intermediary, producing the fixation of the oxygen of the air; since
in no circumstances can pure alcohol, diluted to any degree whatever
with pure water, transform itself into vinegar. But this necessary
intermediary is not, as the German theory would have it, a dead
albuminoid substance; it is a plant, and of all plants one of the
simplest and most minute, which has been known from time immemorial
under the name of flower of vinegar. This little fungus is invariably
present on the surface of a wine which is being transformed into
vinegar. Liebig was not ignorant of this, but he regarded it as a
simple coincidence. Do we not know, said he, that whenever an infusion
of organic matter is exposed to the air it becomes covered with a
cryptogamic vegetation, or is invaded by a crowd of animalculæ? Is not
vinegar a vegetable infusion? Vinegar affords a refuge to the flower of
vinegar, just as it gives refuge to what are called the little eels of
vinegar.
We can appreciate here the uncertainties of pure observation. The
great art--and no one practised it better than Pasteur--consists in
instituting decisive experiments which leave no room for an inexact
interpretation of facts. These decisive proofs of the true part
played by the little microscopic fungus, by this flower of vinegar,
this mycoderma aceti, are thus formulated by Pasteur. It is but
another example of the method which he used in alcoholic, lactic, and
tartaric fermentations. The theories of Berzelius, of Mitscherlich,
and of Liebig were destined again to receive the rudest shocks by the
demonstration of these rigorous facts.
Let us place a little wine in a bottle, then hermetically seal it,
and leave it to itself. In these conditions the wine becomes sour.
But if we take the precaution of putting the bottle into hot water,
so that the wine and the air in the bottle may be heated for some
instants to a temperature of 60° Centigrade, and if, after cooling, we
leave the bottle to itself, the wine in these conditions will never
become transformed into vinegar. The heating, however, must have left
intact the albuminoid or nitrogenous substances contained in the
wine. These, then, cannot constitute the ferment of the vinegar. Can
it be maintained that by heating the wine to 60° we have altered the
albuminoid matter, which is, on this account, no longer able to act as
a ferment, or, in other words, no longer able to determine the union
of the oxygen of the air with the alcohol? This hypothesis falls to
pieces before the following experiment. Open the bottle, blow into it
with bellows, so that the once heated wine shall come into contact with
ordinary air, and the acetification of the wine will take place.
But the master experiment is the following. We have seen that pure
alcoholised water never turns sour unless some albuminoid matter is
introduced into it. Pasteur saw that this albuminoid matter might
be completely suppressed and replaced by saline crystallisable
substances, alkaline and earthy phosphates, to which has been added a
little phosphate of ammonia. In these conditions, especially if the
alcoholised water be acidulated by small quantities of pure acetic
acid, one actually sees the mycoderm developing, and the alcohol
transforming itself into acetic acid. It is not possible to demonstrate
in a more convincing manner that the albuminoid matters of the wine are
not in this case the acetic ferment. These albuminoid matters, however,
contribute to the acetic fermentation, but only as being an aliment to
the mycoderma aceti, and notably a nitrogenous aliment. The true and
only ferment of vinegar is the little fungus; it is the great agent of
the phenomenon; it, indeed, accomplishes all.
Is there not a great charm in seeing an obscure subject clearly
illuminated by facts well understood and well interpreted? If in a
bottle containing wine and air and raised to a temperature of 50° or
60° the wine never turns sour, it is because the germs of the mycoderma
aceti, which the wine and the air hold in suspension, are deprived of
all vitality by the heat. Placed, however, in contact with ordinary
air, this once-heated wine can turn sour; because, though the germs of
the mycoderma aceti contained at first in the wine are killed, this
is not the case with those derived from the surrounding air. Pure
alcoholised water never turns sour, even in contact with ordinary air,
and with whatever germs this air may carry, or that may be found in the
dust of the vessels which receive it. The reason is that these germs
cannot become fertile because of the absence of their indispensable
food. Wine in bottles well filled and laid flat do not acetify; this is
because the mycoderm cannot multiply for lack of oxygen. Without doubt
the air constantly penetrates through the pores of the cork, but always
in such feeble quantities that the colouring matters of the wine, and
other more or less oxydisable constituents, take possession of it
without leaving the smallest quantity for the germs of the mycoderm
which are generally suspended in the wine. When the bottle is upright
the conditions are quite altered. The desiccation of the cork renders
it much more permeable to the air, and the germs of the mycoderm on the
surface of the liquid, if any exist there, are enveloped by air.
