|Preliminary report on experiments
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
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.