tub? What is this yeast, and how did the brewer become possessed of
it? Examine its quantity before and after fermentation. The brewer
introduces, say 10 cwts. of yeast; he collects 40, or it may be 50,
cwts. The yeast has, therefore, augmented from four to five fold
during the fermentation. Shall we conclude that this additional yeast
has been spontaneously generated by the wort? Are we not rather
reminded of that seed which fell into good ground, and brought forth
fruit, some thirty fold, some sixty fold, some an hundred fold? On
examination, this notion of organic growth turns out to be more than a
mere surmise. In the year 1680, when the microscope was still in its
infancy, Leeuwenhoek turned the instrument upon this substance, and
found it composed of minute globules suspended in a liquid. Thus
knowledge rested until 1835, when Cagniard de la Tour in France, and
Schwann in Germany, independently, but animated by it common thought,
turned microscopes of improved definition and heightened powers upon
yeast, and found it budding and sprouting before their eyes. The
augmentation of the yeast alluded to above was thus proved to arise
from the growth of a minute plant now called Torula (or Saccharomyces)
Cerevisiae. Spontaneous generation is therefore out of the question.
The brewer deliberately sows the yeast-plant, which grows and
multiplies in the wort as its proper soil. This discovery marks an
epoch in the history of fermentation.
But where did the brewer find his yeast? The reply to this question
is similar to that which must be given if it were asked where the
brewer found his barley. He has received the seeds of both of them
from preceding generations. Could we connect without solution of
continuity the present with the past, we should probably be able to
trace back the yeast employed by my friend Sir Fowell Buxton to-day to
that employed by some Egyptian brewer two thousand years ago. But you
may urge that there must have been a time when the first yeast-cell
was generated. Granted--exactly as there was a time when the first
barley-corn was generated. Let not the delusion lay hold of you that
a living thing is easily generated because it is small. Both the
yeast-plant and the barley-plant lose themselves in the dim twilight
of antiquity, and in this our day there is no more proof of the
spontaneous generation of the one, than there is of the spontaneous
generation of the other.
I stated a moment ago that the fermentation of grape-juice was
spontaneous; but I was careful to add, in what sense spontaneous will
appear more clearly by-and-by.' Now this is the sense meant. The
wine-maker does not, like the brewer and distiller, deliberately
introduce either yeast; or any equivalent of yeast, into his vats; he
does not consciously sow in them any plant, or the germ of any plant;
indeed, he has been hitherto in ignorance whether plants or germs of
any kind have had anything to do with his operations. Still, when the
fermented grape-juice is examined, the living Torula concerned in
alcoholic fermentation never fails to make its appearance. How is
this? If no living germ has been introduced into the wine-vat, whence
comes the life so invariably developed there?
You may be disposed to reply, with Turpin and others, that in virtue
of its own inherent powers, the grape-juice when brought into contact
with the vivifying atmospheric oxygen, runs spontaneously and of its
own accord into these low forms of life. I have not the slightest
objection to this explanation, provided proper evidence can be adduced
in support of it. But the evidence adduced in its favour, as far as I
am acquainted with it, snaps asunder under the strain of scientific
criticism. It is, as far as I can see, the evidence of men, who
however keen and clever as observers, are not rigidly trained
experimenters. These alone are aware of the precautions necessary in
investigations of this delicate kind. In reference, then, to the life
of the wine-vat, what is the decision of experiment when carried out
by competent men? Let a quantity of the clear, filtered 'must' of the
grape be so boiled as to destroy such germs as it may have contracted
from the air or otherwise. In contact with germless air the
uncontaminated must never ferments. All the materials for spontaneous
generation are there, but so long as there is no seed sown, there is
no life developed, and no sign of that fermentation which is the
concomitant of life. Nor need you resort to a boiled liquid. The
grape is sealed by its own skin against contamination from without. By
an ingenious device Pasteur has extracted from the interior of the
grape its pure juice, and proved that in contact with pure air it
never acquires the power to ferment itself, nor to produce
fermentation in other liquids. [Footnote: The liquids of the healthy
animal body are also sealed from external contamination. Pure blood,
for example, drawn with due precautions from the veins, will never
ferment or putrefy in contact with pure air.] It is not, therefore,
in the interior of the grape that the origin of the life observed in
the vat is to be sought.