Thus, to recapitulate in a few words the principles which have just
been established; it is easy to see that the formation of vinegar is
always preceded by the development, on the surface of the wine, of a
little plant formed of strangulated particles, of an extreme tenuity,
and the accumulation of which sometimes takes the form of a hardly
visible veil, sometimes of a wrinkled film of very slight thickness,
and greasy to the touch, because of the various fatty matters which the
plant contains.
This cryptogam has the singular property of condensing considerable
quantities of oxygen and of provoking the _fixation_ of this gas upon
the alcohol, which is thereby transformed into acetic acid. The little
mycoderm is not less exacting than larger vegetables. It must have
its appropriate aliments. Wine offers them in abundance: nitrogenous
matters, the phosphates of magnesia and of potash. The mycoderm
thrives, moreover, in warm climates. To cultivate it in temperate
regions like ours it is well to warm artificially the places where it
is cultivated. But if wine contains within itself all the elements
necessary to the life of the little mycoderm, this life is further
promoted by rendering the wine more acid through the addition of acetic
acid.
What, then, can be more simple than to produce vinegar from wine--a
manufacture which justly makes the reputation of the town of Orleans?
Take some wine, and after having mixed with it one-fourth or one-third
of its volume of vinegar already formed, sow on its surface the little
plant which does the work of acetification. It is only necessary to
skim off, by means of a wooden spatula, a little of the mycodermic film
from a liquid covered with it, and to transfer it to the liquid to be
acetified. The fatty matters which it contains render the wetting of
it difficult. Thus, when we plunge into the liquid the spatula covered
with the film, the latter detaches itself and spreads out over the
surface instead of falling to the bottom. When we operate in summer,
or in a room heated to 15° or 25° Centigrade in winter, in twenty-four
or forty-eight hours at most, the mycoderm covers the whole liquid,
so easy and rapid is its development. After some days all the wine has
become vinegar.
On one occasion, in a discussion which he was holding at the Academy
of Sciences, Pasteur, wishing to affirm the prodigious activity of the
life and multiplication of this little organism, expressed himself
thus:--
'I would undertake in the space of twenty-four hours to cover with
mycoderma aceti a surface of vinous liquid as large as the hall in
which we are here assembled. I should only have to sow in it the day
before almost invisible particles of newly-formed mycoderma aceti.'
Let the reader try to imagine the millions upon millions of little
mycoderma particles which would come to life in that one day.
But how is the mycoderm seed to be obtained in the first instance?
Nothing more simple. The mycoderma aceti is one of those little
so-called 'spontaneous' productions which are sure to appear of
themselves on the surface of liquids or infusions suitable to their
development. In wine, in vinegar, or suspended in air, everywhere
around us, in our towns, in our houses, there exist germs of this
little plant. If we wish to procure some fresh mycoderm it is only
necessary to put a mixture of wine and vinegar into a warm place. In a
few days, generally, if not always, there appear here and there little
greyish patches scattering the light instead of regularly reflecting
it, as does the surrounding liquid. These specks go on increasing
progressively and rapidly. This is the mycoderma aceti raised from
the seeds which the wine or the added vinegar contained, or which the
air deposited; just as we see a field covered with divers weeds by
seeds naturally distributed in the earth, or which have been brought
to it by the wind or by animals. Even in this last circumstance the
comparison holds good, for after you have put wine or vinegar in a
warm place there soon appear, whence we know not, little reddish
flies, so commonly seen in vinegar manufactories, and in all places
where vegetable matter is turning sour. With their feet, or with their
probosces, these flies transport the seed.
* * * * *
At Orleans the process for the manufacture of vinegar is very simple.
