So far no suspicion appears to have arisen in the minds of those who had occupied themselves with the study of fermentation that this change differed in any essential manner from many other reactions familiar to chemists. The origin and properties of the ferment were indeed remarkable and involved in obscurity, but the uncertainty regarding this substance was no greater than that surrounding many, if not all, compounds of animal and vegetable origin. Although, however, the purely chemical view as to the nature of yeast was generally recognised and adopted, isolated observations were not wanting which tended to show that yeast might be something more than a mere chemical reagent. As early as 1680 in letters to the Royal Society Leeuwenhoek described the microscopic appearance of yeast of various origins as that of small, round, or oval particles, but no further progress seems to have been made in this direction for nearly a century and a half, when we find that Desmazières [[1826]] examined the film formed on beer, figured the elongated cells of which it was composed, and described it under the name of Mycoderma Cerevisiæ. He, however, regarded it rather as of animal than of vegetable origin, and does not appear to have connected the presence of these cells with the process of fermentation.
Upon this long period during which yeast was regarded merely as a chemical compound there followed, as has so frequently occurred in similar cases, a sudden outburst of discovery. No less than three observers hit almost simultaneously upon the secret of fermentation and declared that yeast was a living organism.
First among these in strict order of time was Cagniard-Latour [[1838]], who made a number of communications to the Academy and to the Société Philomatique in 1835–6, the contents of which were collected in a paper presented to the Academy of Sciences on 12 June, 1837, and published in 1838. The observations upon which this memoir was based were almost exclusively microscopical. Yeast was recognised as consisting of spherical particles, which were capable of [p006] reproduction by budding but incapable of motion, and it was therefore regarded as a living organism probably belonging to the vegetable kingdom. Alcoholic fermentation was observed to depend on the presence of living yeast cells, and was attributed to some effect of their vegetative life (quelque effet de leur végétation). It was also noticed that yeast was not deprived of its fermenting power by exposure to the temperature of solid carbonic acid, a sample of which was supplied to Cagniard-Latour by Thilorier, who had only recently prepared it for the first time.
Theodor Schwann [[1837]], whose researches were quite independent of those of Cagniard-Latour, approached the problem from an entirely different point of view. During the year 1836 Franz Schulze [[1836]] published a research on the subject of spontaneous generation, in which he proved that when a solution containing animal or vegetable matter was boiled, no putrefaction set in provided that all air which was allowed to have access to the liquid was previously passed through strong sulphuric acid. Schwann performed a very similar experiment by which he showed that this same result, the absence of putrefaction, was attained by heating all air which came into contact with the boiled liquid. Wishing to show that other processes in which air took part were not affected by the air being heated, he made experiments with fermenting liquids and found, contrary to his expectation, that a liquid capable of undergoing vinous fermentation and containing yeast did not undergo this change after it had been boiled, provided that, as in the case of his previous experiments, only air which had been heated was allowed to come into contact with it.
Schwann's experiments on the prevention of putrefaction were unexceptionable and quite decisive. The analogous experiments dealing with alcoholic fermentation were not quite so satisfactory. Yeast was added to a solution of cane sugar, the flask containing the mixture placed in boiling water for ten minutes, and then inverted over mercury. About one-third of the liquid was then displaced by air and the flasks corked and kept inverted at air temperature. In two flasks the air introduced was ordinary atmospheric air, and in these flasks fermentation set in after about four to six weeks. Into the other two flasks air which had been heated was led, and in these no fermentation occurred. As described, the experiment is quite satisfactory, but Schwann found on repetition that the results were irregular. Sometimes all the flasks showed fermentation, sometimes none of them. This was correctly ascribed to the experimental difficulties, but none [p007] the less served as a point of attack for hostile and damaging criticism at the hands of Berzelius (p. [8]).
The origin of putrefaction was definitely attributed by Schwann to the presence of living germs in the air, and the similarity of the result obtained with yeast suggested the idea that alcoholic fermentation was also brought about by a living organism, a conception which was at once confirmed by a microscopical examination of a fermenting liquid. The phenomena observed under the microscope were similar to those noted by Cagniard-Latour, and in accordance with these observations alcoholic fermentation was attributed to the development of a living organism, the fermentative function of which was found to be destroyed by potassium arsenite and not by extract of Nux vomica, so that the organism was regarded rather as of vegetable than of animal nature. This plant received the name of "Zuckerpilz" or sugar fungus (which has been perpetuated in the generic term Saccharomyces). Alcoholic fermentation was explained as "the decomposition brought about by this sugar fungus removing from the sugar and a nitrogenous substance the materials necessary for its growth and nourishment, whilst the remaining elements of these compounds, which were not taken up by the plant, combined chiefly to form alcohol".
Kützing's memoir, the third of the trio [[1837]], also dates from 1837, and his opinions, like those of Cagniard-Latour, are founded on microscopical observations. He recognises yeast as a vegetable organism and accurately describes its appearance. Alcoholic fermentation depends on the formation of yeast, which is produced when the necessary elements and the proper conditions are present and then propagates itself. The action on the liquid thus increases and the constituents not required to form the organism combine to form unorganised substances, the carbonic acid and alcohol. "It is obvious," says Kützing, in a passage which roused the sarcasm of Berzelius, "that chemists must now strike yeast off the roll of chemical compounds, since it is not a compound but an organised body, an organism."
These three papers, which were published almost simultaneously, were received at first with incredulity. Berzelius, at that time the arbiter and dictator of the chemical world, reviewed them all in his "Jahresbericht" for 1839 [[1839]] with impartial scorn. The microscopical evidence was denied all value, and yeast was no more to be regarded as an organism than was a precipitate of alumina. Schwann's experiment (p. [6]) was criticised on the ground that the fermenting power of the added yeast had been only partially destroyed in the [p008] flasks in which fermentation ensued, completely in those which remained unchanged, the admission of heated or unheated air being indifferent, a criticism to some extent justified by Schwann's statement, already quoted, of the uncertain result of the experiment.
Berzelius himself regarded fermentation as being brought about by the yeast by virtue of that catalytic force, which he had supposed to intervene in so many reactions, both between substances of mineral and of animal and vegetable origin [[1836]], and which enabled "bodies, by their mere presence, and not by their affinity, to arouse affinities ordinarily quiescent at the temperature of the experiment, so that the elements of a compound body arrange themselves in some different way, by which a greater degree of electro-chemical neutralisation is attained".
To the scorn of Berzelius was soon added the sarcasm of Wöhler and Liebig [[1839], [1839]]. Stimulated in part by the publications of the three authors already mentioned, and in part by the report of Turpin [[1838]], who at the request of the Academy of Sciences had satisfied himself by observation of the accuracy of Cagniard-Latour's conclusions, Wöhler prepared an elaborate skit on the subject, which he sent to Liebig, to whom it appealed so strongly that he added some touches of his own and published it in the "Annalen," following immediately upon a translation of Turpin's paper. Yeast was here described with a considerable degree of anatomical realism as consisting of eggs which developed into minute animals, shaped like a distilling apparatus, by which the sugar was taken in as food and digested into carbonic acid and alcohol, which were separately excreted, the whole process being easily followed under the miscroscope.