What then is its true origin? This is Pasteur's answer, which his
well-proved accuracy renders worthy of all confidence. At the time of
the vintage microscopic particles are observed adherent, both to the
outer surface of the grape and of the twigs which support the grape.
Brush these particles into a capsule of pure water. It is rendered
turbid by the dust. Examined by a microscope, some of these minute
particles are seen to present the appearance of organised--cells.
Instead of receiving them in water, let them be brushed into the pure
inert juice of the grape. Forty-eight hours after this is done, our
familiar Torula is observed budding and sprouting, the growth of the
plant being accompanied by all the other signs of active fermentation.
What is the inference to be drawn from this experiment? Obviously
that the particles adherent to the external surface of the grape
include the germs of that life which, after they have been sown in the
juice, appears in such profusion. Wine is sometimes objected to on
the ground that fermentation is 'artificial;' but we notice here the
responsibility of nature. The ferment of the grape clings like a
parasite to the surface of the grape; and the art of the wine-maker
from time immemorial has consisted in bringing--and it may be added,
ignorantly bringing--two things thus closely associated by nature into
actual contact with each other. For thousands of years, what has been
done consciously by the brewer, has been done unconsciously by the
wine-grower. The one has sown his leaven just as much as the other.
Nor is it necessary to impregnate the beer-wort with yeast to provoke
fermentation. Abandoned to the contact of our common air, it sooner
or later ferments; but the chances are that the produce of that
fermentation, instead of being agreeable, would be disgusting to the
taste. By a rare accident we might get the true alcoholic
fermentation, but the odds against obtaining it would be enormous.
Pure air acting upon a lifeless liquid will never provoke
fermentation; but our ordinary air is the vehicle of numberless germs
which act as ferments when they fall into appropriate infusions. Some
of them produce acidity, some putrefaction. The germs of our
yeast-plant are also in the air; but so sparingly distributed that an
infusion like beer-wort, exposed to the air, is almost sure to be
taken possession of by foreign organisms. In fact, the maladies of
beer are wholly due to the admixture of these objectionable ferments,
whose forms and modes of nutrition differ materially from those of the
true leaven.
Working in an atmosphere charged with the germs of these organisms,
you can understand how easy it is to fall into error in studying the
action of any one of them. Indeed it is only the most accomplished
experimenter, who, moreover, avails himself of every means of
checking his conclusions, that can walk without tripping through this
land of pitfalls. Such a man the French chemist Pasteur has hitherto
proved himself to be. He has taught us how to separate the commingled
ferments of our air, and to study their pure individual action. Guided
by him, let us fix our attention more particularly upon the growth and
action of the true yeast-plant under different conditions. Let it be
sown in a fermentable liquid, which is supplied with plenty of pure
air. The plant will flourish in the aerated infusion, and produce
large quantities of carbonic acid gas--a compound, as you know, of
carbon and oxygen. The oxygen thus consumed by the plant is the free
oxygen of the air, which we suppose to be abundantly supplied to the
liquid. The action is so far similar to the respiration of animals,
which inspire oxygen and expire carbonic acid. If we examine the
liquid even when the vigour of the plant has reached its maximum, we
hardly find in it a trace of alcohol. The yeast has grown and
flourished, but it has almost ceased to act as a ferment. And could
every individual yeast-cell seize, without any impediment, free oxygen
from the surrounding liquid, it is certain that it would cease to act
as a ferment altogether.
What, then, are the conditions under which the yeast-plant must be
placed so that it may display its characteristic quality? Reflection
on the facts already referred to suggests a reply, and rigid
experiment confirms the suggestion. Consider the Alpine cherries in
their closed vessel. Consider the beer in its barrel, with a single
small aperture open to the air, through which it is observed not to
imbibe oxygen, but to pour forth carbonic acid. Whence come the
volumes of oxygen necessary to the production of this latter gas? The
small quantity of atmospheric air dissolved in the wort and overlying
it would be totally incompetent to supply the necessary oxygen. In no
other way can the yeast-plant obtain the gas necessary for its
respiration than by wrenching it from surrounding substances in which
the oxygen exists, not free, but in a state of combination. It
decomposes the sugar of the solution in which it grows, produces heat,
breathes forth carbonic acid gas, and one of the liquid products of
the decomposition is our familiar alcohol. The act of fermentation,
then, is a result of the effort of the little plant to maintain its
respiration by means of combined oxygen, when its supply of free
oxygen is cut off. As defined by Pasteur, fermentation is life
without air.