Barrels ranged over each other have on each of their vertically-placed
bottoms a circular opening some centimeters in diameter, and a smaller
hole adjacent, called _fausset_, for the air to pass in and out when
the large opening is closed, either by the funnel, through which the
wine is introduced, or by the syphon, which is used for drawing off the
vinegar. These barrels, of which the capacity is 230 litres, are half
filled. The manual labour consists in keeping up a suitable temperature
in the vessel, and in drawing from it every eight days about eight or
ten litres of vinegar, which are replaced by eight or ten litres of
wine.
A barrel in which this give-and-take of wine and vinegar goes on is
technically called a 'mother.' The starting of a 'mother' is not a
rapid process. We begin by introducing into the barrel 100 litres of
very good and very limpid vinegar; then two litres only of wine are
added. Eight days after, three litres of wine are added, a week later
four or five, until the barrel contains about 180 to 200 litres. Then
for the first time vinegar is drawn off in sufficient quantity to bring
back the volume of the liquid to about 100 litres. At this moment the
labours of the 'mother' begin. Henceforward ten litres of vinegar may
be drawn off every eight days, to be replaced by ten litres of wine.
This is the maximum that a cask can yield in a week. When the casks
work badly, as is often the case, it is necessary to diminish their
production.
This Orleans system has many drawbacks. It requires three or four
months to prepare what is called a 'mother,' which must be nourished
with wine _very_ regularly once a week under penalty of seeing it
lose all its power. Then it is necessary to continue the manufacture
at all times, whether the vinegar be required or not. To reconstitute
a 'mother,' one must begin from the very beginning, a process which
involves a loss of three or four months' time. Lastly--a condition
which is at times very inconvenient--a 'mother' cannot be transported
from one place to another, or even from one part of the same locality
to another. The 'mother,' in fact, must rest immovable.
Pasteur advised the suppression of the 'mothers.' He recommended an
apparatus, which is simply a vat, placed in a chamber the temperature
of which can be raised to 20° or 25° Centigrade. In these vats vinegar
already formed is mixed with wine. On the surface is sown the little
plant which converts the wine into vinegar. The mode of sowing it has
been already explained. The acetification begins with the development
of the plant.
A great merchant of Orleans, who had from the first adopted Pasteur's
process, and who had won the prize offered by the 'Society for the
Encouragement of National Industry' for a manufactory perfected after
these principles, has stated that at the end of nine or ten days,
sometimes even in eight, all the acetified wine is converted into
vinegar. From a hundred litres of wine he drew off ninety-five litres
of vinegar. After the great rise of temperature observed at the moment
of the formation of the vinegar, and which is caused by the chemical
union of the alcohol and the oxygen of the air, the vinegar is allowed
to cool. It may then be drawn from the vat, introduced into barrels,
refined, and straightway delivered, fit for consumption. When the vat
is quite emptied, and well cleaned, a new mixture is made of vinegar
and wine, the little plant is sown as before, and the same facts are
reproduced in the second as in the first operation.
* * * * *
In the vessels where vinegar is preserved, whether in the
manufactories, in private houses, or in grocers' shops, it often
happens that the liquid becomes turbid, and impoverished in an
extraordinary manner; it even ends in putrefaction, if a remedy be
not promptly applied. Pasteur has pointed out the cause of these
phenomena. After the alcohol has become acetic acid by the combustive
action of the mycoderm, the question remains, what becomes of the
mycoderm? Most frequently it falls to the bottom of the vessel,
having no more work to accomplish. This is a phase of the manufacture
which must be watched with care. It is shown by the experiments of
Pasteur that the mycoderma aceti can live on vinegar already formed,
maintaining its power of fixing the oxygen on certain constituents of
the liquid. In this case the acetic acid itself is the seat of the
chemical action--in other words, the oxygen unites with the carbon
of the acetic acid, and transforms it into carbonic acid, and as the
acetic acid has a composition which can be represented by carbon and
water, it follows that if the combustion is allowed to take its course,
instead of vinegar we have eventually nothing but water mixed with a
small proportion of nitrogenous and mineral matters, and the remains
of the mycoderm. We have thus an ordinary organic infusion exempt
from all acidity, and one which could not be better fitted to become
the prey of the vibrios of putrefaction or of the aérobic mucors. By
these mucors, moreover, which form a film on the surface of the liquid
after the mycoderm has fallen, the anaérobic vibrios, protected from
the action of the air, can come into active existence. Here we find
ourselves in presence of one of those double phenomena, of putrefaction
in the deeper parts of the liquid, and of combustion at the surface
which is in contact with the air. Nothing is more prejudicial to the
quality of the vinegar than the setting in of this combustion after
the vinegar has been formed, and when it contains no more alcohol. The
first materials of the vinegar upon which the oxygen transmitted by the
mycoderm fixes are, in fact, the ethereal and aromatic constituents
which give to vinegar its chief value.
