Preliminary report on experiments concerning 
alcoholic fermentation and putrefaction

1837,  Theodore Schwann 

Schwann, Theodore. 1837. Vorläufige Mittheilung betreffend Versuche über die Weingährung und Fäulnis. Annalen der Physik und Chemie, Vol. 41, pages 184-193.

AT THE LAST MEETING OF THE Versammlung der Naturforscher in Jena, I reported experiments concerning spontaneous generation. I showed that if an enclosed glass sphere which contains air and a small amount of an infusion of meat, is heated in boiling water so that the liquid and air of the sphere are warmed to 80°R.[180°R. = 100°C.], then the liquid shows no putrefaction or production of infusoria, even after many months. This is true even when the quantity of meat extract in the sphere is so small that there is no chance that it would have absorbed all of the oxygen out of the air. Even so, it was desirable to modify the experiment so that it would be possible to allow for the entrance of air under conditions where the new air entering the sphere could be heated first. I have accomplished this in the following manner. 
A small flask which contained a small piece of meat and was filled one third full of water, was closed with a stopper which contained two thin glass tubes through it. These glass tubes passed for a distance of three inches through molten metal alloy which was continuously heated almost to the boiling point of mercury [357°C]. One of the glass tubes was connected then to a manometer. Then the liquid in the flask was boiled strongly, so that all of the air which was in the flask and tubes was either driven out or heated to the boiling point of water. After this cooled, a continuous stream of air was passed out of the manometer through the flask, and then out of the flask through the second tube. This air had the chance to be heated considerably on its passage through the tubes. This experiment was performed many times, and in all cases there was no putrefaction or production of infusoria or mold, even after many weeks, and the liquid remained as clear as it had been right after preparation. 
I will argue on another occasion whether this experiment, which has been repeated many times, will end the controversy concerning spontaneous generation. I will only remark here, that this experiment, when viewed from the standpoint of an Opponent of spontaneous generation, can be explained as follows: the germs of molds and infusoria, which according to this theory are present in the air, are killed by the thorough heating of the air. In the same way, putrefaction must be explained as follows: the germs which develop nourish themselves from the organic substances in the meat extract, and the phenomenon of putrefaction is due to the destruction of these substances. This opinion is also supported by the observation that those substances which act as poisons for infusoria and molds, like arsenic or mercuric chloride, are also the most effective in preventing putrefaction, while those substances which are only poisonous for infusoria, like Extractum Nucis vomicae spirituosum, and do not affect molds, prevent all of the manifestations of putrefaction which are associated with the production of infusoria, like hydrogen sulfide production, and only permit those manifestations of putrefaction which are associated with the production of molds.... 
I performed experiments on alcoholic fermentation in the following manner. A solution of cane sugar was mixed with beer yeast and four flasks were filled with it and stoppered. The flasks were then placed in boiling water for about 10 minutes, so that all of the liquid in them had reached this temperature. Then they were removed and inverted under mercury, and after cooling, air was allowed to enter in all four flasks to about 1/3 or 1/4 of the volume of liquid. This occurred in two of the flasks through a thin glass tube which was brought to red heat. In the other two the air which entered was not heated. An analysis had shown that air which had passed through a red-hot tube still contained 19.4 per cent oxygen.... The flasks were then stoppered and then incubated upright at a temperature of 10° to 14°R. [13° to 18°C]. After 4 to 6 weeks a fermentation began in the two flasks which had received the unheated air . . . while the two other flasks remain even now, after twice the time, completely unaffected.
Thus, in alcoholic fermentation as in putrefaction, it is not the oxygen of the air which causes this to occur, but a substance in the air which is destroyed by heat. 
One is forced to think that perhaps alcoholic fermentation is also a destruction of sugar which is caused by the development of infusoria or some other type of plant. Since Extr. Nucis vom. spir. is a poison for infusoria and not for molds, while arsenic is a poison for both, I employed these substances first, in order to see if my attention should be directed to infusoria or plants. I found that the Extr. Nucis vom. had no effect on alcoholic fermentation, while several drops of a solution of potassium arsenate abolished it completely. Thus I should probably look for a plant as the responsible agent. 
Microscopic examination of the beer yeast showed the familiar little grains  [Körnchen] which the ferment forms, but the majority of these were connected together in chains. They were partly round, but mostly oval grains of a light yellow colour, which occasionally occurred singly, but most often in chains of two to eight or more. From such chains came ordinarily one or more other chains at oblique angles. Frequently I could see between two units of a chain, a small grain attached at one side, as the beginning of a new chain, and generally at the last grain of the chain was a small grain which from time to time became elongated. In brief, the whole appearance was quite similar to many other articulated fungi, and it is without a doubt a plant. 
Herr Prof. Meyen, who observed these things at my request, was of the same opinion as myself, and found it difficult to say whether this was more like an alga or a filamentous fungus, but favored the latter because of the lack of green pigment. 
The beer yeast consists almost exclusively of these fungi. In freshly pressed grape juice, nothing of this kind can be seen. But if this juice is placed at a temperature of about 20°R. [25°C] one finds already after 36 hours such plants in it, which first consist of only a few units. The growth of these can be watched under the microscope, so that one can see already after 1/2 to 1 hour the increase in volume of a very small unit which is connected to a larger one. It is only several hours later that one can see the development of gas bubbles, since the first carbon dioxide formed remains dissolved in the water. The formation of these plants increases during the course of the fermentation, and at the end they can be seen in large amount as a light yellow powder at the bottom. They show only slight differences from the fungi in the beer yeast. Only a few of them are identical with those of the beer yeast. In the alcoholic fermentation of grapes the units are rounder and do not remain so frequently in straight chains. Also the number of units which remain single or with only a second small grain is much larger than is the case in beer yeast. The observation of their growth leaves no doubt of their plant-like nature. 
From these experiments the following main points can be established: 
(I) A boiled organic substance or a boiled fermentable liquid does not putrefy or ferment, respectively, even when air is admitted, so long as the air has been heated. 
(2) For putrefaction or fermentation or other processes in which new animals or plants appear, either unboiled organic substance or unheated air must be present. 
(3) In grape juice the development of gas is a sign of fermentation, and shortly thereafter appears a characteristic filamentous fungus, which can be called a sugar fungus [Note the derivation here of the Latin name Saccharomyces, which is the genus name for common yeast]. Throughout the duration of the fermentation, these plants grow and increase in number. 
(4) If ferments which already contain plants are placed in a sugar solution, the fermentation begins very quickly, much quicker than when these plants must first develop. 
(5) Poisons which only affect infusoria and do not affect lower plants (Extr. Nucis vom. spir.) prevent the manifestations of putrefaction which are characteristic of infusoria, but do not affect alcoholic fermentation or putrefaction with molds. Poisons which affect both animals and plants (arsenic) prevent putrefaction as well as alcoholic fermentation. 
The connection between the alcoholic fermentation and the development of the sugar fungus should not be misunderstood. It is highly probable that the development of the fungus causes the fermentation. Because a nitrogen containing substance is also necessary for the fermentation, it appears that nitrogen is necessary for the life of this plant, as it is probable that every fungus contains nitrogen. The alcoholic fermentation must be considered to be that decomposition which occurs when the sugar fungus utilizes sugar and nitrogen containing substances for its growth, in the process of which the elements of these substances which do not go into the plant are preferentially converted into alcohol. Most of the observations on the alcoholic fermentation fit quite nicely with this explanation. 