But here the knowledge of that thorough investigator comes to our aid
to warn us against errors which have 'been committed over and over
again. It is not all yeast-cells that can thus live without air and
provoke fermentation. They must be young cells which have caught
their vegetative vigour from contact with free oxygen. But once
possessed of this vigour the yeast may be transplanted into a
saccharine infusion absolutely purged of air, where it will continue
to live at the expense of the oxygen, carbon, and other constituents
of the infusion. Under these new conditions its life, as a plant,
will be by no means so vigorous as when it had a supply of free
oxygen, but its action as a ferment will be indefinitely greater.
Does the yeast-plant stand alone in its power of provoking alcoholic
fermentation? It would be singular if amid the multitude of low
vegetable forms no other could be found capable of acting in a similar
way. And here again we have occasion to marvel at that sagacity of
observation among the ancients to which we owe so vast a debt. Not
only did they discover the alcoholic ferment of yeast, but they had to
exercise a wise selection in picking it out from others, and giving it
special prominence. Place an old boot in a moist place, or expose
common paste or a pot of jam to the air; it soon becomes coated with a
blue-green mould, which is nothing else than the fructification of a
little plant called Penicillium glaucum. Do not imagine that the
mould has sprung spontaneously from boot, or paste, or jam; its germs,
which are abundant in the air, have been sown, and have germinated, in
as legal and legitimate a way as thistle-seeds wafted by the wind to a
proper soil. Let the minute spores of Penicillium be sown in a
fermentable liquid, which has been previously so boiled as to kill all
other spores or seeds which it may contain; let pure air have free
access to the mixture; the Penicillium will grow rapidly, striking
long filaments into the liquid, and fructifying at its surface. Test
the infusion at various stages of the plant's growth, you will never
find in it a trace of alcohol. But forcibly submerge the little
plant, push it down deep into the liquid, where the quantity of free
oxygen that can reach it is insufficient for its needs, it immediately
begins to act as a ferment, supplying itself with oxygen by the
decomposition of the sugar, and producing alcohol as one of the
results of the decomposition. Many other low microscopic plants act
in a similar manner. In aerated liquids they flourish without any
production of alcohol, but cut off from free oxygen they act as
ferments, producing alcohol exactly as the real alcoholic leaven
produces it, only less copiously. For the right apprehension of all
these facts we are indebted to Pasteur.
In the cases hitherto considered, the fermentation is proved to be the
invariable correlative of life, being produced by organisms foreign to
the fermentable substance. But the substance itself may also have
within it, to some extent, the motive power of fermentation. The
yeast-plant, as we have learned, is an assemblage of living cells; but
so at bottom, as shown by Schleiden and Schwann, are all living
organisms. Cherries, apples, peaches, pears, plums, and grapes, for
example, are composed of cells, each of which is a living unit. And
here I have to direct your attention to a point of extreme interest.
In 1821, the celebrated French chemist, Bérard, established the
important fact that all ripening fruit, exposed to the free
atmosphere, absorbed the oxygen of the atmosphere and liberated an
approximately equal volume of carbonic acid. He also found that when
ripe fruits were placed in a confined atmosphere, the oxygen of the
atmosphere was first absorbed, and an equal volume of carbonic acid
given out. But the process did not end here. After the oxygen had
vanished, carbonic acid, in considerable quantities, continued to be
exhaled by the fruits, which at the same time lost a portion of their
sugar, becoming more acid to the taste, though the absolute quantity
of acid was not augmented. This was an observation of capital
importance, and Bérard had the sagacity to remark that the process
might be regarded as a kind of fermentation.
Thus the living cells of fruits can absorb oxygen and breathe out
carbonic acid, exactly like the living cells of the leaven of beer.
Supposing the access of oxygen suddenly cut off, will the living
fruit-cells as suddenly die, or will they continue to live as yeast
lives, by extracting oxygen from the saccharine juices round them?