Another cause of the deterioration of the quality of vinegar, which is
sometimes very annoying to the manufacturer, consists in the frequent
presence of little eel-like organisms, very curious when viewed with a
strong magnifier. Their bodies are so transparent that their internal
organs can be easily distinguished. These eel-like creatures multiply
with extraordinary rapidity. Certainly there is not a single barrel
of vinegar manufactured by the Orleans system which does not contain
them in alarming numbers. Prior to Pasteur's investigations, the
ignorance regarding these organisms was such that they were actually
considered necessary to the production of the vinegar; whereas they
are, on the contrary, most inimical to it, and must, if possible, be
got rid of. This is, moreover, rendered desirable by the repugnance
which is naturally felt to using a liquid defiled by the presence of
such animalcules--a repugnance which becomes almost insurmountable to
anyone who has once seen through a microscope the swarms contained
in a drop of vinegar. The mischief wrought by these little beings in
the manufacture of vinegar results from the fact that they require
air to live. The effect can easily be perceived by filling to the
brim a bottle of vinegar, corking it, and then comparing it with a
similar bottle half filled with the same vinegar, and left uncorked in
contact with the air. In the first bottle, the motions of the eel-like
creatures become gradually slower, until after a few days they cease to
multiply and fall lifeless to the bottom of the vessel. In the second
bottle, on the contrary, they continue to swarm and move about. This
need of oxygen is further demonstrated by the fact that, if the vinegar
reaches a certain depth in the bottle, life is suspended in the lower
parts, and the little eel-like organisms, in order to breathe more
freely, form a crawling zone in the upper layers of the liquid.
Connecting these observations with the other fact that the vinegar
is formed by the action of the mycodermic film on its surface, we can
understand at once that the mycoderm and the little eels continually
carry on a struggle for existence, since both of these living
things--the one animal the other vegetable--imperiously demand the same
aliment, oxygen. They live, moreover, in the same superficial layers, a
circumstance which gives rise to very curious phenomena. When, for one
reason or another, the film of mycoderm is not formed, or when there
is any delay in its production, the little eels invade in such great
numbers the upper layers of the liquid that they absorb all the oxygen.
The little plant has in consequence great difficulty in developing
itself or even in beginning its life. Reciprocally, when the work of
acetification is active, and when the mycoderm has occupied the upper
layers, it gradually drives away the eels, which take refuge, not deep
down, where they would perish, but against the moist sides of the
barrel or the vat. There they form a thick whitish scum all in motion.
It is a very curious spectacle. Here their enemy, the mycoderm, can no
longer injure them to the same extent, since they are surrounded with
air; and here they wait with impatience for the moment when they can
again take their place in the liquid, and, in their turn, fight against
the mycoderm. In Pasteur's process, where the vats are very often
cleansed, it is easy to keep them free from these little animalcules;
they have not time to multiply to a hurtful extent. Indeed, if the
operation be well conducted, they do not make their appearance at all.
* * * * *
Nearly all Pasteur's publications have had from the moment of their
appearance to undergo the severest criticism. Their novelty caused
them to clash with the prejudices and errors current in science. His
researches on fermentation provoked lively opposition. Liebig did not
accept without recrimination a series of researches which concurred in
upsetting the theory he had enunciated and defended in all his works.
