentation then ceases. In beer wort or vegetable juices, on the other hand, more ferment is continually formed in the manner already described from the nitrogenous constituents of the juice, and hence the sugar is completely fermented away and unexhausted ferment left behind. Liebig's views were reiterated in his celebrated "Chemische Briefe," and became the generally accepted doctrine of chemists. There seems little doubt that both Berzelius and Liebig in their scornful rejection of the results of Cagniard-Latour, Schwann and Kützing, were influenced, perhaps almost unconsciously, by a desire to avoid seeing an important chemical change relegated to the domain of that vital force from beneath the sway of which a large part of organic chemistry had just been rescued by Wöhler's brilliant synthetical production of urea and by the less recognised synthesis of alcohol by Hennell (see on this point Ahrens [[1902]]). A strong body of evidence, however, gradually accumulated in favour of the vegetable nature of yeast, so that it may be said that by 1848 a powerful minority adhered to the views of Cagniard-Latour, Schwann, and Kützing [see Schrohe, [1904], p. 218, and compare Buchner, [1904]]. Among these must be included Berzelius [[1848]], who had so forcibly repudiated the idea only ten years before, whereas Liebig in the 1851 edition of his letters does not mention the fact that yeast is a living organism (Letter XV).
The recognition of the vegetable nature of yeast, however, by no [p010] means disproved Liebig's view of the nature of the change by which sugar was converted into carbon dioxide and alcohol, as was carefully pointed out by Schlossberger [[1844]] in a research on the nature of yeast, carried out in Liebig's laboratory but without decisive results.
Mitscherlich was also convinced of the vegetable character of yeast, and showed [[1841]] that when yeast was placed in a glass tube closed by parchment and plunged into sugar solution, the sugar entered the glass tube and was there fermented, but was not fermented outside the tube. He regarded this as a proof that fermentation only occurred at the surface of the yeast cells, and explained the process by contact action in the sense of the catalytic action of Berzelius, rather than by Liebig's transference of molecular instability. Similar results were obtained with an animal membrane by Helmholtz [[1843]], who also expressed his conviction that yeast was a vegetable organism.
In 1854 Schröder and von Dusch [[1854], [1859, 1861]] strongly reinforced the evidence in favour of this view by succeeding in preventing the putrefaction and fermentation of many boiled organic liquids by the simple process of filtering all air which had access to them through cotton-wool. These experiments, which were continued until 1861, led to the conclusion that the spontaneous alcoholic fermentation of liquids was due to living germs carried by the air, and that when the air was passed through the cotton-wool these germs were held back.
At the middle of the nineteenth century opinions with regard to alcoholic fermentation, notwithstanding all that had been done, were still divided. On the one hand Liebig's theory of fermentation was widely held and taught. Gerhardt, for example, as late as 1856 in the article on fermentation in his treatise on organic chemistry [[1856]], gives entire support to Liebig's views, and his treatment of the matter affords an interesting glimpse of the arguments which were then held to be decisive. The grounds on which he rejects the conclusions of Schwann and the other investigators who shared the belief in the vegetable nature of yeast are that, although in some cases animal and vegetable matter and infusions can be preserved from change by the methods described by these authors, in others they cannot, a striking case being that of milk, which even after being boiled becomes sour even in filtered air, and this without showing any trace of living organisms. The action of heat, sulphuric acid, and filtration on the air is to remove, or destroy, not living organisms but particles of decomposing matter, that is to say, ferments which would add their activity to that of the oxygen of the air. Moreover, many ferments, as for example diastase, act without [p011] producing any insoluble deposit whatever which can be regarded as an organism.
"Evidemment," he concludes, "la théorie de M. Liebig explique seule tous les phénomènes de la manière la plus complète et la plus logique; c'est à elle que tous les bons esprits ne peuvent manquer de se rallier."
On the other hand it was held by many to have been shown that Liebig's view of the origin of yeast by the action of the air on a vegetable infusion was erroneous, and that fermentation only arose when the air transferred to the liquid an active agent which could be removed from it by sulphuric acid (Schulze), by heat (Schwann), and by cotton-wool (Schröder and von Dusch). Accompanying alcoholic fermentation there was a development of a living organism, the yeast, and fermentation was believed, without any very strict proof, to be a phenomenon due to the life and vegetation of this organism. This doctrine seems indeed [Schrohe, [1904]] to have been widely taught in Germany from 1840–56, and to have established itself in the practice of the fermentation industries.
In 1857 commenced the classical researches of Pasteur which finally decided the question as to the origin and functions of yeast and led him to the conclusion that "alcoholic fermentation is an act correlated with the life and organisation of the yeast cells, not with the death or putrefaction of the cells, any more than it is a phenomenon of contact, in which case the transformation of sugar would be accomplished in presence of the ferment without yielding up to it or taking from it anything" [[1860]]. It is impossible here to enter in detail into Pasteur's experiments on this subject, or indeed to do more than indicate the general lines of his investigation. His starting-point was the lactic acid fermentation.