Comment by Thomas Brock
From the simplicity and clarity of Schwann's observations, it would seem obvious that his conclusions should quickly become universally accepted. However, this was not to be, as we shall see. In his first experiment Schwann confirms Spallanzani's experiments, at the same time answering any question that might arise about the availability of oxygen for the putrefaction process. He devises an ingenious system to prevent any unheated air from reaching his vessels, which surely required considerable patience and endurance. Many later workers could not confirm Schwann's experiment, and this may have been due merely to their lack of care in running the experiment. Schwann then proceeds to draw the conclusion that putrefaction is a by-product of the process of the growth of organisms which use the organic materials as food. He makes clever use of general and specific metabolic poisons in deciding what organisms were responsible for what phenomena. 
His experiments on alcoholic fermentation are also outstanding. He was the first to observe yeast in the process of growing. He accurately describes this process, and draws the obvious conclusion that this growth which he sees is of a living organism. He shows that alcoholic fermentation and the appearance of yeast cells are always associated events, and draws the conclusion, later denied by Liebig, that the yeast cells make the alcohol. 
Schwann was many years ahead of his time, and it was not until Pasteur's work of 1860 that the scientific world generally accepted these early conclusions. Two years after the preceding paper was published, Schwann published his classic work, "Microscopical researches on the similarity in structure and growth of animals and plants," in which he described for the first time the cellular nature of the higher animals.