This is a question of extreme theoretic significance. It was first
answered affirmatively by the able and conclusive experiments of
Lechartier and Bellamy, and the answer was subsequently confirmed and
explained by the experiments and the reasoning of Pasteur. Bérard
only showed the absorption of oxygen and the production of carbonic
acid; Lechartier and Bellamy proved the production of alcohol, thus
completing the evidence that it was a case of real fermentation,
though the common alcoholic ferment was absent.
*****
So full was Pasteur of the idea that the cells of a fruit would
continue to live at the expense of the sugar of the fruit, that once
in his laboratory, while conversing on these subjects with M. Dumas,
he exclaimed, 'I will wager that if a grape be plunged into an
atmosphere of carbonic acid, it will produce alcohol and carbonic acid
by the continued life of its own cells--that they will act for a time
like the cells of the true alcoholic leaven.' He made the experiment,
and found the result to be what he had foreseen. He then extended the
'enquiry. Placing under a bell-jar twenty-four plums, he filled the
jar with carbonic acid gas; beside it he placed twenty-four similar
plums uncovered. At the end of eight days, he removed the plums from
the jar, and compared them with the others. The difference was
extraordinary. The uncovered fruits had become soft, watery, and very
sweet; the others were firm and hard, their fleshy portions being not
at all watery. They had, moreover, lost a considerable quantity of
their sugar. They were afterwards bruised, and the juice was
distilled. It yielded six and a half grammes of alcohol, or one per
cent. of the total weight of the plums. Neither in these plums, nor
in the grapes first experimented on by Pasteur, could any trace of the
ordinary alcoholic leaven be found. As previously proved by
Lechartier and Bellamy, the fermentation was the work of the living
cells of the fruit itself, after air had been denied to them. When,
moreover, the cells were destroyed by bruising, no fermentation
ensued. The fermentation was the correlative of a vital act, and it
ceased when life was extinguished.
Luedersdorf was the first to show by this method that yeast acted,
not, as Liebig had assumed, in virtue of its organic, but in virtue of
its organised character. He destroyed the cells of yeast by rubbing
them on a ground glass plate, and found that with the destruction of
the organism, though its chemical constituents remained, the power to
act as a ferment totally disappeared.
One word more in reference to Liebig may find a place here. To the
philosophic chemist thoughtfully pondering these phenomena, familiar
with the conception of molecular motion, and the changes produced by
the interactions of purely chemical forces, nothing could be more
natural than to see in the process of fermentation a simple
illustration of molecular instability, the ferment propagating to
surrounding molecular groups the overthrow of its own tottering
combinations. Broadly considered, indeed, there is a certain amount
of truth in this theory; but Liebig, who propounded it, missed the
very kernel of the phenomena when he overlooked or contemned the part
played in fermentation by microscopic life. He looked at the matter
too little with the eye of the body, and too much with the spiritual
eye. He practically neglected the microscope, and was unmoved by the
knowledge which its revelations would have poured in upon his mind.
His hypothesis, as I have said, was natural--nay it was a striking
illustration of Liebig's power to penetrate and unveil molecular
actions; but it was an error, and as such has proved an ignis fatuus
instead of a pharos to some of his followers.
*****
I have said that our air is full of the germs of ferments differing
from the alcoholic leaven, and sometimes seriously interfering with
the latter. They are the weeds of this microscopic garden which often
overshadow and choke the flowers. Let us take an illustrative case.
Expose milk to the air. It will, after a time, turn sour, separating
like blood into clot and serum. Place a drop of this sour milk under
a powerful microscope and watch it closely. You see the minute
butter-globules animated by that curious quivering motion called the
Brownian motion. But let not this attract your attention too much,
for it is another motion that we have now to seek. Here and there you
observe a greater disturbance than ordinary among the globules; keep
your eye upon the place of tumult, and you will probably see emerging
from it a long eel-like organism, tossing the globules aside and
wriggling more or less rapidly across the field of the microscope.