After having kept silence for ten years, he published, at Munich, where
he was professor, a long memoir entirely directed against Pasteur's
results. In 1870, on the eve of the war, Pasteur, who was at that time
returning from a scientific journey into Austria, determined to pass
by Munich, with the view of attempting to convince his distinguished
adversary. Liebig received him with great courtesy, but, hardly
recovered from an illness, he alleged his convalescence as a reason for
declining all discussion.
Then followed the Franco-German war. Hardly was it terminated when
Pasteur brought before the Academy of Sciences at Paris a defence
of what he had published, as a sort of challenge to his illustrious
opponent. The memoir of Liebig was filled with the most skilful
arguments.
'I pondered it for nearly ten years before producing it,' he wrote.
Pasteur, putting aside all subtleties of argument, went straight to
the two objections of the German chemist which lay at the root of the
discussion.
It may be remembered that one of the most decisive proofs by which
Pasteur overthrew Liebig's theory resulted from the experiments in
which by the aid of mineral bodies and fermentable matter he produced
a special living ferment for each definite fermentation. By removing
all nitrogenous organic matter, which in Liebig's theory constitutes
the ferment, Pasteur established, at one and the same time, the life
of the ferment and the absence of all action of albuminoid matter in
process of alteration. Liebig here formally contested the fact that
Pasteur had been able to produce yeast and alcoholic fermentation in a
sweetened mineral medium by sowing therein an infinitesimal quantity of
yeast. It is certain that, ten years previously, when Pasteur announced
the production of yeast life and alcoholic fermentation under such
conditions, his experiment was one so difficult to perform that it
sometimes happened to Pasteur himself to be unable to reproduce it. The
cells of yeast sown in the sweetened mineral medium found themselves
often associated with other microscopic organisms, which were
singularly hurtful to the life of the yeast. Pasteur was at this period
far from being familiarised with the delicacy which such experiments
require, and he did not yet know all the precautions indicated later
on, which were indispensable to success. Though in his original memoir
of 1860 Pasteur had pointed out the difficulties of his experiment,
these difficulties existed nevertheless. Liebig took hold of them
with skill, exaggerated them; saw, so to speak, nothing but them; and
declared that the results announced never could have been obtained. But
in 1871 the fundamental experiment of Pasteur, on the life of yeast
in a sweetened mineral medium, had become a trifle for him. He knew
exactly how to form media deprived of all foreign germs, how to prepare
pure yeast, and how to prevent the introduction of new germs, which
could develop in the liquids and hinder the life of the yeast.
'Choose,' said he to Liebig, 'from the members of the Academy one or
several, and ask them to decide between you and me. I am ready to
prepare before you and before them, in a sweetened mineral medium, as
much yeast as you can reasonably ask for, and with substances provided
by yourself.'
Liebig's second objection had reference to acetic fermentation. The
process of acetification known as that of 'beech shavings' is widely
practised in Germany and even in France. It consists in causing alcohol
diluted with water and with the addition of some _millièmes_ of acetic
acid to trickle slowly into barrels or vats filled with shavings of
beech, either massed together without order or disposed in layers
after having been rolled up like the spring of a watch. Openings
formed in the sides of the barrel, and in a double bottom upon which
the shavings rest, permit the access of the air, which rises into the
barrel as it would in a chimney, and yields all or part of its oxygen
to the alcohol to convert it into acetic acid. All writers prior to
Pasteur, and Liebig in particular, maintained that the shavings acted
like porous bodies in the same manner as finely divided platinum. The
acetic acid, they said, was formed by a direct oxidation, without any
other influence than the porosity of the wood. This view of the subject
was rendered plausible by the fact that in many manufactories the
alcohol employed is that of distillation, which contains no albuminoid
substances. Moreover, the duration of the shavings is in a sense
indefinite.
According to Pasteur, the shavings perform only a passive part in
the manufacture. They promote the division of the liquid and cause
a considerable augmentation of the surface exposed to the air. They
moreover serve as a support for the ferment, which is still, according
to him, the mycoderma aceti, under the mucous form proper to it when
submerged.