Familiar with one sample of this organism, which from its motions
receives the name of vibrio, you soon detect numbers of them. It is
these organisms, and other analogous though apparently motionless
ones, which by decomposing the milk render it sour and putrid. They
are the lactic and putrid ferments, as the yeast-plant is the
alcoholic ferment of sugar. Keep them and their germs out of your
milk and it will continue sweet. But milk may become putrid without
becoming sour. Examine such putrid milk microscopically, and you find
it swarming with shorter organisms, sometimes associated with the
vibrios, sometimes alone, and often manifesting a wonderful alacrity
of motion. Keep these organisms and their germs out of your milk and
it will never putrify. Expose a mutton-chop to the air and keep it
moist; in summer weather it soon stinks. Place a drop of the juice of
the fetid chop under a powerful microscope; it is seen swarming with
organisms resembling those in the putrid milk. These organisms, which
receive the common name of bacteria, [Footnote: Doubtless organisms
exhibiting grave specific differences are grouped together under this
common name.] are the agents of all putrefaction. Keep them and their
germs from your meat and it will remain for ever sweet. Thus we begin
to see that within the world of life to which we ourselves belong,
there is another living world requiring the microscope for its
discernment, but which, nevertheless, has the most important bearing
on the welfare of the higher life-world.
And now let us reason together as regards the origin of these
bacteria. A granular powder is placed in your hands, and you are
asked to state what it is. You examine it, and have, or have not,
reason to suspect that seeds of some kind are mixed up in it. To
determine this point you prepare a bed in your garden, sow in it the
powder, and soon after find a mixed crop of docks and thistles
sprouting from your bed. Until this powder was sown neither docks nor
thistles ever made their appearance in your garden. You repeat the
experiment once, twice, ten times, fifty times. From fifty different
beds after the sowing of the powder, you obtain the same crop. What
will be your response to the question proposed to you? 'I am not in a
condition,' you would say, 'to affirm that every grain of the powder
is a dock-seed, or a thistle-seed; but I am in a condition to affirm
that both dock and thistle-seeds form, at all events, part of the
powder.' Supposing a succession of such powders to be placed in your
hands with grains becoming gradually smaller, until they dwindle to
the size of impalpable dust particles; assuming that you treat them
all in the same way, and that from every one of them in a few days you
obtain a definite crop--may be clover, it may be mustard, it may be
mignonette, it may be a plant more minute than any of these, smallness
of the particles, or of the plants that spring from them, does not
affect the validity of the conclusion. Without a shadow of misgiving
you would conclude that the powder must have contained the seeds or
germs of the life observed. There is not in the range of physical
science, an experiment more conclusive nor an inference safer than
this one.
Supposing the powder to be light enough to float in the air, and that
you are enabled to see it there just as plainly as you saw the heavier
powder in the palm of hand. If the dust sown by the air instead of by
the hand produce a definite living crop, with the same logical rigour
you would conclude that the germs of this crop must be mixed with the
dust. To take an illustration: the spores of the little plant
Penicillium glaucum, to which I have already referred, are light
enough to float in the air. A cut apple, a pear, a tomato, a slice of
vegetable marrow, or, as already mentioned, an old moist boot, a dish
of paste, or a pot of jam, constitutes a proper soil for the
Penicillium. Now, if it could be proved that the dust of the air when
sown in this soil produces this plant, while, wanting the dust,
neither the air, nor the soil, nor both together can produce it, it
would be obviously just as certain in this case that the floating dust
contains the germs of Penicillium as that the powders sown in your
garden contained the germs of the plants which sprung from them.
But how is the floating dust to be rendered visible? In this way.
Build a little chamber and provide it with a door, windows, and
window-shutters. Let an aperture be made in one of the shutters
through which a sunbeam can pass. Close the door and windows so that
no light shall enter save through the hole in the shutter. The track
of the sunbeam is at first perfectly plain and vivid in the air of the
room. If all disturbance of the air of the chamber be avoided, the
luminous track will become fainter and fainter, until at last it
disappears absolutely, and no trace of the beam is to be seen. What
rendered the beam visible at first? The floating dust of the air,
which, thus illuminated and observed, is as palpable to sense as dust
or powder placed on the palm of the hand. In the still air the dust
gradually sinks to the floor or sticks to the walls and ceiling, until
finally, by this self-cleansing process, the air is entirely freed
from mechanically suspended matter.